Testing to Evaluate 

Extended Battery 

Operation in Nuclear 

Power Plants

Office of Nuclear Regulatory Research

NUREG/CR-7188

BNL-NUREG-106282-2014
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Testing to Evaluate 

Extended Battery 

Operation in Nuclear 

Power Plants

Manuscript Completed: October 2014

Date Published: May 2015

Prepared by: 

W. Gunther, G. Greene, Y. Celebi, K. Sullivan and J. Higgins

Brookhaven National Laboratory

Nuclear Science and Technology Department

Systems Engineering Group

Upton, NY 11973-5000

Liliana Ramadan, NRC Project Manager

Office of Nuclear Regulatory Research

NUREG/CR-7188

BNL-NUREG-106282-2014

iii

ABSTRACT

The Japanese earthquake and tsunami event on March 11, 2011, illustrated the fact that 

restoration of the alternating current (AC) power supply at nuclear power plants (NPPs) can be 

significantly impacted by external events and can take a longer time to recover under certain 

scenarios. Therefore, a lesson learned from the Japanese event that may be applicable to U.S. 

NPPs is the need to examine the extended loss of alternating current power (ELAP) conditions 

and determine if the existing vented lead acid batteries can function beyond their defined design 

basis (or beyond-design basis if existing Station Blackout (SBO) coping analyses were utilized) 

duty cycles in order to support core cooling. The NRC’s Office of Nuclear Regulatory Research 

sponsored testing to evaluate the battery’s performance availability and capability to supply the 

necessary direct current (DC) loads to support core cooling and instrumentation requirements 

for extended periods of time. Plant profiles from four NPPs [3 Pressurized Water Reactors 

(PWRs) and 1 Boiling Water Reactor (BWR)] were obtained from the nuclear industry through 

the Nuclear Energy Institute (NEI) and were used for this test program. The testing provided an 

indication of the amount of time available (depending on the actual load profile) for batteries to 

continue to supply core cooling equipment beyond the original duty cycles for a representative 

plant. Testing also demonstrated that battery availability can be significantly extended using 

load shedding techniques to allow more time to recover AC power. The projected availability of 

a battery can be accurately calculated using the IEEE Standard 485-2010, “IEEE 

Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications,” or using the 

empirical algorithm described in this report. This report provides detailed test results and 

analysis regarding the battery’s performance availability and capability for the tested load 

profiles.

v

FOREWORD

This NUREG report documents the testing that was sponsored by the U.S. Nuclear Regulatory 

Commission (NRC) and conducted at Brookhaven National Laboratory (BNL) to determine the 

ability of the vented lead-acid batteries used in nuclear power plants (NPPs) to operate during 

an extended loss of alternating current power (ELAP) event which is similar to a Station 

Blackout (SBO) with the exception that alternate alternating current (AAC) power sources are 

not available and the emergency diesel generators and offsite power are assume nonrecoverable.

The Japanese earthquake and tsunami event on March 11, 2011, illustrated the fact that 

restoration of alternating current (AC) power at NPPs can be significantly impacted by external 

events and can take a long time to recover under certain scenarios. Therefore, a lesson learned 

from the Japanese event that may be applicable to U.S. NPPs is the need to examine ELAP 

conditions and determine if the existing safety-related Class 1E batteries can function beyond 

their design basis (or beyond-design basis if existing Station Blackout (SBO) coping analyses 

were utilized) duty cycles to support core cooling. 

The purpose of this research project was to test the batteries’ response to ELAP conditions and 

to determine the batteries’ performance capability. The testing simulated four load profiles that 

were longer periods of time than typical SBO coping times and, in some respects, with less 

severe low load profiles. As a result, this testing provided information on the maximum duration 

available from typical NPP batteries to support decay heat removal (natural circulation) during 

an ELAP event. The four typical load profiles tested were developed in cooperation with the 

Nuclear Energy Institute and the Electric Power Research Institute, along with voluntary 

participation from several nuclear power plants. The profiles were collectively derived from

typical SBO loading profiles at several NPPs (three pressurized water reactors and one boiling 

water reactor) and modified to address the impacts of the time to initiate load shedding and the 

effect of intermittent loads.

BNL retained three sets of nuclear qualified batteries from a previous test arrangement. The 

three battery vendors were Enersys, Exide/GNB, and C&D Technologies. Each battery 

consisted of twelve battery cells that are the same models typically used in NPP safety-related 

Class 1E direct current (DC) power systems. The production and control of the batteries met 

requirements set forth in Appendix B of Title 10 of part 50 of the Code of Federal Regulations 

and Regulatory Guide 1.129 that are installed in safety-related Class 1E DC power systems at 

NPPs. The test setup was consistent with a typical nuclear power plant’s Class 1E battery 

system design except for seismic supports, the number of cells in the battery strings used, and 

an aging factor to simulate a near-end-of life condition. The batteries used in this testing were 

deep cycled many times, which was equivalent to the number of deep discharges that a nuclear 

grade battery would typically see during its typical 20-year life. Nonetheless, the most 

significant uncertainty associated with the test results is that the batteries were not pre-aged to 

simulate an end-of-life condition. In order to account for the fewer number of cells in a battery 

string, a scaling factor was used in the analysis of the test results. 

The testing described in this NUREG report provides an indication of the amount of time 

available (depending on the load profile) for batteries to continue to supply core cooling 

equipment and vital instrumentation during an ELAP event. This extended power supply may be
vi

helpful in managing the recovery of AC power to the battery chargers and/or AC power in 

general to maintain or restore core cooling during an ELAP event. 

Overall, the measured battery availability varied from 22 to 48 hours. Nevertheless, several 

plant-specific factors can reduce the extended battery times. These factors include aging due to 

the length of time a battery is in service and the battery’s ambient operating temperature. 

Conversely, testing confirmed that the batteries can be operated up to 72 hours at a constant 

load. 

The testing also demonstrated that battery performance is consistent with manufacturer 

performance data. This enabled the staff to conclude that battery performance can be 

accurately predicted using the IEEE Standard 485-2010, “IEEE Recommended Practice for 

Sizing Lead-Acid Batteries for Stationary Applications” or using the empirical algorithm 

described in this report. All of this information can be valuable to nuclear power plant licensees 

in planning strategies for an ELAP event to maintain key safety functions.

 
vii

TABLE OF CONTENTS

Section Page

ABSTRACT .................................................................................................................................iii

FOREWORD.................................................................................................................................v

LIST OF FIGURES ......................................................................................................................ix

LIST OF TABLES........................................................................................................................xi

EXECUTIVE SUMMARY...........................................................................................................xiii

ACKNOWLEDGMENTS............................................................................................................xix

ABBREVIATIONS AND ACRONYMS ......................................................................................xxi

1 INTRODUCTION..................................................................................................................1

1.1 Objectives ..................................................................................................................1

1.2 Research Approach ...................................................................................................2

1.3 Assumptions and Measures of Uncertainty................................................................2

1.4 Test Facility ................................................................................................................3

1.4.1 Laboratory Space ...........................................................................................3

1.4.2 Battery Test Setup..........................................................................................3

1.4.3 Data Acquisition Equipment ...........................................................................4

1.5 Quality Plan................................................................................................................4

1.6 Test Plan....................................................................................................................4

2 EXTENDED BATTERY OPERATION (SEQUENCE 1) TESTING RESULTS ..................13

2.1 Background..............................................................................................................13

2.2 Test results-Time versus current response..............................................................13

2.3 Test results-Time versus voltage response .............................................................15

2.4 Test results-Time versus voltage response-special case ........................................17

2.5 Evaluation of Test Anomaly .....................................................................................19

2.6 Test results –Time versus specific gravity ...............................................................21

2.7 Test results –Time versus temperature....................................................................24

2.8 Test Results – Battery Recharge Characteristics ....................................................26

2.9 Comparison to Vendor Curves.................................................................................27

2.10 Summary of Sequence 1 Test Results.....................................................................28

3 LOAD PROFILE (SEQUENCE 2) TESTING RESULTS ...................................................31

3.1 Introduction ..............................................................................................................31

3.2 Background Information – ELAP..............................................................................31

3.3 Sequence 2 Load Profiles ........................................................................................35

3.4 Sequence 2 Test Results .........................................................................................39

3.4.1 Test Series #1: Load Profile EBR-5..............................................................40

3.4.2 Test Series #2 (Load Profile EBR-4) ............................................................43

3.4.3 Test Series #3 (Load Profile EBR-5 and EBR-3)..........................................46

3.4.4 Test Series #4 (Load Profiles EBR-5 and EBR-2)........................................51

3.5 Recharge Characteristics of Load-Profile Tests.......................................................56
viii

3.6 Summary of the Load Profile Testing (Sequence 2) ................................................57

4 SUMMARY AND CONCLUSIONS ....................................................................................61

5 REFERENCES...................................................................................................................63
ix

LIST OF FIGURES

Figure Page

Figure ES-1 Extended Battery Availability Test Results............................................................ xiv

Figure ES-2 Enersys load profile EBR-2 - Average Cell Voltage .............................................. xvi

Figure ES-3 Average End of Test Specific Gravity - Sequence 1 Tests ................................... xvi

Figure ES-4 Average End of Test Specific Gravity - Sequence 2 Tests ...................................xvii

Figure ES-5 Recharge Current Curves for C&D cycles 15-19 (Sequence 1 Tests) .................xviii

Figure 1-1 BNL Battery Facility..................................................................................................4

Figure 1-1 Battery availability for other battery types using algorithm .......................................7

Figure 2-1 Battery availability versus load from 1 to 72 hours.................................................15

Figure 2-2 Typical Lead-Acid Battery Cell Voltage Response During Constant Current 

Discharge (EPRI 1006757 [20]) .............................................................................15

Figure 2-3 C&D Cell #10 Response during 72-hour discharge test.........................................16

Figure 2-4 GNB Cell #4 Response during 48-hour discharge test ..........................................16

Figure 2-5 Enersys Cell #3 Response during 36-hour discharge test .....................................17

Figure 2-6 Enersys Cell #2 Voltage Response during 72-hour test.........................................17

Figure 2-7 Cell #2 Voltage Response during attempted recharge...........................................18

Figure 2-8 Cell #2 Temperature Response during attempted recharge ..................................18

Figure 2-9 Enersys Cell Voltages at the conclusion of the 72-hour performance test.............19

Figure 2-10 Average End of Test Specific Gravity - Sequence 1 Tests ....................................22

Figure 2-11 Cell Temperature Monitoring Using Surface Mounted Thermocouple ...................25

Figure 2-12 Effect of Temperature on Capacity (Adapted from 

IEEE Standard 485 -2010 [10]) ..............................................................................25

Figure 2-13 C&D Capacity Curves for the LCR-33 battery........................................................28

Figure 2-14 Average “end-of-test” cell midpoint specific gravity................................................29

Figure 2-15 C&D Cell #2 Voltage Response during the 72-hour performance test...................29

Figure 2-16 C&D recharge characteristics for tests from 8 hours to 72 hours...........................30

Figure 3-1 EBR-2 Load Profile Test Results for Enersys Run #15..........................................36

Figure 3-2 EBR-3 Load Profile Test Results for Enersys Run #12..........................................37

Figure 3-3 EBR-4 Load Profile Test Results for GNB Run #8 .................................................38

Figure 3-4 EBR-5 Load Profile Test Results for C&D Run #4 .................................................39

Figure 3-5 Voltage Change over Time (C&D battery test run #4)............................................42

Figure 3-6 Voltage Change over Time (Enersys battery test run #9) ......................................45

Figure 3-7 Test Run #10 (C&D LCR-33 Battery).....................................................................48

Figure 3-8 Test Run #11 (GNB NCN-21 Battery) ....................................................................48

Figure 3-9 Test Run #12 (Enersys 2GN-23 Battery) ...............................................................49

Figure 3-10 Voltage Change over Time (Enersys cell #6- test run #12)....................................49

Figure 3-11 Test Run #13 (C&D LCR-33 Battery).....................................................................53

Figure 3-12 Test Run #14 (GNB NCN-21 Battery) ....................................................................53

Figure 3-13 Test Run #15 (Enersys 2GN-23 Battery) ...............................................................54

Figure 3-14 Voltage Change over Time (C&D cell #6- test run #13).........................................54

Figure 3-15 Enersys Battery Recharge Characteristics during Load Profile Testing.................56

Figure 3-16 GNB Voltage Response to Load Profile #4 (EBR-2)..............................................59

Figure 3-17 Average “end-of-test” midpoint specific gravity readings for Sequence 2 tests......59

xi

LIST OF TABLES

Table Page

Table ES-1 Summary of Sequence 1- Extended Battery Operational Testing ......................... xiv

Table ES-2 Summary of Test Duration Times Compared to Calculated Estimates................... xv

Table 1-1 Battery Vendor Data – Rated Discharge Current vs.Time End Voltage ...................5

Table 1-2 Test Plan of Loads and Estimated Test Duration (Sequence 1)...............................6

Table 1-3 Battery availability for other battery types using algorithm........................................7

Table 1-4 Summary of Load Profile Tests ................................................................................9

Table 2-1 Summary of Sequence 1 Performance Tests.........................................................14

Table 2-2 Actual Ampere-Hours discharged as a function of test duration.............................14

Table 2-3 Enersys cell voltages at the conclusion of the extended duration tests..................20

Table 2-4 GNB cell voltages at the conclusion of the extended duration tests.......................20

Table 2-5 C&D cell voltages at the conclusion of the extended duration tests .......................21

Table 2-6 C&D cell specific gravity at the conclusion of the extended duration tests.............22

Table 2-7 GNB cell specific gravity at the conclusion of the extended duration tests.............23

Table 2-8 Enersys cell specific gravity at the conclusion of the extended duration tests........24

Table 2-9 Enersys Extended Service Recharge Data ............................................................26

Table 2-10 GNB Extended Service Recharge Data..................................................................27

Table 2-11 C&D Extended Service Recharge Data..................................................................27

Table 3-1 Test Series #1 Loading (Rated Current and Time).................................................41

Table 3-2 Test Series #1 Comparison of estimated to actual battery availability....................41

Table 3-3 Impact of load shed timing on battery availability ...................................................41

Table 3-4 First End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)........42

Table 3-5 Specific Gravity readings for Test Series #1...........................................................43

Table 3-6 Test Series #2 Loading (Rated Current and Time).................................................44

Table 3-7 Test Series #2 Comparison of estimated to actual battery availability....................44

Table 3-8 Comparison of battery rating to its availability ........................................................45

Table 3-9 Second End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)...45

Table 3-10 Specific Gravity readings for Test Series #2...........................................................46

Table 3-11 Test Series #3 Loading (Rated Current and Time).................................................47

Table 3-12 Test Series #3 Comparison of estimated to actual battery availability....................47

Table 3-13 Third End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts).......50

Table 3-14 Voltage changes with application of intermittent loads ...........................................50

Table 3-15 Specific Gravity readings for Test Series #3...........................................................51

Table 3-16 Test Series #4 Loading (Rated Current and Time).................................................52

Table 3-17 Test Series #4 Comparison of estimated to actual battery availability....................52

Table 3-18 Impact of load shedding time on battery availability ...............................................53

Table 3-19 Fourth End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts).....55

Table 3-20 Specific Gravity readings for Test Series #4...........................................................55

Table 3-21 Recharge Characteristics Observed/Calculated During Sequence 2 Testing ........57

Table 3-22 Summary of Estimated To Actual Battery Availability Times (Hours) .....................58

xiii

EXECUTIVE SUMMARY

The Japanese earthquake and tsunami event on March 11, 2011, illustrated the fact that 

restoration of AC power at nuclear power plants (NPPs) can be significantly impacted by 

external events and can take a long time to recover under certain scenarios. Therefore, a lesson 

learned from the Japanese event that may be applicable to the U.S. NPPs is the need to 

examine ELAP conditions and determine if the existing safety-related Class 1E batteries can 

function beyond their design basis (or beyond-design basis if existing Station Blackout (SBO) 

coping analyses were utilized) duty cycles to support core cooling. This report describes the 

research approach that was taken, the testing activities that were performed, and the results 

and conclusions drawn from this research effort.

BNL used the three sets of nuclear qualified batteries from a previous test arrangement as 

described in NUREG/CR-7148 [1]. Each battery string consisted of 12 cells. The production and 

control of the cells met 10 CFR 50 Appendix B Criteria and Regulatory Guide 1.129 that are 

installed as safety-related Class 1E DC power systems at nuclear plants. Test equipment was 

calibrated to national standards to reduce the parametric uncertainty associated with 

measurements of current, voltage, specific gravity, temperature and time. The batteries were 

tested in a manner consistent with how they would be operated in a typical nuclear power 

plant’s safety-related Class 1E DC power system using a resistive load bank. The most 

significant uncertainty associated with the test results is that the batteries were not pre-aged to 

simulate an end-of-life condition for the batteries. Further discussion of the test assumptions

and uncertainty can be found in section 1.3 of this report. 

Two sets of test results are provided in this report denoted as Sequence 1 and Sequence 2. In 

Sequence 1, a series of tests were conducted to verify various lengths of time that the tested 

batteries can operate at reduced discharge rates. The battery vendors provide capacity 

specifications for their batteries at various load conditions. For example, Enersys has published 

capacity data to 72 hours, C&D to 12 hours, and GNB to 8 hours. Sequence 1 testing at BNL 

acquired battery capacity data to fill in the data gaps of the vendor data out to 72 hours with 

discharge intervals of 8, 12, 24, 36, 48, and 72 hours. BNL compared the tested batteries 

against the published data for the 8-hour performance test that served as the baseline readings. 

The tests allowed BNL to develop an algorithm to predict the expected availability for a battery 

at any given load. This algorithm was used along with IEEE Standard 485-2010 to predict the 

battery run times for the load profiles tested in sequence 2. 

In Sequence 2, ELAP conditions were simulated by a series of load profiles and load shedding 

schemes to determine how long the battery could carry the anticipated plant loads. These tests 

used four different load profiles that were supplied by the Nuclear Energy Institute (NEI) and 

approved by NRC. The four load profiles were from four different (NPPs) who had analyzed the 

equipment needed to assure core cooling during an ELAP event while providing the critical 

instrumentation required by plant operators and decision makers. These load profiles were 

designated as EBR-2 through EBR-5 to preserve the anonymity of the NPPs who volunteered 

their load profile information. In each of these tests, the test terminated when the overall battery 

string reached 21.0 volts (an average of 1.75 volts/cell). Some NPPs have calculated higher 

battery end voltages due to specific load demands or the length of cable from the battery to the 

loads, so their expected battery availability times would be less but calculable from the test data. 

Four observations of relevance to understanding battery performance for extended battery duty 

cycles (i.e., those greater than 8 hours which is the maximum credited battery coping time to 

satisfy the existing SBO Rule) are summarized below:
xiv

Observation 1: In Sequence 1 of the test program, a series of performance tests were 

conducted to verify various lengths of time that the tested batteries can operate at reduced 

discharge rates. The battery vendors provide capacity specifications for their cell types at 

various load conditions as depicted in Table ES-1. For example, Enersys published data to 72 

hours, C&D to 12 hours, and GNB to 8 hours. The testing at BNL acquired battery data to fill in 

the gaps of vendor data as identified by the shading and parentheses in Table ES-1. BNL also 

tested the batteries to confirm the 8 hour data point that was available from each vendor.

Table ES-1 Summary of Sequence 1- Extended Battery Operational Testing

Test Duration 

(amps)/Battery Type

1 

hour

2 

hours

4 

hours

8 hours 

(Baseline)

12 

hours

24 

hours

36 

hours

48 

hours

72 

hours

Enersys 2GN-23 925 605 385 225 160 89 (60) (47) 31.9

Exide/GNB NCN-21 750 515 317 187 (132) (71) (50) (39) (27)

C&D LCR-33 1167 799 500 290 205 (111) (77) (60) (42)

The resulting composite information of battery availability for a given load from 1 to 72 hours is 

displayed in Figure ES-1. The C&D battery LCR-33 battery has an 8-hour rating of 2320 

ampere-hours; the Enersys 2GN-23 battery has an 8-hour rating of 1800 ampere-hours; the 

Exide/GNB NCN-21 battery has an 8-hour rating of 1496 ampere-hours. The testing showed 

that batteries typically used in NPPs could safely operate for 72 hours when there is load 

reduction between (25 and 45 amps in this case), depending on the ampere-hour capacity of 

the battery.

Figure ES-1 Extended Battery Availability Test Results

In Sequence 2 of the testing program, battery duration times obtained during testing were 

consistent with the estimates obtained using the approach provided in IEEE Standard 485-2010 

[10] and an algorithm that was developed by BNL using battery vendor data supplemented by 

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

Load (Amps)

Availability (Hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33
xv

the extended operational testing results achieved in the Sequence 1 testing (see Section 1.5 for 

the algorithm development).

Table ES-2 contains the results of the Sequence 2 tests along with the estimates that were 

made of battery availability prior to the test. Load shedding was initiated at various times and 

the loads applied to the batteries were based on the data provided by the NEI. The table 

illustrates how accurately battery availability can be calculated once the loads and their duration 

are obtained. 

Table ES-2 Summary of Test Duration Times Compared to Calculated Estimates

TEST # BATTERY

TYPE

Estimate

(Hours); 

IEEE 485

Estimate

(Hours); 

BNL 

Test 

Measurement 

(Hours)

Temp. (°F); 

(start of 

test)

4 C&D 27.0 25.3 26.6 77

5 GNB 29.2 30.4 32.61 78

6 Enersys 29.2 28.4 27.7 78

7 C&D 40.6 37.7 39.9 77

8 GNB 20.3 21.2 22.21 77

9 Enersys 26.3 27.2 26.3 77

10 C&D 30.4 28.5 30.61 77

11 GNB 36-40 42.8 45.21 78

12 Enersys 36-40 42.7 41.2 78

13 C&D 32.0 30.1 32.21 78

14 GNB 42.0 44.9 47.71 78

15 Enersys 43.8 45.7 44.2 77 1

Measured time exceeded both estimates

Observation 2: The voltage profiles for the cell and overall string voltage were linear through 

most of the discharge until it approached 1.85 volts where it decreased more rapidly. This nonlinear response at the point where the overall battery voltage is about 111 volts (1.85V x 60 

cells) has important operational implications since the ability to supply critical loads will be 

significantly diminished. This consistently observed response indicates the need to more 

carefully monitor the battery as it approaches its end voltage. An example of the voltage 

response is illustrated in Figure ES-2.
xvi

Figure ES-2 Enersys load profile EBR-2 - Average Cell Voltage

Observation 3: Specific gravity was routinely measured on all cells prior to each test, at the 

completion of the discharge cycle, and following the recharge of the battery. Specific gravity is 

a measure of a cell’s state of charge. An important observation made following the longer 

duration test is that the specific gravity had decreased to a very low level as compared to the 4-

hour and 8-hour tests that are typically used to measure battery capacity. The data that is of 

interest relates to the specific gravity readings obtained at the end of the discharge cycles, 

specifically the relationship of the midpoint specific gravity readings as a function of the length of 

the discharge test. That is, the longer the test, the lower the value of the specific gravity at the 

end of the test. The end point of 1.020 obtained on the 72-hour discharge tests for all three 

vendors is equivalent to about 3% acid; the normal fully charged acid concentration is 29% for a 

specific gravity of 1.215 and the typical acid concentration at the end of an 8-hour performance 

test is 12%. The average cell midpoint specific gravity measured at end of the various tests is 

illustrated in Figures ES-3 and ES-4 for Sequences 1 and 2, respectively.

Figure ES-3 Average End of Test Specific Gravity - Sequence 1 Tests

1.75

1.85

1.95

2.05

2.15

2.25

0 10 20 30 40 50

Average cell voltage (volts)

Test Duration (Hours)

1.00

1.02

1.04

1.06

1.08

1.10

0 20 40 60 80

Specific gravity

Test duration (hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33
xvii

Figure ES-4 Average End of Test Specific Gravity - Sequence 2 Tests

The implication of this observation is that the ability of the battery to be recharged once ac 

power becomes available could be jeopardized if the specific gravity remains extremely low for 

several hours. One of the battery vendors noted that in low specific gravity circumstances, the 

sulfate on the face of the plate can dissolve and re-crystallize back onto the surface in a 

continuous layer, essentially forming a lead sulfate/lead hydrate surface coating over the whole 

plate. This, in combination with the very low specific gravity of the electrolyte between the 

plates, can combine to create a sufficiently high resistance to impede current flow making the 

battery inoperable.

Observation 4: The time to recharge the battery to a fully charged state is longer following the 

long duration discharge tests. Following each discharge during the Sequence 1 testing, the 

battery was placed on a float charge as recommended by the battery vendors (27 volts or 2.25 

V/cell) and the recharge current monitored for 24 hours. Figure ES-5 illustrates the recharge 

current curve for each of the extended operational tests of the C&D battery. The other two 

battery types exhibited a similar response.

1.00

1.02

1.04

1.06

1.08

1.10

1.12

0 10 20 30 40 50

Specific gravity

Test duration (Hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33
xviii

Figure ES-5 Recharge Current Curves for C&D cycles 15-19 (Sequence 1 Tests)

Duration of Discharge Increasing
xix

ACKNOWLEDGMENTS

The authors wish to thank Liliana Ramadan, NRC Project Manager, for her guidance and input 

during the development, conduct of testing and writing of this report. We thank her NRC 

colleagues Matthew McConnell, Kenn Miller, Mike Waterman and Roy Mathew for their insights 

and helpful comments during their review of the draft document. We very much appreciate the 

support and involvement of Richard Reister of DOE/NE-72. We are thankful for the technical 

views and insights provided by our colleagues and EPRI and NEI in the development and 

implementation of the extended battery testing program particularly the contributions of Kyle 

Floyd, Wayne Johnson, Ken Caraway, and Gordon Clefton. We appreciate the work by Paul 

Giannotti and Randy Weidner of BNL in providing instrumentation and graphical support, 

respectively. We acknowledge and appreciate the administrative efforts of Maria Anzaldi in 

producing this document.

xxi

ABBREVIATIONS AND ACRONYMS

BNL Brookhaven National Laboratory

BWR Boiling Water Reactor

EPRI Electric Power Research Institute

ELAP Extended Loss of AC Power

IEEE Institute of Electrical and Electronics Engineers

LOCA Loss of Coolant Accident

LOOP Loss of Off-site Power

MOV Motor Operated Valve

NEI Nuclear Energy Institute

NPP Nuclear Power Plant

NRC Nuclear Regulatory Commission

PWR Pressurized Water Reactor

PRA Probabilistic Risk Assessment

SBO Station Blackout

1

1 INTRODUCTION

In 1991, the NRC published NUREG 1150, “Severe Accident Risks: An Assessment for Five 

U.S. Nuclear Power Plants.” In this study the NRC used new and improved Probabilistic Risk 

Analysis (PRA) techniques for consequence analysis. The study was a significant turning point 

in the use of PRA in all regulatory matters. As a result, risk informed regulation such as 10 CFR 

50.63, Loss of all alternating current power [station blackout (SBO)], was implemented. The 

NRC also issued guidance for addressing severe accidents and PRA in the following document: 

“NRC Policy Statement on Severe Reactor Accidents regarding future designs and existing 

plants.” This guidance contains four items for new and operating reactors. One of the four items 

was a special focus on assuring the reliability of decay heat removal systems and reliability of 

alternating current (AC) and direct current (DC) operated steam driven/diesel driven cooling 

systems. The DC-operated core cooling systems are generally relied on for SBO conditions.

The Japanese earthquake and tsunami event on March 11, 2011, illustrated the fact that 

restoration of AC power at nuclear power plants (NPPs) can be significantly impacted by 

external events and can take a longer time to recover under certain scenarios. Therefore, a 

lesson learned from the Japanese event that is applicable to the U.S. reactors is the need to 

examine extended loss of AC power (ELAP) conditions and determine if the existing safetyrelated Class 1E batteries can function beyond their design basis (or beyond-design basis if 

existing Station Blackout (SBO) coping analyses were utilized) duty cycles to support core 

cooling. 

This Report describes the efforts undertaken and the results achieved to satisfy the objectives 

of this research.

1.1 Objectives

The purpose of this research project was to model and test the response of representative 

samples of nuclear qualified batteries to ELAP conditions to determine battery capacity and 

capability under these conditions. The testing involved operating the batteries at the lower loads 

needed to extend the availability of the battery, and simulating four sample load profiles 

provided by the NEI. The four load profiles were from four different NPPs who had analyzed the 

equipment needed to assure core cooling during an ELAP event while providing the critical 

instrumentation required by plant operators and decision makers. As a result, this project 

provided information on the durations available from typical safety-related Class 1E batteries to 

support decay heat removal and supply critical instrumentation during an ELAP event.

BNL used the three sets of nuclear qualified batteries from a previous test arrangement as 

described in NUREG/CR-7148 [1]. The three battery types are:

• Enersys type 2GN-23 cells with an 8-hour rating of 1800 Ampere-hours (A-hrs) at 225 

Amperes (A),

• Exide GNB type NCN-21 cells with an 8-hour rating of 1496 A-hrs at 187 A, and

• C&D Technologies type LCR-33 cells with an 8-hour rating of 2320 A-hrs at 290 A. 

Each battery string consisted of 12 battery cells. The production and control of the cells met 

10 CFR 50 Appendix B Criteria and Regulatory Guide 1.129 that apply to batteries that are 

installed as safety-related Class 1E at NPPs. The test arrangement is consistent with a typical 

NPP’s safety-related Class 1E battery design except for seismic supports and the fact that a 
2

battery string at a NPP typically consists of 60-cell battery strings for a 125 Volt (V) DC power 

system. While these batteries were not pre-aged they had each had been subjected to fifteen 

deep discharges (4-hour capacity tests) prior to the start of this program. This is an equivalent 

number of deep discharges that a nuclear grade battery would typically see during its typical 

twenty-year life. 

1.2 Research Approach

The approach taken in this research project involved testing battery models that are typically 

used in commercial NPPs in the U.S. The batteries were subjected to a series of tests to 

simulate a range of loads that are representative of a variety of SBO conditions in which nonessential or redundant equipment is de-energized to extend the output of the battery. Actions of 

this nature are known as load shedding.

A terminal voltage of 1.75 volts/cell was used as the desired end state for each of the load tests. 

The 1.75 volts per cell is equivalent to 105 volts for a 60-cell battery string typically used in a 

NPP and reflects a common end state used by battery vendors and NPPs to evaluate battery

functionality. Other battery parameters were monitored such as electrolyte temperature and 

specific gravity to further assess the changes in battery state as the various loads were applied. 

Float current and specific gravity were measured to determine when the battery had returned to 

a fully charged state following each discharge test. Following each series of tests, the data were 

compiled and transmitted to the NRC and industry representatives to discuss preliminary 

observations regarding the battery performance during extended operation and to make minor 

adjustments to the test plan as necessary.

1.3 Assumptions and Measures of Uncertainty

In conducting the testing to assess the extended operation of batteries beyond their typical 

design times, certain assumptions were made that could contribute to slight uncertainties in the 

final test outcome:

a. The testing was performed under relatively constant temperature and humidity 

laboratory conditions. In a postulated ELAP event, there is a potential for other factors 

such as extreme temperatures and humidity to exist in the battery room. These 

conditions could impact the ability of the battery to perform its function for the length of 

time that was achieved in the laboratory. 

b. Three battery models from three battery manufacturers were used in this test program. 

It is assumed that other models and types of vented lead acid batteries used in NPPs 

would perform similarly to the batteries tested.

c. The batteries tested were not pre-aged raising the uncertainty that older batteries could 

perform differently. It should be noted that the NPP load profiles that were provided did 

apply an aging correction factor of 1.25 that effectively reduces the expected battery 

availability (see section 3.3). 

d. A resistive load bank was used to simulate plant loads. Some DC loads in a NPP are 

motors which have an inrush current when started. For the load profile testing (EBR-3), 

the inrush current value was assumed to exist for the entire one minute of MOV 

operation. This higher than expected value still did not affect overall battery availability

but did cause a temporary voltage dip that could affect the supplied equipment.

e. The endpoint of the tests was when the average cell voltage of the battery reached 1.75 

volts (21.0 volts for the BNL 12-cell battery string). It is recognized that not all plants can 

tolerate an end voltage of 1.75 volts due to cable runs or specific equipment voltage 
3

needs. Therefore, it is likely that the BNL battery availability times exceed what most 

plants will experience simply due to the plant-specific end voltage requirements. 

1.4 Test Facility

BNL established a controlled area for this testing that achieved the needed environmental and 

electrical safety parameters contained in IEEE and manufacturer’s standards, and meets good 

practices and BNL safety procedures. The battery laboratory contains temperature, humidity, 

and hydrogen monitoring and control, electrolyte spill control measures, and adequate air flow

to prevent hydrogen accumulation. Access to the testing area is limited to those directly involved 

with the battery testing program. Data acquisition equipment is installed to acquire and store the 

measured parameters during testing.

1.4.1 Laboratory Space

Testing was performed in a high bay area located in Building 526 at BNL. The dedicated space 

for the battery testing is approximately 800 square feet, has a controlled heating and ventilation 

system and adequate electrical power sources to support the battery charging and associated 

test equipment. In addition, the space was upgraded in accordance with battery vendor 

recommendations to include a containment system to capture any spilled electrolyte from the 

cells and a hydrogen monitor to detect any accumulation of explosive gasses. The room 

temperature can be maintained or adjusted as needed to provide the same ambient conditions 

for all tests as closely as reasonably practical to avoid having to make temperature corrections 

to compare results.

1.4.2 Battery Test Setup 

Battery racks are used that replicate a NPP installation with the exception that the racks are not 

seismically qualified. There are three racks of 12 cells each. Adequate spacing between the 

racks is provided to ensure that test leads can be attached and specific gravity measurements 

can be taken for all of the cells. The installation is shown in Figure 1-1.
4

Figure 1-1 BNL Battery Facility

1.4.3 Data Acquisition Equipment

The testing employed an automated capacity test set, manufactured by the Alber Corporation 

(Alber), to control the load on the battery string. This test equipment monitors and displays cell 

voltages and was programmed to discharge the battery strings at various defined load 

conditions to simulate the desired operational conditions of the battery’s extended duty cycle. A 

600 amp (A) load bank was used in conjunction with the Alber BCT-128 capacity test set.

In addition to test equipment that acquires a continuous stream of data during testing, manual 

specific gravity readings were taken using a calibrated digital hydrometer (Model SBS-2500 with 

an accuracy of .0001g/cm3

) were taken to assess battery condition [19]. Float current was 

monitored to verify the state-of-charge of the battery following each discharge test. The 

monitoring of the recharge process and recording of the float current response provided useful 

information about the battery string’s capacity and capability. 

1.5 Quality Plan

Existing BNL quality control procedures were used to ensure that the test results are 

reproducible and accurate. Instruments were calibrated using standards that are traceable to a 

national standard. 

1.6 Test Plan

The testing was comprised of two sequences. Sequence 1 verified various lengths of time that 

the tested batteries could operate at reduced discharge rates. These series of tests provided a 
5

set of performance expectations for extended battery operation under lower load conditions and 

provided data that could be used to assess the state of health of the batteries such as specific 

gravity, temperature, and recharge characteristics. The battery vendors provide general 

specifications for their batteries at various load conditions as depicted in Table 1-1. For 

example, Enersys had data to 72 hours, C&D to 12 hours, and GNB to 8 hours. Sequence 1 

testing acquired the battery data to fill in the data gaps of the battery manufacturer’s published 

capacity data as identified by the shaded areas in Table 1-1, and allowed a comparison of the 

tested batteries against the published data for the 8-hour performance tests that BNL performed 

to serve as a set of baseline readings.

Table 1-1 Battery Vendor Data – Rated Discharge Current vs. Time for a 1.75V End 

Voltage

Battery/Extended 

Test Load (amps) 1 hr 2 hrs 4 hrs 8 hrs (Baseline)

12 

hrs

24 

hrs

36 

hrs

48 

hrs

72

hrs

Enersys 2GN-23 925 605 385 225 160 89 (60) (47) 31.9

Exide/GNB 

NCN-21 750 515 317 187 (132) (71) (50) (39) (27)

C&D LCR-33 1167 799 500 290 205 (111) (77) (60) (42)

The beginning state for each test was a fully-charged battery. A fully-charged battery is 

represented by a stabilized float current below 2.0 amperes (amps) and a nominal midpoint 

specific gravity reading of 1.215 for all cells (the bench-mark value established by each of the 

three battery vendors). Previous testing of these three batteries at BNL had shown that when a 

stable float current below 2.0 amps is achieved, the battery can meet its design performance 

requirements. The end state for the discharge tests was when the overall battery string voltage 

reached 21.0 volts (average of 1.75 volts per cell, a common end-point value) or if any one cell 

reached 1.30 volts (to avoid a possible cell voltage reversal). In only one case (the 72 hour test 

of the Enersys battery) did the test terminate when a cell reached 1.30 volts. This test and the 

resulting analyses by the vendor are described in section 2.4. All other tests were terminated 

on the 21.0 volt criterion.

Sequence 1: Table 1-2 outlines the testing planned for Sequence 1. This series of 15 tests 

were intended to subject each battery to performance tests of different durations while allowing 

the battery to recover to a charged condition prior to the next test. These performance tests 

were conducted in accordance with IEEE 450-2010 and were used to fill in the data that were

missing in Table 1-1 (shaded area). In addition, each of the three batteries was tested at their 8-

hour rated current to provide a baseline for confirming the capacity of each battery. The Enersys 

battery was tested at its 72-hour rated current to provide confirmation of the vendor’s published 

data.

The discharge current for the tests in Sequence 1 was calculated by developing an equation 

based upon the discharge currents and times that were published by the vendors for the three 

batteries to be tested.
6

Table 1-2 Test Plan of Loads and Estimated Test Duration (Sequence 1)

Test Battery 

Type

Calculated 

Discharge Current 

(Amps)

Vendor 

Discharge 

Current 

(Amps)

Expected 

Duration 

(hours)

1 GNB 189 187 8

2 C&D 292 290 8

3 Enersys 227 225 8

4 GNB 132 (N/A)* 12

5 C&D 111 (N/A)* 24

6 Enersys 60 (N/A)* 36

7 GNB 71 (N/A)* 24

8 C&D 77 (N/A)* 36

9 Enersys 47 (N/A)* 48

10 GNB 50 (N/A)* 36

11 C&D 60 (N/A)* 48

12 Enersys 32.5 31.9 72

13 GNB 39 (N/A)* 48

14 C&D 42 (N/A)* 72

15 GNB 27 (N/A)* 72

*(N/A) These data were not available from the vendors at the time of the testing

The approach that was taken was to use the available performance data to develop an empirical 

relationship that could be used to estimate the missing data. The first step was to normalize 

each of the seven discharge current data that were provided by the battery vendors in Table 1-1 

for 8 to 72 hours by the 8-hr rated current. This resulted in a set of normalized currents vs. time, 

in which the normalized current was bounded between unity and zero. Examination of the trend 

of the data suggested that a curve fit to the collective data from the three batteries would be 

best represented by a power law function of the form,

i*(t) = K•tb

where the normalized current is i* = i(t)/i(t=8 hr), t = time in hours, and “K” and “b” are constants 

derived from the curve fit to the data. The empirical relationship that was derived to interpolate 

the rated current data that were missing from Table 1-1 is as follows:

i*(t) = 6.345•t-0.8842

In order to solve for the rated current for one of the batteries at a specified discharge time, insert 

the specified time in hours into the equation above, solve for i*(t), and then multiply the value of 

i*(t) by the battery’s rated discharge current at 8 hours, the normalizing value. The test matrix for 

Sequence 1 that was developed is given in Table 1-2. Note that the calculated discharge 

currents listed in Table 1-2 for tests 1, 2, 3, and 12 are the rated discharge currents as 

published by the battery vendors. The remaining numbers are the calculated discharge currents

using the algorithm previously described.
7

While the algorithm was developed using the data on the BNL test batteries, it turns out to be 

applicable to other battery types as well. To demonstrate this, the battery specifications for the 

battery types identified by NEI in the load profiles that were tested in Sequence 2 of this 

research were evaluated using the above the algorithm. Figure 1-1 illustrates how the general 

form of the capacity curves can be replicated using the algorithm out to 72 hours. Table 1-3

provides the data for this figure.

Figure 1-1 Battery availability for other battery types using algorithm

Table 1-3 Battery availability for other battery types using algorithm

Availability (Hours) 8 12 24 36 48 72

Battery Mfg./Model Spec

amps

Calc. ampsSpec.

amps

Calc.

amps

Calc.

amps

Calc.

amps

Calc.

amps

Calc.

amps

Exide/GNB NCN-33 302 302 N/A 212 115 81 63 43

C&D LCR-21 190 190 134 132 73 51 39 27

C&D LCR-27 244 244 174 172 93 65 50 35

C&D LCR-29 254 254 180 180 97 68 53 37

C&D LCY-39 300 300 212 212 115 80 62 43

The results of the Sequence 1 testing are provided in Section 2 of this report.

Sequence 2: Simulation of ELAP conditions: In this series of service tests, representative load 

profiles with load shedding schemes were replicated to determine how long the battery could 

carry the specified loads. Four different load profiles that were provided by the NEI and 

approved by NRC were used. These load profiles represent a cross section of inputs from NPPs 

who have analyzed what equipment is needed to assure core cooling while providing the critical 

instrumentation needed by plant operators and decision makers. The test profiles factor in a 

response time for initiating the shedding of loads and reflect the impact of various loads on 

battery performance. Each of the three battery types available at BNL were used equally in this 

testing sequence. That is, for the eighteen test runs associated with Sequence 2, each battery 

was tested six times. Because the sizes of the BNL batteries are different than what was 

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

Load (Amps)

Availability (Hours)

NCN33

LCR21

LCR27

LCR29

LCY39
8

provided by NEI with the load profiles, scaling of the load profiles was performed to relate the 

plant specified battery type to the different capacities of the three battery types actually being 

tested (based on the ratio of their amp-hour ratings). Following a series of discussions with NEI, 

EPRI, and NRC, load profiles were adjusted to evaluate different load shedding initiation times

and assess the impacts of intermittent loads on the batteries. In addition, it was decided to 

conduct one series of 3 tests without scaling the loads in order to observe each of the batteries 

response to an identical load profile. 

The series of service test runs conducted is briefly described below and summarized in Table 1-

4. Note that a battery string was tested no sooner than every third week so that it could be 

placed on a 100-hour equalize charge followed by a two-week float charge to restore the battery 

to a fully-charged state prior to its next test. As noted previously, a fully-charged battery is 

represented by a stabilized float current below 2.0 amps and a nominal midpoint specific gravity 

reading of 1.215 for all cells. Battery conditions were recorded prior to the start of each cycle 

that included individual cell voltages, overall float current, and the specific gravity of all cells. 

Three measurements of specific gravity (top, midpoint, and bottom) were taken on two cells to 

document any electrolyte stratification prior to testing. Each battery was subjected to an 8-hour 

performance test prior to the start of sequence 2 (test runs 1-3) and at the end of sequence 2 

testing (test runs 16-18) in order to assess battery capacity and capability over the test runs. 

Four load profiles were selected that represent several of the nuclear power plant DC load 

responses to an ELAP condition. As noted previously, variations in load shedding timing were 

also incorporated into the load profiles to assess the impact of this parameter on battery 

availability. These load profiles are summarized in Table 1-4. A brief description provided by 

NEI of the basis for the load profiles follows the table.

The test results from Sequence 2 are described in detail in Section 3 of this report.
9

Table 1-4 Summary of Load Profile Tests

TEST RUN BATTERY TYPE LOAD STEP 1 LOAD STEP 2 LOAD STEP 3 L

TEST SERIES #1 (Based on EBR-5); load scaling for battery type

4 C&D 514A for 1 minute 333A for 120 

minutes

118A for 120 

minutes 86A 

5 GNB 332A for 1 minute 214A for 30 

minutes 76A for 30 minutes 55A 

6 Enersys 399A for 1 minute 258A for 60 

minutes

91A for 60 

minutes 67A 

TEST SERIES #2 (Based on EBR-4; no battery load scaling)

7 C&D 443A for 1 minute 334A for 52 

minutes 328 for 15 minutes 67A 

8 GNB 443A for 1 minute 334A for 52 

minutes 328 for 15 minutes 67A 

9 Enersys 443A for 1 minute 334A for 52 

minutes 328 for 15 minutes 67A 

TEST SERIES #3 (Based on EBR-5 for C&D/60 minute load shed; EBR-3 for Enersys and GNB); includes load scaling

10 C&D 514A for 1 minute 333A for 60 

minutes

118A for 60 

minutes 86A 

11 GNB 160A for 90.2 

minutes 39A for 10.5 hours 39A with 1 minute loads ohours to 21.0 volts

12 Enersys 192A for 90.2 

minutes 47A for 10.5 hours 47A with 1 minute loads ohours to 21.0 volts

TEST SERIES #4 (Based on EBR- 5 for C&D/30 minute load shed; EBR-2 for Enersys and GNB); includes load scaling

13 C&D 514A for 1 minute 333A for 30 

minutes

118A for 30 

minutes 86A 

14 GNB 112A for 120 

minutes 38A to 21.0 volts No Load Step 3 No 

15 Enersys 469A for 1 minute 135A for 120 

minutes 45A to 21.0 volts No 

Battery Test Runs 1-3 and 16-18 (Baseline performance test for each battery type)

• Each battery was fully charged. 

• An 8-hour performance test was conducted to establish a battery capacity baseline for each battery type
10

Battery Test Runs 4- 6 (Load Profile #1 [EBR-5] for each battery type – initiation of the load 

shedding sequence for the C&D battery was 120 minutes for comparison to tests 10 and 13)

EBR-5 is representative of a BWR using reactor core isolation cooling (RCIC) for decay heat 

removal that employed their procedures and guidance to determine the amount of non-essential 

loads that could be shed. This profile assumes RCIC is switched to manual mode once the 

procedures and guidance are activated. This removes several automatic DC Motor-Operated 

Valve (MOV) operations.

• Each battery was fully charged prior to the test. 

• The extended load profile (EBR-5) was run with the loads scaled to each battery’s rating 

to determine how the battery functions over time. 

Battery Test Runs 7-9 (Load Profile #2 [EBR-4] for each battery type)

EBR-4 is representative of a PWR where automatic tripping of 4kVAC and 600 VAC breakers 

occurs during the first minute. Plant SBO procedures are assumed for the first 53 minutes until 

load shedding is started. These procedures were also used to identify the non-essential loads 

to be shed during this event.

• Each battery was fully charged prior to the test.

• The extended load profile (EBR-4) was simulated without applying any load scaling.

Battery Test Run 10 (Load Profile #1 [EBR-5] but with 60-minute load shed for C&D battery)

• The C&D battery was fully charged prior to the test. 

• The EBR-5 load profile was applied to the C&D battery, but with the load shed initiation 

set to 60 minutes in order to check the impact that the initiation of load shedding has on 

battery availability.

Battery Test Runs 11 and 12 (Load Profile #3 [EBR-3] for the GNB and Enersys Batteries)

EBR-3 represents a PWR where ac loads are stripped from their respective buses. At 90.2 

minutes, the operators are able to open breakers to selected panels and shed non-essential DC 

loads. Only critical safe shutdown loads are supplied by this battery after 90 minutes.

• Each battery was fully charged prior to the test. 

• The extended load profile (EBR-3) was simulated with the loads scaled to each battery’s 

rating to determine how the battery functions over time. In this profile, the GNB and 

Enersys battery test runs were used to simulate five (5) intermittent loads every 2 hours 

after 12 hours. The battery string was then operated at a steady load until the battery 

string reached 21.0 volts or any one cell reaches 1.3 volts.

Battery Test Run 13 (Load Profile #1 [EBR-5] with 30-minute load shed for C&D battery)

• The C&D battery was fully charged prior to the test. 

• The EBR-5 load profile was applied to the C&D battery, but with the load shed initiation 

time reduced to 30 minutes in order to check the impact that the initiation of load 

shedding has on battery availability.
11

Battery Test Runs 14-15 (Load Profile #4 [EBR-2] for the GNB and Enersys Batteries)

EBR-2 is representative of a PWR in which normal SBO loading is assumed for the first two 

hours before load shedding is initiated. Loads shed include non-Class 1E loads and other nonessential loads in accordance with the ELAP operating procedures/guidance.

• Each battery was fully charged prior to the test. 

• The extended load profile (EBR-2) was simulated with the loads scaled to each battery’s 

rating to determine how the battery functions over time.

13

2 EXTENDED BATTERY OPERATION (SEQUENCE 1) TESTING 

RESULTS

2.1 Background

Sequence 1 was directed at establishing a bounding set of performance expectations for 

extended battery operation. In general, a higher discharge rate (increased load current) will 

result in reduced capacity (shorter time to discharge) while a low discharge rate will result in 

increased capacity (longer time to discharge). However, the relationship between load and 

capacity is not linear; reducing the load by half will not double the time to discharge. For this 

reason, manufacturers normally specify several discharge rates (in amperes) along with their 

associated discharge times (in hours).

Sequence 1 tests were conducted to verify how long the batteries could operate at specified 

reduced discharge rates. The battery vendors provide capacity specifications for their batteries 

at various load conditions as previously depicted in Table 1-1. Understanding the battery 

response to lower discharge current and longer duration testing was the key objective of 

Sequence 1 testing. In general, battery capacity varies with the discharge rate; the higher the 

discharge rate, the lower the cell capacity. Manufacturers’ literature on the three battery types 

used at BNL specified several discharge rates (in amperes) along with the associated discharge 

time (in hours).

2.2 Test results-Time versus current response

In the Sequence 1 series of tests, a total of 16 tests were conducted; 8-hour baseline tests on 

each of the batteries, 11 tests to fill in the data gaps from the published vendor tables, and 2 

tests of the Enersys battery at 72 hours, one of which (Enersys-19) was conducted with only 10 

cells in the string. In each of these tests, we endeavored to start the test with the ambient 

temperature as close to 77°F as possible in order to minimize the amount of correction to the 

desired test current. 

Table 2-1 provides a summary of the specified discharge current and nominal duration and the 

associated ampere-hours expected to be expended by the battery during the discharge 

(columns 2 and 3). It also indicates the actual measured current and duration of the test and 

the associated ampere-hours discharged (columns 4 and 5). From these data, a battery 

capacity can be calculated in two ways- by comparing the measured time to the specified time 

or by comparing the measured ampere-hours discharged to the specified ampere-hours. As 

indicated in column 6 of Table 2-1, these measurements of capacity are fairly uniform, although 

the ratio of ampere-hours expended is generally higher than the ratio of the time. The Alber 

capacity test set uses the time ratio to calculate capacity, which is the more commonly used 

method in the nuclear industry.
14

Table 2-1 Summary of Sequence 1 Performance Tests

(1) (2) (3) (4) (5) (6) (7)

TestBattery/Tes

t Cycle

Specified 

Current/Discharg

e Duration

Specified 

Amp·hrs 

Discharged

Measured 

Current/Discharge 

Duration (Alber)

Measured 

Amp·hrs 

Discharged

Battery 

Capacit Initial 

Temp 

(°F) Time1 Amphrs2

GNB-15 187 A /8 hr = 1496 A·hr 187 A/7.97 hr = 1490 A·hr 99.6% 99.6% 76

C&D-15 290 A /8 hr = 2320 A·hr 291 A/8.26 hr = 2404 A·hr 103.3

%

103.6

% 77

Enersys-15 225 A /8 hr = 1800 A·hr 226 A/8.01 hr = 1810 A·hr 100.1

%

100.6

% 77

GNB-16 132 A/12 hr = 1584 A·hr 135 A/11.65 hr = 1573 A·hr 97.1% 99.3% 80

C&D-16 111 A/24 hr = 2664 A·hr 113 A/25.83 hr = 2919 A·hr 107.6

%

109.6

% 80

Enersys-16 60 A/36 hr = 2160 A·hr 62 A/35.13 hr = 2178 A·hr 97.6% 100.8

% 80

GNB-17 71 A/24 hr = 1704 A·hr 73 A/25.96 hr = 1895 A·hr 108.2

%

111.2

% 79

C&D-17 77 A/36 hr = 2772 A·hr 79 A/38.08 hr = 3008 A·hr 105.8

%

108.5

% 80

Enersys-17 47 A/48 hr = 2256 A·hr 49 A/44.28 hr = 2170 A·hr 92.3% 96.2% 78

GNB-18 50 A/36 hr = 1800 A·hr 52 A/37.17 hr = 1933 A·hr 103.3

%

107.4

% 78

C&D-18 60 A/48 hr = 2880 A·hr 61 A/50.27 hr = 3066 A·hr 104.7

%

106.4

% 77

Enersys-18 31.9 A /72 hr = 2297 A·hr 33 A/66.21 hr = 2185 A·hr 92.0% 95.1% 77

GNB-19 39 A/48 hr = 1872 A·hr 41 A/48.40 hr = 1984 A·hr 100.8

%

106.0

% 78

C&D-19 42 A/72 hr = 3024 A·hr 43 A/72.76 hr = 3129 A·hr 101.1

%

103.5

% 75

GNB-20 27 A/72 hr = 1944 A·hr 28 A/68.39 hr = 1915 A·hr 95.0% 98.5% 75

Enersys-19

(10 cells) 31.9 A/72 hr = 2297 A·hr 33 A/62.67 hr = 2068 A·hr 87.0 % 90.0 % 76

1

Battery capacity is the ratio of measured discharge duration ÷ specified discharge duration. 2

Battery capacity is the ratio of measured A·hr discharged ÷ specified A·hr discharged

By plotting the information derived from these tests along with published vendor data, one can 

extrapolate the increased efficiency of the battery at lower loads and the expected battery 

availability for any steady state current from one to 72 hours. These are illustrated in Table 2-2

and Figure 2-1, respectively.

Table 2-2 Actual Ampere-Hours discharged as a function of test duration

Battery/Test 

Duration 8 Hours 12 Hours 24 Hours 36 Hours 48 Hours 72 Hours

Enersys 1810 No Test No Test 2178 2170 2185/2068

GNB 1490 1573 1895 1933 1984 1915

C&D 2404 No Test 2919 3008 3066 3129

Table 2-2 shows the nearly 30% increase in battery efficiency at lower loads as compared to the 

8-hour rating that results in longer battery availability.
15

Figure 2-1 illustrates the significant decrease in loads in order to achieve longer duration 

availability of the batteries.

Figure 2-1 Battery availability versus load from 1 to 72 hours

2.3 Test results-Time versus voltage response

EPRI Report TR- 1006757 [20] presents the expected voltage profile for a lead-calcium cell 

undergoing a constant current discharge as Figure 2-2. This figure illustrates the initial drop in 

voltage that the cell experiences when it begins to discharge. After the voltage stabilizes from its 

initial drop, the voltage gradually decreases with time as the active materials and sulfuric acid 

are consumed in the chemical reaction. Toward the end of discharge, insufficient quantities of 

active material or sulfuric acid exist to sustain the chemical reaction and the voltage declines 

more rapidly. A weak cell in a battery string may exhibit a steeper decline in voltage over time. 

This could be detected during a performance test by monitoring the individual cell voltages. The 

impact of a weak cell in a battery string is discussed further in Section 2.4. 

Figure 2-2 Typical Lead-Acid Battery Cell Voltage Response During Constant Current Discharge 

(EPRI 1006757 [20])

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76

Load (Amps)

Availability (Hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33
16

As noted earlier, the testing conducted at BNL used an average cell voltage of 1.75 volts at the 

point of test termination (21.0 volts for our 12 cell battery strings). Manufacturers and users 

often evaluate cell performance using 1.75 V per cell as a reference point. However, the system 

design could require a higher voltage (or allow a lower voltage) to meet the voltage 

requirements of all components. Manufacturers rarely provide discharge ratings below 1.5 V per 

cell; once the voltage has fallen to this level, the rate of voltage decline is very rapid and little 

additional energy can be removed from the battery. 

Figures 2-3, 2-4, and 2-5 are the cell voltage responses for the three BNL batteries that were 

experienced during three 72-hour performance tests. In each case, the cell exhibited an initial 

drop from 2.25 to about 2.05 volts when the load was applied, a linear decrease over the major 

part of the test, and an accelerated decrease in voltage at approximately 1.85 volts to its 

terminal voltage of about 1.75 volts. 

Figure 2-3 C&D Cell #10 Response during 72-hour discharge test

Figure 2-4 GNB Cell #4 Response during 48-hour discharge test
17

Figure 2-5 Enersys Cell #3 Response during 36-hour discharge test

In summary, the results from this extended discharge testing are consistent with the expected 

response of a battery during a normal discharge. These characteristics are:

• An initial prompt voltage drop (known as Coup de Fouet)

• A voltage recovery during the first few minutes of discharge

• A nearly linear voltage drop during most of the discharge period

• A rapid nonlinear voltage drop at the end of discharge (~1.85V)

2.4 Test results-Time versus voltage response-special case

In every case except one, the extended operational testing ended automatically when the 

overall battery string voltage reached 21.0 volts (1.75V/cell on average). The other mechanism 

to automatically stop a test was if any one cell reached a voltage of 1.3 volts. This was a 

conservative setting to ensure that a cell did not go into reversal which could have damaged the 

cell and jeopardized the test program. 

During the conduct of the 72 hour test of the Enersys battery, cell #2 reached 1.3 volts at about 

the 66 hour mark during the night when the test lab was unattended and the test was 

automatically stopped. The voltage versus time response is illustrated in Figure 2-6.

Figure 2-6 Enersys Cell #2 voltage response during 72-hour test
18

Approximately 10 hours after the test terminated, recharge of the battery string commenced. At 

about 3.5 hours into the recharge at 27.0 volts (2.25 V/cell), an abnormally high voltage and 

high temperature was observed on cell #2. A manual action was taken to disconnect the battery 

from the charger so the cause of these abnormal conditions could be evaluated and to prevent a 

potential thermal runaway condition that could have severely damaged the cell. Figure 2-7 

illustrates the voltage response on cell #2 during the recharge and Figure 2-8 displays the 

corresponding temperature response. 

Thermal runaway conditions can develop because of variations in cell-to-cell voltages. In that 

situation, the lower voltage cells are slowly discharging and building sulfate crystals on the 

plates, and the high voltage cells are gassing excessively, with the potential for thermal 

runaway. Gassing that results in more heat generation than the cell can dissipate occurs 

somewhere above a cell voltage of 2.33 volts when the ambient temperature is 77°F [3]. 

Figure 2-7 Cell #2 Voltage Response during attempted recharge

Figure 2-8 Cell #2 Temperature Response during attempted recharge

75

80

85

90

95

100

105

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Temperature (°F)

Time from start of recharge (hours)

cell 1 cell 2 cell 3 cell 4

cell 5 cell 6 cell 7 cell 8

cell 9 cell 10 cell 11 cell 12
19

2.5 Evaluation of Test Anomaly

The jar containing cells 1 and 2 were shipped to Enersys for further analysis. Their test report 

provided the following description regarding the observed problem. A definitive root cause for 

the spike in charge voltage on cell 2 was not uncovered in the recovery efforts and testing 

performed at the Enersys laboratory. When the recharge was initiated at BNL, the charger came 

on at full power. Cell 2 could not absorb the full current, and its voltage increased until its 

current-voltage relationship was in line with the driving current. This was substantially higher 

than the other cells and generated significantly more heat due to the high resistance of the cell. 

It can only be speculated that the timing from the end of the discharge to the beginning of the 

recharge allowed a sufficient, but not excessive, highly resistive layer of lead sulfate and/or lead 

hydrate to develop. Cell 2 was the lowest in specific gravity based on the voltage seen in the 

discharge results. In low gravity circumstances, the sulfate on the face of the plate can dissolve 

and re-crystallize back onto the surface in a continuous layer, essentially forming a lead sulfate/ 

lead hydrate surface coating over the whole plate. This, in combination with the very low specific 

gravity of the electrolyte between the plates, combines to create a sufficiently high resistance to 

impede current flow. The action of removal and shipment may have redistributed sufficient acid 

to reduce that resistance to allow current flow when the recovery effort commenced at Enersys. 

Enersys recommended that we modify our recharge process to ensure that the recharge is 

sufficient to bring the cells back to a fully charged condition. This is required because the 

frequency of the deep discharge events are sufficiently in excess of that expected in the field 

operation that normal field charge routines are insufficient to fully recover the battery. As a result 

of this incident and the vendor’s recommendation, subsequent testing incorporated a 100-hour 

equalizing charge and at least 2 weeks on a float charge to ensure that the battery was 

sufficiently charged before embarking on the next extended operational test.

Additional BNL Analysis: Looking back over the test data, we noted that cell #2 was often the 

lowest voltage cell when the test was terminated. Figure 2-9 depicts the cell voltages at the 

conclusion of the 72 hour test. The overall cell voltage was 21.135 volts, yielding a cell average 

voltage of 1.761 volts. 

Figure 2-9 Enersys Cell Voltages at the conclusion of the 72-hour performance test

For each of the extended operation tests, cell #2 was the lowest voltage when the test was 

automatically terminated. This is illustrated in Table 2-3.

1.000

1.100

1.200

1.300

1.400

1.500

1.600

1.700

1.800

1.900

2.000

1 2 3 4 5 6 7 8 9 10 11 12

Volts

CELL No.
20

Table 2-3 Enersys cell voltages at the conclusion of the extended duration tests

Cell No. 8-hour test 36-hour test 48-hour test 72-hour test

1 1.776 1.758 1.752 1.786

2 1.655 1.708 1.675 1.299

3 1.775 1.746 1.743 1.784

4 1.755 1.752 1.752 1.792

5 1.755 1.744 1.752 1.795

6 1.730 1.722 1.723 1.772

7 1.793 1.784 1.786 1.821

8 1.781 1.790 1.804 1.827

9 1.775 1.779 1.785 1.816

10 1.766 1.775 1.777 1.809

11 1.793 1.791 1.799 1.824

12 1.763 1.768 1.775 1.810

Perhaps this consistent performance could have been an indication that this cell required 

attention prior to the very long duration tests. Battery monitoring systems and periodic capacity 

testing might give the nuclear plant operator warning that a cell could potentially degrade the 

performance of the entire battery string.

Looking back at the cell voltages at the conclusion of the extended performance tests for the 

other two battery strings revealed that the GNB and C&D battery strings also had a cell that 

affected the overall performance of the battery strings. As illustrated in Table 2-4, the GNB cell 

(#7) was consistently lower than the rest of the cells at the conclusion of the test. Only on the 

72-hour test did its value drop below 1.7 volts. 

Table 2-4 GNB cell voltages at the conclusion of the extended duration tests

Cell No 8 hour test 12 hour test 24 hour test 36 hour test 48 hour test 72 hour 

test

1 1.769 1.755 1.745 1.757 1.760 1.776

2 1.786 1.783 1.785 1.792 1.792 1.795

3 1.767 1.765 1.755 1.755 1.748 1.751

4 1.753 1.765 1.755 1.751 1.747 1.749

5 1.739 1.752 1.750 1.751 1.758 1.763

6 1.759 1.759 1.761 1.770 1.779 1.785

7 1.728 1.741 1.740 1.723 1.712 1.686

8 1.767 1.760 1.749 1.755 1.765 1.775

9 1.751 1.756 1.755 1.755 1.751 1.741

10 1.792 1.787 1.789 1.795 1.798 1.798

11 1.743 1.750 1.754 1.751 1.749 1.747

12 1.755 1.750 1.756 1.751 1.756 1.759

For the C&D battery, the lowest end voltages varied only slightly from test to test. This is 

illustrated in table 2-5. This table shows that cell #10 had the lowest voltage at the conclusion 
21

of four of the five tests, and was the second lowest cell on the fifth (72 hour test), but in no case 

did any cell end at a voltage less than 1.7 volts. 

Table 2-5 C&D cell voltages at the conclusion of the extended duration tests

Cell No. 8 hour test 24 hour test 36 hour test 48 hour test 72 hour test

1 1.785 1.785 1.775 1.787 1.782

2 1.752 1.736 1.720 1.720 1.700

3 1.749 1.758 1.757 1.765 1.769

4 1.753 1.764 1.770 1.765 1.772

5 1.778 1.778 1.766 1.773 1.767

6 1.737 1.732 1.742 1.730 1.744

7 1.752 1.764 1.777 1.774 1.782

8 1.780 1.793 1.797 1.793 1.798

9 1.771 1.758 1.748 1.758 1.736

10 1.737 1.715 1.720 1.709 1.714

11 1.765 1.761 1.761 1.765 1.766

12 1.762 1.766 1.778 1.779 1.787

Others have noted that cell variability in a discharge test is common [13] and is not necessarily 

problematic; there are always weaker and stronger cells. It was also noted that sharp knees in 

the voltage response curve are usually indicative of acid limitation. This is likely what happened 

with the Enersys Cell #2 due to not being fully recharged prior to the 72-hour test. End-of-test 

cell voltage readings were continued during the Sequence 2 testing where it was observed that 

Enersys cells #2 and #10 and C&D cell #10 reached voltage levels below 1.7 volts at the time 

when the overall battery string reached an end-of-test voltage of 21.0 volts (average of 

1.75V/cell). These readings are provided in Tables 3.4, 3.9, 3.13, and 3.19. 

2.6 Test results –Time versus specific gravity

Specific gravity measurements of each cell were taken prior to each performance test, following 

the performance test, and following the recharge of the battery that was conducted at a float 

voltage of 2.25 V/cell. On two cells, additional specific gravity readings were taken at the top of 

the cell and at the bottom of the cell in order to determine the extent of electrolyte stratification. 

Tables 2.6, 2.7, and 2.8 illustrate the end-point specific gravity readings on each cell for the 

extended operations tests on the C&D, GNB, and Enersys batteries, respectively. Following 

each set of specific gravity readings, a calibration check was performed using distilled water to 

ensure that the digital hydrometer was operating within specifications. This is indicated in the 

table as H20. 

The data that are of particular interest relate to the specific gravity readings obtained at the end 

of the discharge cycle, specifically the relationship of the midpoint specific gravity readings as a 

function of the length of the discharge test. That is, the longer the test, the lower the value of the 

specific gravity at the end of the test. The end point of 1.020 obtained on the 72-hour discharge 

tests for all three vendors is equivalent to about 3% acid; the normal acid concentration at a 

specific gravity of 1.215 is 29%, and the typical acid concentration at the end of an 8-hour 

performance test is 12%. The midpoint readings from each of these tables was averaged and 

plotted versus the performance test duration time. The results are shown in figure 2-10 that 

follows the three tables. 
22

Figure 2-10 Average End of Test Specific Gravity - Sequence 1 Tests

Table 2-6 C&D cell specific gravity at the conclusion of the extended duration tests

Battery/Test C&D 15 (8 

hour test)

C&D 16 (24 

hour test)

C&D 17 (36 

hour test)

C&D 18 (48 

hour test)

C&D 19 (72 

hour test)

Cell/Position Post Discharge

Specific Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

1-Midpoint 1.09 1.04 1.03 1.03 1.02

2-Midpoint 1.09 1.04 1.02 1.02 1.01

3-Midpoint 1.09 1.04 1.03 1.03 1.02

4-Midpoint 1.09 1.04 1.03 1.03 1.02

5-Midpoint 1.09 1.04 1.03 1.03 1.02

6-Midpoint 1.09 1.04 1.02 1.03 1.02

7-Midpoint 1.09 1.04 1.03 1.03 1.02

8-Midpoint 1.09 1.04 1.03 1.03 1.02

9-Midpoint 1.09 1.04 1.02 1.03 1.02

10-Midpoint 1.09 1.04 1.02 1.02 1.02

11-Midpoint 1.09 1.04 1.03 1.03 1.02

12-Midpoint 1.09 1.04 1.03 1.03 1.02

9-Top 1.08 1.03 1.02 1.02 1.01

11-Top 1.08 1.04 1.02 1.02 1.02

9-Bottom 1.09 1.05 1.03 1.04 1.03

11-Bottom 1.09 1.05 1.03 1.04 1.03

H2O 1.00 1.00 1.00 1.00 1.00

1.072

1.02

1.094

1.09

1.00

1.02

1.04

1.06

1.08

1.10

0 20 40 60 80

Specific gravity

Test duration (hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33
23

Table 2-7 GNB cell specific gravity at the conclusion of the extended duration tests

BatteryTestGNB 15 (8 

hour test)

GNB 16 (12 

hour test)

GNB 17 (24 

hour test)

GNB 18 (36 

hour test)

GNB 19 (48

hour test)

GNB 20 (72 

hour test)

CellPosition

Post Discharge 

Specific 

Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific 

Gravity

Post Discharge 

Specific 

Gravity

Post Discharge 

Specific 

Gravity

Post Discharge 

Specific 

Gravity

1-Midpoint 1.09 1.07 1.04 1.03 1.03 1.02

2-Midpoint 1.10 1.08 1.05 1.03 1.04 1.02

3-Midpoint 1.09 1.07 1.05 1.03 1.03 1.02

4-Midpoint 1.10 1.07 1.05 1.03 1.03 1.02

5-Midpoint 1.09 1.07 1.05 1.03 1.03 1.02

6-Midpoint 1.09 1.07 1.05 1.03 1.03 1.02

7-Midpoint 1.09 1.07 1.04 1.03 1.03 1.02

8-Midpoint 1.09 1.07 1.04 1.03 1.03 1.02

9-Midpoint 1.09 1.07 1.05 1.03 1.03 1.02

10-Midpoint 1.10 1.07 1.05 1.03 1.04 1.03

11-Midpoint 1.09 1.07 1.05 1.03 1.03 1.02

12-Midpoint 1.09 1.07 1.04 1.03 1.03 1.02

9-Top 1.09 1.06 1.04 1.03 1.02 1.02

11-Top 1.09 1.06 1.04 1.03 1.02 1.02

9-Bottom 1.10 1.08 1.06 1.05 1.05 1.09

11-Bottom 1.10 1.08 1.06 1.05 1.05 1.08

H2O 1.00 1.00 1.00 1.00 1.00 1.00
24

Table 2-8 Enersys cell specific gravity at the conclusion of the extended duration tests

Battery/Test Enersys 15 (8 

hour test)

Enersys 16 (36 

hour test)

Enersys 17 

(48 hour test)

Enersys 18 

(72 hour test)

Cell/Position Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

Post Discharge 

Specific Gravity

1-Midpoint 1.07 1.02 1.02 1.02

2-Midpoint 1.07 1.02 1.01 1.01

3-Midpoint 1.07 1.02 1.02 1.02

4-Midpoint 1.07 1.02 1.02 1.02

5-Midpoint 1.07 1.02 1.02 1.02

6-Midpoint 1.07 1.02 1.02 1.02

7-Midpoint 1.07 1.02 1.02 1.03

8-Midpoint 1.08 1.03 1.03 1.03

9-Midpoint 1.07 1.02 1.02 1.03

10-Midpoint 1.07 1.02 1.02 1.02

11-Midpoint 1.08 1.03 1.03 1.03

12-Midpoint 1.07 1.02 1.02 1.03

9-Top 1.07 1.02 1.02 1.02

11-Top 1.07 1.02 1.02 1.03

9-Bottom 1.08 1.03 1.04 1.07

11-Bottom 1.08 1.03 1.04 1.08

H2O 1.00 1.00 1.00 1.00

2.7 Test results –Time versus temperature

The temperature of each of the cells was monitored continuously using surface mounted 

thermocouples (Fig. 2-11) whose output was transmitted to a data acquisition system [17]. This 

was done primarily to ensure that the capacity measurements were referenced to 77°F. For the 

sake of standardization, battery performance ratings are normally specified at a temperature of 

77°F (25°C). This allows battery capability information provided by the vendors to be based on 

the same criterion.

The characteristics of a lead-acid battery vary with temperature; heat accelerates chemical 

activity and cold slows it down. A change in the electrolyte temperature from the reference 

temperature has two significant effects on lead-acid battery performance:

• Battery capacity decreases as the temperature drops below 77°F (25°C).

• Battery life decreases as the temperature rises above 77°F (25°C).
25

Figure 2-11 Cell Temperature Monitoring Using Surface Mounted Thermocouple

At temperatures lower than 77°F (25°C), the battery will not be able to provide its rated capacity. 

In general, a lower temperature increases the viscosity of the electrolyte, and thus restricts its 

ability to circulate into the plates. Also, the efficiency of the chemical reaction decreases as 

temperature decreases. Figure 2-12 provides typical battery capacity factors at temperatures 

other than the reference temperature of 77°F (25°C). For example, at 60°F a battery can 

provide only 90% of its rated capacity. The data contained in Figure 2-12 are based on a vented 

lead-acid battery with a specific gravity of 1.215. As previously illustrated in Table 2-1, the 

ambient temperature at the start of the Sequence 1 tests was maintained between 75°F and 

80°F in order to minimize the correction factor that needed to be applied. 

Figure 2-12 Effect of Temperature on Capacity (Adapted from IEEE Standard 485 -2010 [10]) 

Surface Mounted 

Thermocouple 
26

2.8 Test Results – Battery Recharge Characteristics

In the Phase 1 test program (NUREG/CR-7148), a series of 4-hour performance tests were 

conducted to validate the use of float current as a means of determining battery state-of-charge. 

Recharge data continued to be acquired during the extended battery operational testing. During 

the Sequence 1 tests, recharge was conducted at a float voltage of 2.25 volts per cell or 27.0 

volts for the 12 cell battery string. 

As illustrated in Tables 2-9, 2-10, and 2-11, discharging the battery at lower loads for extended 

periods of time increases the amount of ampere-hours that can be obtained from the battery 

and alters the recharge characteristics with regard to the time needed to recharge the battery. 

Columns 3, 5, and 9 in each of the tables are based upon comparisons of the measured Ah 

discharged (col. 2) to the rated Ah capacity (col. 1) for a discharge performance test from 2.25 

Volts/cell (V/cell) to an end voltage of 1.75 V/cell.

For example, Table 2-9 shows that the 4-hour performance test of the Enersys battery 

discharged 1530 Ah at a discharge rate of 385 A. The 72-hour performance test of the Enersys 

battery with a discharge rate of 31.9 A yielded 2068 ampere-hours, a 35% increase in its 

ampere-hour output. The time to fully charge the battery was 64 hours at the float voltage 

recharge rate.

Similarly, Table 2-10 indicates that the 4-hour performance test of the GNB battery discharged 

1201Ah at a discharge rate of 317A, while a 72-hour test of the same battery at a discharge rate 

of 27A yielded a total of 1915Ah, a 60% increase in its Ah output. The time to reach a float 

current of 2A was 24 hours as compared to 15.8 hours for the 4-hour performance test. 

And finally, as illustrated in Table 2-11, the 4-hour performance test of the C&D battery 

conducted at a discharge rate of 500A produced 2050Ah, while a 72-hour performance test 

conducted at 42A produced 3129Ah, an increase of 52% in the Ah output. During recharge at 

2.25 V/cell, it took 68 hours to reach a float current of 2A as compared to 20.6 hours when the 

battery was recharged at the float voltage value following the 4-hour performance test.

Table 2-9 Enersys Extended Service Recharge Data 

CycleParametersAh 

Discharge

d

Time to 

100% 

Recharge 

(hr)

Time 

to 2 A 

(hr)

% 

Recharged 

at 2 A

Time 

Constant, 

tC (hr)

Time at 

Current 

Limit, ∆tCL

(hr)

∆tCL

+ 

5*tC

% 

Recharge

d at ∆tCL

+ 5*tC

1 (385A, 4 

hr) 1530 Ah 11.2 11.9 100.6% 1.98 6.50 16.4 101.3%

15 (225A, 8 

hr) 1810 Ah 17.5 19.7 100.4% 3.20 7.15 23.15 100.6%

16 (60A, 36 

hr) 2178 Ah 27.2 36.5 101.3% 4.06 7.95 28.25 100.2%

17 (47A, 48 

hr) 2170 Ah 47.2 46.0 98.9% 4.19 7.28 28.23 97.0%

19 (31.9A, 72 

hr) 2068 Ah 64.0 62.3 99.8% 4.36 5.96 27.76 93.6%
27

Table 2-10 GNB Extended Service Recharge Data

Table 2-11 C&D Extended Service Recharge Data

CycleParametersAh 

Discharge

d

Time (hr) 

to 100% 

Recharge 

Time 

to 2 A 

(hr)

% 

Recharged 

at 2 A

Time 

Constant, 

tC (hr)

Time at 

Current 

Limit, ∆tCL

(hr)

∆tCL

+ 5*tC

% 

Recharge

d at ∆tCL

+ 5*tC

1 (500A, 4 

hr)

2050 Ah 14.6 20.6 101.9% 3.03 8.87 24.0 102.2%

15 (290A, 8 

hr)

2404 Ah 18.5 25.6 102.2% 3.60 10.20 28.2 102.4%

16 (111A, 24 

hr)

2919 Ah 27.6 40.3 102.1% 4.61 11.62 34.7 101.5%

17 (77A, 36 

hr)

3008 Ah 27.1 >48 >103.5% 4.97 12.00 36.9 102.4%

18 (60A, 48 

hr)

3066 Ah 36.3 >47.9 >101.5% 5.04 11.85 37.1 100.1%

19 (42A, 72 

hr)

3129 Ah 40.9 68.0 >102.9% 5.20 11.72 37.7 99.4%

2.9 Comparison to Vendor Curves 

Figure 2-13 is a set of curves for the C&D battery used in this testing program. It illustrates the 

capacity ratings for the LCR cell type for 8 to 72 hour reserve times with various end point cell 

voltages (1.75 to 2.00 volts). 

The load is expressed as Amperes per Positive Plate. For example, an LCR-33 has 16 positive 

plates and 17 negative plates (33 total plates), so the values on the horizontal axis would be 

multiplied by 16 to determine the load that the LCR-33 cells can supply for a given time to a 

given end voltage. The output derived from this process compares very closely to the data 

acquired from the normalized curve fit calculation and the actual measured results described in 

the previous sections. 

CycleParametersAh 

Discharge

d

Time (hr) 

to 100% 

Recharge 

Time 

to 2 A 

(hr)

% 

Recharged 

at 2 A

Time 

Constant, 

tC (hr)

Time (hr) 

at Current 

Limit, ∆tCL

∆tCL

+ 5*tC

% 

Recharge

d at ∆tCL

+ 5*tC

4 (317A, 4 

hr)

1201 Ah 12.0 15.8 101.0% 2.51 4.25 16.8 101.0%

15 (187A, 8 

hr)

1490 Ah 11.3 14.5 101.8% 2.47 6.17 18.5 102.1%

16 (132A, 12 

hr)

1573 Ah 10.8 19.9 107.6% 3.23 6.27 22.4 107.9%

17 (71A, 24 

hr)

1895 Ah 20.1 29.6 102.1% 3.61 7.00 25.1 101.4%

18 (50A, 36 

hr)

1933 Ah 28.0 27.6 101.5% 3.85 6.57 25.8 99.5%

19 (39A, 48 

hr)

1984 Ah 32.8 46.9 102.2% 3.97 6.52 26.4 98.3%

20 (27A, 72 

hr)

1915 Ah 16.9 24.0 103.1% 3.28 7.57 24.0 103.1
28

Figure 2-13 C&D Capacity Curves for the LCR-33 battery

2.10 Summary of Sequence 1 Test Results

As described in Section 2.1, Sequence 1 testing at BNL acquired battery data to fill in the data 

gaps of vendor data out to 72 hours. We compared the tested batteries against the published 

data for the 8-hour performance test that served as the baseline readings. The tests allowed 

BNL to develop an algorithm to predict the expected time up to 72 hours for a battery to reach 

1.75V/cell at any given loading. This algorithm compared favorably to battery vendor data that 

was subsequently made available to the industry.

Operation of the batteries at these extended times revealed the following characteristics:

1. Batteries used in NPPs can safely operate out to 72 hours when the loading is reduced 

(between 25 and 45 amps in this case), depending upon the ampere-hour capacity of 

the battery.

2. The electrolyte depletes significantly in very long discharges. Acid concentrations as low 

as 3% (nominal concentration for a fully charged battery is 29%) was obtained on the 

long duration tests as illustrated in Figure 2-14.
29

Figure 2-14 Average “end-of-test” cell midpoint specific gravity

3. The battery voltage response during the long discharge at constant current is nearly 

linear until the cell voltage nears 1.85 volts (111 volts for a 60-cell battery string) at 

which time it decreases more rapidly. Figure 2-15 is an example of this response for cell 

#2 of the C&D battery during its 72 hour performance test. This characteristic is 

important for the operator to be aware of in SBO situations where the battery is 

supplying power to critical plant equipment. The supplied equipment could become 

unavailable sooner than the operator would expect if the voltage response was entirely 

linear. 

Figure 2-15 C&D Cell #2 Voltage Response during the 72-hour performance test

4. The length of time necessary to recharge a battery increases in proportion to the length 

of time that it has been discharged. This is reflected in the recharge current response 

depicted in figure 2-16 which shows that the battery is recharged to full capacity in a 

shorter time for the short duration tests and increases in time with the longer duration 

tests. 

1.00

1.02

1.04

1.06

1.08

1.10

0 20 40 60 80

Specific gravity

Test duration (hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33

Specific Gravity of 1.02 is ~ 3% acid; nominal specific gravity is 1.215 (29% acid)

Specific gravity of 1.08 at the end of the 8-hour 

test is ~12% acid concentration
30

Figure 2-16 C&D recharge characteristics for tests from 8 hours to 72 hours

5. Individual cell voltage differences observed at the end of the extended performance test 

supports the practice of monitoring individual cell voltages during required performance 

testing in order to detect a weak cell or cells that could affect the overall performance of 

the battery string. 

6. Aside from the test anomaly associated with the 72-hour test of the Enersys battery, no 

new battery failure mechanisms were observed during extended operational testing. 

However, the depletion of the electrolyte during very long battery discharges (36-72 

hours) could impact the ability to recharge the battery under some conditions such as if 

there was a delay between the time when the battery is fully discharged and the initiation 

of recharge. 

Duration of Discharge Increasing
31

3 LOAD PROFILE (SEQUENCE 2) TESTING RESULTS

3.1 Introduction

In Sequence 2, ELAP conditions were simulated by a series of load profiles and load shedding 

schemes to determine how long the battery could carry the projected plant loads. The batteries 

were tested using four different load profiles that were supplied by the NEI and approved by the 

NRC. The four load profiles were from four different NPPs (3 PWRs and 1 BWR) who had 

analyzed the equipment they needed to assure core cooling while providing the critical 

instrumentation required by plant operators and decision makers. These profiles incorporate 

load shedding to extend the battery operating time during the ELAP. For one of the load profiles, 

the assumed time for the initiation of load shedding was varied from 30 minutes to 120 minutes 

to understand the impact of the time it takes the operators to initiate load shedding. For 

another of the profiles, five one-minute loads were applied after ten hours to simulate the effects 

of periodic motor-operated valve (MOV) or pump operation. And finally, one of the load profiles 

was tested without scaling the loads to the tested batteries in order to account for that 

parameter in the test results. 

3.2 Background Information – ELAP

Alternating current (AC) electric power is primarily supplied by the offsite power transmission 

system to which the plant is connected. In the event offsite sources of power are lost, redundant 

and independent emergency ac power systems (typically diesel generators), are provided to 

permit the functioning of equipment and systems that are essential to reactor shutdown, reactor 

core cooling, containment isolation, and heat removal. In addition to the emergency ac power 

systems, each plant also has two (or more) redundant trains of DC power that are capable of 

powering essential safety loads for a specified period of time. The size of the DC batteries are 

based on the amount of starting and running current each load draws from the battery and the 

length of time each load needs to be supplied from the batteries during an accident. The battery 

duty cycles, or accident load profiles, are created from the list of design loads by plotting the 

total current drawn by those loads versus time. 

For a loss of offsite power (LOOP) only event, the time the batteries are needed is very short 

(<1 minute). In the event of an SBO, the batteries may be required to remain operable for 

several hours. As observed from the events at Fukushima, an ELAP caused by other external 

events such as flooding may prevent restoration of normal or backup AC power. The current 

NRC regulatory framework for SBO (10 CFR 50.63) requires each plant to demonstrate that it 

can cope with and recover from an SBO that lasts for a specified duration. For plants that rely 

solely on the use of batteries to power essential equipment, the SBO coping time is typically 

between 4 and 8 hours. These time frames are based on the assumption that at least one 

source of AC power (offsite or onsite) will be successfully restored within four hours. The 

common-cause damage to the emergency power supplies and electrical distribution systems 

that occurred at Fukushima triggered an ELAP that ultimately led to core damage in three 

reactors.

The earthquake and tsunami at the Fukushima Dai-ichi nuclear power plant in March 2011, 

highlighted the possibility that extreme natural phenomena could challenge the prevention, 

mitigation and emergency preparedness defense-in-depth layers already in place in nuclear 

power plants. At Fukushima, limitations in time and unpredictable conditions associated with 

the accident significantly challenged attempts by the responders to preclude core damage and 
32

containment failure. During the events in Fukushima, the challenges faced by the operators 

were beyond any faced previously at a commercial nuclear reactor and beyond the anticipated 

design-basis of the plants. 

Following the events at the Fukushima Dai-ichi nuclear power plant on March 11, 2011, the 

NRC established a senior-level agency task force referred to as the Near-Term Task Force 

(NTTF). The NTTF was tasked with conducting a systematic and methodical review of the NRC 

regulations and processes and determining if the agency should make additional improvements 

to these programs in light of the events at Fukushima Dai-ichi. As a result of this review, the 

NTTF developed a comprehensive set of recommendations, documented in SECY-11-0093, 

“Near-Term Report and Recommendations for Agency Actions Following the Events in Japan,” 

dated July 12, 2011. Following interactions with stakeholders, these recommendations were 

enhanced by the NRC staff and presented to the Commission.

On February 17, 2012, the NRC staff provided SECY-12-0025, “Proposed Orders and Requests 

for Information in Response to Lessons Learned from Japan’s March 11, 2011, Great Tohoku 

Earthquake and Tsunami,” to the Commission, including the proposed order to implement the 

enhanced mitigation strategies. As directed by SRM-SECY-12-0025, the NRC staff issued 

Order EA-12-049, “Order Modifying Licenses with Regard to Requirements for Mitigation 

Strategies for Beyond-Design-Basis External Events.” This order directed licensees to develop, 

implement, and maintain guidance and strategies to maintain or restore core cooling, 

containment, and spent fuel pool cooling capabilities in the event of a beyond-design-basis 

external event (BDBEE). In particular, licensee strategies need to be capable of mitigating a 

simultaneous loss of all AC power and loss of normal access to the ultimate heat sink (LUHS) 

resulting from a BDBEE by providing the capability to maintain or restore core cooling, 

containment, and spent fuel pool cooling capabilities at all units on the WBNP site. Order EA12-049 applies to all power reactor licensees and all holders of construction permits for power 

reactors.

Order EA-12-049 requires that operating power reactor licensees and construction permit 

holders use a three-phase approach for mitigating BDBEEs. The initial phase requires the use 

of installed equipment and resources to maintain or restore core cooling, containment and spent 

fuel pool cooling capabilities. The transition phase requires providing sufficient, portable, onsite 

equipment and consumables to maintain or restore these functions until they can be 

accomplished with resources brought from off site. The final phase requires obtaining sufficient 

offsite resources to sustain those functions indefinitely. Specific requirements of the order are 

listed below:

1) Licensees or construction permit (CP) holders shall develop, implement, and 

maintain guidance and strategies to maintain or restore core cooling, 

containment, and spent fuel pool capabilities following a beyond-design-basis 

external event.

2) These strategies must be capable of mitigating a simultaneous loss of all AC

power and loss of normal access to the ultimate heat sink [UHS] and have 

adequate capacity to address challenges to core cooling, containment, and spent 

fuel pool capabilities at all units on a site subject to Order EA-12-049.
33

3) Licensees or CP holders must provide reasonable protection for the associated 

equipment from external events. Such protection must demonstrate that there is 

adequate capacity to address challenges to core cooling, containment, and spent 

fuel pool capabilities at all units on a site subject to Order EA-12-049.

4) Licensees or CP holders must be capable of implementing the strategies in all 

modes.

5) Full compliance shall include procedures, guidance, training, and acquisition, 

staging, or installing of equipment needed for the strategies.

On August 21, 2012, following several submittals and discussions in public meetings with NRC 

staff, the Nuclear Energy Institute (NEI) submitted document NEI 12-06, “Diverse and Flexible 

Coping Strategies (FLEX) Implementation Guide,” Revision 0 to the NRC to provide 

specifications for an industry-developed methodology for the development, implementation, and 

maintenance of guidance and strategies in response to the Mitigation Strategies order. On 

August 29, 2012, the NRC staff issued its final version of JLD-ISG-2012-01, “Compliance with 

Order EA-12-049, Order Modifying Licenses with Regard to Requirements for Mitigation 

Strategies for Beyond-Design-Basis External Events,” endorsing NEI 12-06, Revision 0, as an

acceptable means of meeting the requirements of Order EA-12-049, and published a notice of 

its availability in the Federal Register (77 FR 55230).

While the initiating event is undefined, it results in an extended loss of ac power (ELAP) with 

loss of normal access to the ultimate heat sink (LUHS), which is considered a surrogate for a 

BDBEE. The initial conditions and assumptions for the analyses are stated in NEI 12-06, 

Section 3.2.1, and include the following:

1. The reactor is assumed to have safely shutdown with all rods inserted (subcritical).

2. The DC power supplied by the plant batteries is initially available, as is the ac power 

from inverters supplied by those batteries; however, over time the batteries may be 

depleted.

3. There is no core damage initially.

4. There is no assumption of any concurrent event.

5. Because the loss of AC power presupposes random failures of safety-related equipment 

(emergency power sources), there is no requirement to consider further random failures.

On February 28, 2013, licensees submitted their draft overall integrated plans (OIPs) that 

described their strategy for satisfying NRC Order EA 12-049. While these OIPs are still under 

staff review, the initial assessment found that all licensees are relying on their DC power 

system, specifically the safety-related Class 1E batteries, as part of their Phase 1 coping 

strategy. The duration which licensees are crediting the safety-related Class 1E batteries varies 

from several minutes to as many as 34 hours before transitioning to Phase 2. Therefore, a 

lesson learned from the Japanese event that may be applicable to the U.S. NPPs is the need to 

examine ELAP conditions and determine if the existing safety-related Class 1E batteries can 

function beyond their design basis (or beyond-design basis if existing Station Blackout (SBO) 

coping analyses were utilized) duty cycles to support core cooling, containment, and spent fuel 

pool cooling. 
34

Battery sizing is important to ensure that the loads being supplied are adequately supported by 

the battery for the expected coping times. An accepted method for sizing a lead-acid battery is 

described in IEEE Standard 485-2010, “IEEE Recommended Practice for Sizing Lead-Acid 

Batteries for Stationary Applications” [10] as endorsed by Regulatory Guide 1.212, “Sizing of 

Large Lead-Acid Storage Batteries”.

Factors that govern the size (number of cells and rated capacity) of a battery include: 

1. the maximum system voltage, 

2. the minimum system voltage, 

3. correction factors (Temperature, Design Margin, Aging)

4. the duty cycle.

If factors 1- 3 are equal, battery size is based on the duty cycle, which is defined as loads a 

battery is expected to supply for specified time periods (IEEE Standard 485-2010 [10]).

As indicated in IEEE Standard 485-2010, momentary loads can occur one or more times during 

the duty cycle but do not exceed 1 min at any occurrence. When several momentary loads 

occur within the same 1 min period, the load for the 1 min period should be assumed to be the 

sum of all momentary loads occurring within that minute. Sizing for a load lasting only a fraction 

of a second, based on the battery’s 1 min performance rating, results in a conservatively sized 

battery. Typical momentary loads are: a) Switchgear operations, b) Motor-driven valve 

operations (stroke time≤ 1 min), c) Motor starting currents and d) Inrush currents. 

For an ELAP, the battery duty consists of various loads applied and removed during the period 

of time AC power is lost. Examples of such loads are:

• Switchgear and load center control, tripping, closing, and indicating devices; 

• Certain automatically / remotely actuated valves (solenoid and DC MOVs)

• Inverter loads; 

• Emergency turbine lube-oil pump; 

• Protective relaying; 

• Fire protection loads; 

• Annunciators; 

• Turbine-generator excitation breakers and controls; 

• DC emergency lighting; 

• Communications. 

The application, removal, and duration of such loads produce a load current profile of the 

battery known as the battery duty cycle. Removing loads that are not important for preserving 

the integrity of the reactor core and the containment building during an ELAP can significantly 

extend the battery duty cycle. Since the present regulatory framework does address actions to 

extend the battery duty cycle beyond those considered under the SBO Rule, the amount of time 

batteries could remain available in an ELAP scenario has not been previously analyzed. 
35

3.3 Sequence 2 Load Profiles

To address this issue, the NRC requested BNL to evaluate the length of time a sample of 

batteries could remain available when supplying representative load profiles provided by the 

NEI. While load profile calculations can be performed, actual testing of batteries using these 

loads had not been performed in the past.

In Sequence 2 of the battery testing program, ELAP conditions were simulated by a series of 

load profiles and load shedding schemes to determine how long the battery could carry the 

desired load. Four different load profiles were proposed by the NEI and approved by NRC. The 

four load profiles are summarized below:

EBR-2 represents the ELAP load profile for “Train B” of a PWR having an independent SBO 

generator (i.e., AAC power source1) and the SBO is assumed for the first two hours. In a station 

SBO, both trains of batteries are required for the first hour. Critical momentary (1 minute) loads 

(629 amps total or 10.5 A-hrs) are supplied by the Train A battery which has a capacity of 2532 

A-hrs. The current SBO scenario assumes that the Train A will be restored via the SBO

generator in 1 hour. If after 1 hour, the SBO generator is not available (ELAP condition), 

operators would begin the process of load shedding in accordance with ELAP operating 

procedures/guidance. Required operator actions are assumed to be completed by end of the 

second hour.

In this scenario, the Train B battery is run to depletion or to the point that some Train B power is 

restored. It is on this basis that the initiation of load shedding was set to 2 hours. Achieving safe 

shutdown during the ELAP condition requires operators to implement a set of manual actions 

that would bring the plant to a stable condition using secondary cooling and maintain natural 

circulation for core cooling. Secondary cooling is not interrupted by the loss of AC since the 

motive force for the cooling water (Auxiliary Feed Water) is steam driven and sources of steam 

(steam generators) are available as long as decay heat is available. A minimum amount of 

instrumentation was selected to be available to make it possible for control room operators to 

safely manage the plant. To maintain power to these instruments, with limited DC power 

availability, a set of load shedding actions is assumed to be initiated early in the event.

Other assumptions that were made in determining this load profile involved the inverter loading. 

For this profile, the overall inverter load is based on past field measurements and adjusted for 

voltage, assuming a constant power characteristic for the inverter. 50% efficiency is assumed 

for the inverter, resulting in a 50 A constant contribution to the long term load profile. This 

particular plant based its battery loading on an end voltage of 1.78 volts per cell, or 106.8V for 

their 60-cell string.

Using IEEE Standard 485-2010, the calculated runtime by the plant for this battery load 

condition was 29 hours. This is factoring in a minimum design temperature of 60°F and a 1.25 

aging correction factor. One of the actual tests using this profile is illustrated in Figure 3-1. The 

test duration to 1.75 volts per cell was 44.2 hours.

 1 One acceptable means of complying with the requirements in 10 CFR 50.63 involves the provision of an Alternate 

AC (AAC) power source (typically a diesel generator) that will be available on a sufficiently timely basis to permit 

operation of systems necessary to achieve and maintain a safe-shutdown condition. 
36

Figure 3-1 EBR-2 Load Profile Test Results for Enersys Run #15

EBR-3 represents the ELAP load profile of a “Train B” battery at a PWR. During the SBO all AC

loads are stripped from their respective buses. These loads would normally be sequenced on 

but since there is no AC bus voltage available, the sequencer will not start. All active 

components powered by DC are assumed to be available for the first 90 minutes. At 90 

minutes, the battery coping strategy will focus on reducing the DC loads (by operator action at 

distribution panels) by 194 amps, including 165 amps of inverter feeds to redundant 

instrumentation and controls, diesel generator sequencing, and Heating, Ventilation, and Air 

Conditioning (HVAC); 27 amps to control panels for the HVAC, and 22 amps to non-essential 

valves and actuators. Only the following loads would be supplied by the Train B battery: 

• Reactor Protection (43 amps); 

• 4160 and 480 volt Switchgear instrumentation (10 amps) and 

• Auxiliary Relay Panels and Diesel Generator Engine Control Panel (9 amps). 

Thus, after 90 minutes, only critical safe shutdown loads totaling 62 amps would be supplied by 

this battery. The core cooling strategy is the same as EBR-2 (Auxiliary Feedwater System and 

natural circulation cool down of the core). For EBR-3, the inverter demand in amps was 

assumed to be at the lowest input voltage of 1.86 volts per cell. It was also noted that battery 

voltage and amps are instrumented in the main control room and would be available to the 

operator to know when it was “approaching depletion”. The percent capacity of the battery on 

the most recent performance test is another factor used in the procedure to determine battery 

depletion. A switch over to the other train could be initiated by the operator from the control 

room when this point is reached.

The plant calculated the expected runtime for this battery load condition to be 21.8 hours using 

a plant specific end voltage of 1.86 volts, an aging correction factor of 1.25, a minimum design 

temperature of 70°F, and a design margin of 10%. 

One of the actual tests using this profile is illustrated in figure 3-2. Note that five MOV/pump 

operations were simulated every 2 hours after approximately 10.5 hours and the battery was 

available for 41.2 hours to an average cell voltage of 1.75 volts.

0

50

100

150

200

250

300

350

400

450

500

-5 5 15 25 35 45

Load (Amps)

Duration of Test (Hours)

0

469A for 1 minute, 

135A for 2 hours, and 

45A to 21.0 volts
37

Figure 3-2 EBR-3 Load Profile Test Results for Enersys Run #12

EBR-4 represents an ELAP load profile of a “Train A” battery at a PWR. Automatic tripping of 

4kV and 600 V breakers occurs during the first minute. For this load profile, normal SBO 

loading is assumed for the first 53 minutes until load shedding is started. All loads except critical 

instrumentation are shed to support ELAP.

To maximize battery runtime, the strategy employed at this site is to run the Train A battery first 

and minimize the load on the Train B battery initially. When the Train A battery is approaching 

depletion, the alternate Train B instrumentation would be re-energized from the Train B battery. 

This plant also used field measurements to determine the inverter loads and then assumed the 

ampere loading at the lowest input battery voltage, which in this case is 1.85 volts per cell (60-

cell string). This plant requires a minimum battery voltage of 111 volts to ensure adequate 

voltage at its critical loads. They indicated that they use a special procedure to estimate the 

remaining battery availability that incorporates battery voltage and current measurements that 

are available in the control room along with battery temperature and % capacity. The plant 

calculated runtime for this battery load condition is 15.5 hours using an end voltage of 1.85 volts 

per cell, a minimum design temperature of 60°F, an aging correction factor of 1.25, and a design 

margin of 5%.

One of the actual tests using this profile is illustrated in Figure 3-3. The test duration to an 

average cell voltage of 1.75 volts was 22.2 hours.

0

50

100

150

200

250

-5 5 15 25 35 45

Load (Amps)

Test Duration (hours)

192A for 90.2 minutes; 47A 

for 10.5 hours; 47A with 5-1 

minute loads of 192A every 2 

hours to 21.0 volts
38

Figure 3-3 EBR-4 Load Profile Test Results for GNB Run #8

EBR-5 represents an ELAP load profile for Train A of a BWR. This profile assumes that the 

Reactor Core Isolation Cooling System (RCIC) is switched to manual mode once the extended 

ELAP procedure/guidance is activated. This removes several automatic DC MOV operations. It 

is assumed that the normal SBO loads are only required for the first 61 minutes. The SBO and 

ELAP procedures provide operator actions to bring the plant to a stable condition using the 

RCIC system taking suction from the Condensate Storage Tank and operating in manual mode. 

The procedures call for non-essential loads being shed within 30 minutes. This includes the 

Main Turbine Emergency Bearing Oil Pump, the 125V DC cabinet feeding non-1E loads, and 

the redundant Control Building Emergency Lighting Cabinet.

Critical instrumentation powered by this battery includes reactor water level and pressure. Other 

important instrumentation such as containment integrity parameters and spent fuel pool level 

are provided by other power sources

The plant calculated a runtime for this battery load condition of 21 hours based on an end 

voltage of 1.75 volts per cell, a minimum design temperature of 65°F, an aging factor of 1.25, 

and a design margin of 5%. One of the actual tests using this profile is illustrated in Figure 3-4. 

The test duration to an average cell voltage of 1.75 volts was 32.6 hours.

0

50

100

150

200

250

300

350

400

450

500

-5 0 5 10 15 20 25

Load (Amps)

Test Duration (hours)

443A for 1 minute; 334A for 

52 minutes; 328 for 15 

minutes; 67A to 21.0 volts
39

Figure 3-4 EBR-5 Load Profile Test Results for C&D Run #4

Evaluation of load profiles: In May 2012, when the NRC requested the NEI to assist with 

providing representative load profiles that could be used in this testing program, benefits to both 

the NRC and the nuclear industry were anticipated. Specifically, the letter stated:

“Participation will provide the NRC staff with direct current (DC) load profiles to be modeled and 

tested, which should yield test results more closely representing existing station battery SBO 

coping capabilities. The goal is to use the insights gained through the cooperative research 

program to potentially develop and provide guidance to extend battery performance during a 

prolonged SBO event. The nuclear industry could benefit from the research data by the 

development of coping strategies for extended SBO events.” [12]

The goal of obtaining realistic load profiles that could be tested using the existing battery 

laboratory at BNL was achieved through the cooperative effort with NEI, the Electric Power 

Research Institute (EPRI), and participating NPPs. NEI not only provided the load profiles that 

their analyses indicated were achievable, they also provided information about the genesis of 

the profiles, with specific information about the equipment that could be shed as well as the 

associated loads for the equipment that would be called upon to achieve a safe shutdown 

condition. Individual plant assumptions when estimating battery availability may contain other 

criteria such as a battery specific gravity value in accordance with the minimum allowed by their 

technical specifications and/or specific operator actions conducted in a certain time frame.

3.4 Sequence 2 Test Results

The results of Sequence 2 testing indicate that battery availability can be significantly extended 

beyond 4-8 hours when used to meet the load demands anticipated in an ELAP condition. In 

each of these tests, the test terminated when the overall battery string reached 21.0 volts 

(average of 1.75 volts/cell). A second criterion for test termination was a cell voltage of 1.3 volts 

but that was never experienced in the Sequence 2 testing. Since some NPPs have calculated 

higher battery end voltages due to specific load demands or the length of cable, the expected 

0

100

200

300

400

500

600

-5 5 15 25 35

Load (Amps)

Test Duration (Hours)

514A for 1 minute; 333A for 

2 hours; 118A for 2 hours; 

86A to 21.0 volts
40

battery availability would be somewhat less. The battery voltage response curve (see Figure 3-

5 for example) can be used to determine the battery availability based on the plant specific end

voltage value.

Prior to the start of the load profile testing, an 8-hour baseline performance test was run on each 

of the three battery strings. These were identified as test runs 1, 2, and 3. The battery 

capacities obtained from these tests were:

• C&D LCR-33: 100.5%

• GNB NCN-21: 95.5%

• Enersys 2GN-23: 94.8%

These batteries had been subjected to more than 20 deep discharge cycles prior to the 

Sequence 2 testing. The resulting difference in capacity accounts for some of the minor 

differences seen in the performance of the batteries on identical load profiles.

This section will describe the results from the load profiles, highlighting five important areas:

1. The extended duration capability of the batteries,

2. The ability to predict the duration using IEEE 485 or vendor supplied information,

3. The generally linear voltage response during battery discharge until near the cell 

approaches 1.85 volts when the voltage decreases more rapidly, 

4. The depletion of electrolyte during the longer duration tests, possibly impacting the 

ability of the battery to be recharged, and

5. The slight variation in the cell voltages at the end of the discharge that could be 

worthwhile monitoring to ensure that no thermal runaway conditions develop.

Following the load profile testing, 8-hour baseline performance tests were again conducted on 

each of the battery strings. The battery capacities obtained from these tests, designated as 

cycles 16-18 were:

• C&D LCR-33: 101.5%

• GNB NCN-21: 97.2%

• Enersys 2GN-23: 95.8%

These results show no apparent degradation of the batteries as a result of conducting the load 

profile testing.

3.4.1 Test Series #1: Load Profile EBR-5

Subsequent to the baseline test runs, the first load profile tested was identified by NEI as EBR5. The nominal test parameters provided by NEI were based on a profile for a C&D model LCR29 battery and includes four different battery loads applied over time with an assumed load shed 

initiation time of 30 minutes. The load profile was tested on each of the three battery strings at 

BNL, but was scaled linearly according to the sizes (amp-hour ratings) of the batteries as 

compared to the LCR-29 battery. The load shed time for the C&D battery was modified to 120 

minutes and the load shed time for the Enersys battery to 60 minutes to determine the impact of 

the load shedding initiation times on the overall battery availability time. The ampere loading 

and load shed times for the test runs are summarized in table 3-1. Step 4 was voltage limited 
41

rather than time limited by using an average end voltage of 1.75 volts per cell (21.0 volts for the 

12-cell battery string) as the testing endpoint. 

Table 3-1 Test Series #1 Loading (Rated Current and Time)

Battery Type Scaling 

Factor

Step 1 (1 

minute)

Step 2 (variable) Step 3 (variable) Step 4 (to 

21.0 Volts)

C&D LCR-29 (NEI) ---- 450 amps 291 amps (30 

minutes)

103 amps (30 

minutes)

75 amps

C&D LCR-33 (test #4) 1.143 514 amps 333 amps (120 

minutes)

118 amps (120 

minutes)

86 amps

GNB NCN-21 (test #5) 0.737 332 amps 214 amps (30 

minutes)

76 amps (30 

minutes)

55 amps

Enersys 2GN-23 (test #6) 0.887 399 amps 258 amps (60 

minutes)

91 amps (60 

minutes)

67 amps

The Alber Battery Capacity Test Set was used to control and monitor the test. The Alber unit 

provided real-time data on the discharge current, battery string overall voltage, and individual 

cell voltages. The capacity test set automatically adjusts the load to 77°F. In test run #4, the 

actual temperature was 77°F so no adjustment was required. Test runs #5 and #6 were 

initiated with the battery at 78°F, requiring a very minor adjustment to the applied test current. 

The Alber test set was also used to record the cell voltages for the first 24 hours following 

initiation of recharge at an equalizing voltage of 28.0 volts (2.33 volts per cell).

The results of the tests are in line with the estimates that had been made based on a battery 

sizing calculation derived from IEEE Standard 485-2010 and one that used a curve fit calculated 

by BNL. This information is illustrated in Table 3-2.

Table 3-2 Test Series #1 Comparison of estimated to actual battery availability

Test Run # Estimate 1 Estimate 2 Actual

4 (C&D) 25.3 hours 27.0 hours 26.6 hours

5 (GNB) 30.4 hours 29.2 hours 32.6 hours

6 (Enersys) 28.4 hours 29.2 hours 27.7 hours

Estimate 1 – Estimated test duration based on curve fit derived by BNL from battery vendor and BNL test data

Estimate 2 – Estimated test duration based on 14-step battery sizing spread sheet derived from IEEE 485

The impact of battery availability times as a function of how quickly the load shedding occurs is 

illustrated in table 3-3. By initiating the load shedding as soon as possible, the loading on the 

battery is reduced and its availability extended by several hours.

Table 3-3 Impact of load shed timing on battery availability

Test Run # Load Shed Time Battery Availability

5 (GNB) 30 minutes 32.6 hours

6 (Enersys) 60 minutes 27.7 hours

4 (C&D) 120 minutes 26.6 hours

Voltage Response: Figure 3-5 was developed by taking the data acquired from the Alber 

battery capacity test set directly to a spread sheet. This provides a smoother curve fit that is 

more indicative of the actual voltage change that occurred over time. It is expected that this 

type of curve could be used to determine the battery availability for any desired end voltage at 

or higher than 1.75 volts per cell. The end voltage is a plant-specific determination based on 

the supplied equipment voltage requirements and the lengths of the cable runs.
42

Figure 3-5 Voltage Change over Time (C&D battery test run #4)

While the end point for each of the tests is when the overall battery voltage reaches 21.0 volts, it 

should be noted that the individual cell voltage characteristics vary among the three battery 

types. Table 3-4 shows the end voltages for each of the three battery strings for this first load 

profile test. The high and low end cell voltages for each battery string are highlighted in the 

table and quantified in the right column. The span of approximately 10% of the end voltage 

readings for the Enersys battery string (.180/1.75V) is larger than the other two battery strings 

and this points out the need for NPPs to monitor cell voltages as a means to detect and mitigate 

weak cells. 

Table 3-4 First End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)

Battery Type 

and Run # Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10

Cell 

11

Cell 

12

End 

voltage 

span

C&D ( #4) 1.787 1.728 1.747 1.747 1.794 1.711 1.763 1.780 1.781 1.709 1.781 1.784 .085

GNB (#5) 1.749 1.781 1.773 1.772 1.750 1.752 1.741 1.743 1.746 1.792 1.768 1.767 .051

Enersys (#6) 1.771 1.701 1.749 1.746 1.742 1.635 1.801 1.804 1.796 1.790 1.815 1.789 .180

Specific Gravity Results: The specific gravity readings vary slightly as well among the three 

battery types. Readings taken prior to the discharging the battery, at the completion of the 

discharge, and following the 100-hour equalizing charge are shown in table 3-5 for each cell. 

For cells # 9 and 11 on each string, two additional specific gravity measurements were taken as 

well. These readings were at the top and bottom of the cell and provide an indication of the 

amount of electrolyte stratification that existed in the C&D and GNB batteries even after the 

100-hour equalizing charge. Highly stratified electrolyte in the cells will impact the batteries 

capacity and capability. Note that after each set of readings, a calibration check of the 

instrument was conducted using distilled water. These data are also included in the table.

21

21.5

22

22.5

23

23.5

24

24.5

0.0 5.0 10.0 15.0 20.0 25.0 30.0

Overall Voltage

Test Duration (Hours)

Voltage changes 

coincident with load 

changes at 1 minute, 2 

hours, and 4 hours 
43

Table 3-5 Specific Gravity readings for Test Series #1

Battery C&D (Test Run #4) GNB (Test Run #5) Enersys (Test Run #6)

Cell/Position PreTest

Post 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge Pre-Test Post Discharge

Post 

Recharge

1-Midpoint 1.227 1.049 1.229 1.205 1.048 1.219 1.219 1.031 1.202

2-Midpoint 1.225 1.043 1.224 1.211 1.053 1.225 1.220 1.028 1.200

3-Midpoint 1.227 1.047 1.229 1.207 1.051 1.223 1.206 1.028 1.195

4-Midpoint 1.227 1.046 1.225 1.208 1.051 1.224 1.214 1.030 1.201

5-Midpoint 1.229 1.048 1.227 1.209 1.049 1.222 1.221 1.030 1.209

6-Midpoint 1.223 1.043 1.223 1.207 1.049 1.223 1.206 1.025 1.195

7-Midpoint 1.226 1.046 1.225 1.206 1.048 1.223 1.222 1.034 1.210

8-Midpoint 1.232 1.049 1.229 1.205 1.047 1.221 1.228 1.038 1.211

9-Midpoint 1.228 1.045 1.227 1.206 1.048 1.225 1.221 1.034 1.206

10-Midpoint 1.226 1.043 1.224 1.210 1.053 1.224 1.219 1.033 1.208

11-Midpoint 1.225 1.046 1.222 1.208 1.050 1.223 1.222 1.037 1.208

12-Midpoint 1.229 1.049 1.227 1.207 1.050 1.223 1.215 1.032 1.205

9-Top 1.214 1.042 1.183 1.174 1.043 1.131 1.217 1.033 1.201

11-Top 1.221 1.043 1.190 1.177 1.046 1.136 1.218 1.035 1.205

9-Bottom 1.246 1.048 1.278 1.260 1.060 1.276 1.263 1.038 1.278

11-Bottom 1.233 1.048 1.273 1.256 1.058 1.273 1.264 1.040 1.281

H2O 1.000 1.001 1.002 1.000 1.000 1.002 1.002 1.001 1.001

3.4.2 Test Series #2 (Load Profile EBR-4)

The second load profile tested was identified by NEI as EBR-4. This load profile represents a 

PWR in which automatic tripping of the 4kV and 600 V breakers occur during the first minute. 

Normal SBO loading is assumed for the first 53 minutes until load shedding is started. “Loss of 

all ac power” procedures were used to determine the non-essential loads to be shed during this 

event. During Step 3 all loads except critical instrumentation are shed to support ELAP. 

Step 4 loads consist of critical instrumentation. 

The nominal test parameters for EBR-4 provided by NEI were based on a profile for a C&D 

model LCR-27 battery and include four different battery loads applied over time. As agreed 

upon in the development of the test plan, this load profile was tested on each of the three 

battery strings at BNL without scaling for the sizes (amp-hour ratings) of the batteries being 

tested at BNL as compared to the LCR-27 battery. It was felt that one test series without scaling 

would be warranted to observe each battery’s response to the same load profile. The load shed 

timing for each test run was also maintained the same for each battery test. The ampere 

loading and load shed times for these test runs are summarized in Table 3-6.
44

Table 3-6 Test Series #2 Loading (Rated Current and Time)

Battery Type Scaling 

Factor

Step 1

(1 minute)

Step 2

(52 minutes)

Step 3

(15 minutes)

Step 4

(to 21.0 Volts)

C&D LCR-27 (NEI) ---- 443 amps 334 amps 328 amps 67 amps

C&D LCR-33 (test #7) 1.0 443 amps 334 amps 328 amps 67 amps

GNB NCN-21 (test #8) 1.0 443 amps 334 amps 328 amps 67 amps

Enersys 2GN-23 (test 

#9) 1.0 443 amps 334 amps 328 amps 67 amps

As illustrated in table 3-7, the results of the tests are in line with the estimates that had been 

made based on a battery sizing calculation derived from IEEE Standard 485-2010 [10]and one 

that used a curve fit derived from battery vendor supplied battery specifications supplemented 

by BNL test data. 

Table 3-7 Test Series #2 Comparison of estimated to actual battery availability

Test Run # Estimate 1 Estimate 2 Actual

7 (C&D) 37.7 hours 40.6 hours 39.9 hours

8 (GNB) 21.2 hours 20.3 hours 22.2 hours

9 (Enersys) 27.2 hours 26.3 hours 26.3 hours

Estimate 1 – Estimated test duration based on curve fit derived from battery vendor and BNL test curves

Estimate 2 – Estimated test duration based on 14-step battery sizing spread sheet derived from IEEE 485

Scaling factor analysis: This test series yielded the predicted response that the largest battery 

(C&D) would have the longest availability while the smallest battery (GNB) would have the 

shortest availability. Table 3.8 illustrates the ratio of the 8-hour amp-hour ratings of the batteries 

(normalized to the GNB battery which is the smallest) and compares it to the ratio of the test 

duration times normalized to the GNB battery. While the 8- hour amp-hr rating was used for 

scaling purposes throughout this testing program, it should be noted that this ratio is consistent 

with the longer duration tests completed in Sequence 1. Namely, the amp-hr ratios of the C&D 

battery to the GNB battery at 36 and 72 hours are1.54 and 1.56, respectively and the amp-hr 

ratios of the Enersys to the GNB battery are 1.2 and 1.18, respectively. There is a close 

correlation between these two ratios for the Enersys battery with the GNB battery, however the 

ratio of the C&D battery test duration exceeded the ratio of the ampere-hour ratings (1.80 to 

1.55). For the purposes of this testing, the scaling factor applied to the battery loads turned out 

to not be a significant variable when assessing long duration capabilities of the batteries.
45

Table 3-8 Comparison of battery rating to its availability

Test Run # 8- hour amphr rating

8- hour amphr Ratio Test Duration Test Duration 

Ratio

7 (C&D) 2320 1.55 39.9 hours 1.80

8 (GNB) 1496 1.0 22.2 hours 1.0

9 (Enersys) 1800 1.20 26.3 hours 1.18

Voltage Response: Figure 3-6 takes the data from the Alber test set for test #9 directly to a 

spread sheet and provides a smoother curve fit that is more indicative of the actual voltage 

change that occurs over time. The actual end voltage would be a plant-specific determination 

based on the supplied equipment voltage requirements and the lengths of the cable runs.

Figure 3-6 Voltage Change over Time (Enersys battery test run #9)

While the end point for each of the tests is when the overall battery voltage reaches 21.0 volts, it 

should be noted that the individual cell voltage characteristics vary among the three battery 

types. Table 3-9 shows the end voltages for each of the three battery strings. The high and low 

end cell voltages for each battery string are highlighted in the table and quantified in the right 

column.

Table 3-9 Second End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)

Battery Type 

and Run # Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10 Cell 11 Cell 12

End 

voltage 

span

C&D ( #7) 1.792 1.738 1.768 1.758 1.788 1.710 1.763 1.785 1.770 1.697 1.770 1.778 0.095

GNB (#8) 1.733 1.788 1.779 1.768 1.740 1.763 1.736 1.744 1.747 1.795 1.765 1.763 0.062

Enersys (#9) 1.775 1.720 1.742 1.722 1.722 1.674 1.795 1.795 1.794 1.785 1.809 1.784 0.135

20

21

22

23

24

25

26

27

28

0 2 4 6 8 10 12 14 16 18 20 22 24 26

Voltage

Test Duration (Hours)

Test Ends at 21.0 volts
46

Specific Gravity Results: As previously described, the specific gravity readings were taken at 

the midpoint of each cell prior to the load profile discharge, following the discharge, and 

following the 100-hour equalizing charge. The readings for the second load profile (EBR-4) are 

illustrated in table 3-10. The midpoint readings are denoted as M while the top and bottom cell 

specific gravity readings are denoted as T and B, respectively. In this test, each of the 3 battery 

types exhibited some stratification of the electrolyte following the recharge, although the 

Enersys battery had the least (due to better mixing during recharge).

Table 3-10 Specific Gravity readings for Test Series #2 

Battery C&D (Test Run #7) GNB (Test Run #8) Enersys (Test Run #9)

Cell/Position PreTestPost 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge

1-Midpoint 1.226 1.032 1.231 1.207 1.048 1.214 1.210 1.033 1.194

2-Midpoint 1.223 1.026 1.226 1.211 1.053 1.219 1.211 1.030 1.196

3-Midpoint 1.225 1.030 1.228 1.209 1.051 1.218 1.209 1.032 1.196

4-Midpoint 1.224 1.030 1.226 1.208 1.051 1.219 1.214 1.034 1.191

5-Midpoint 1.226 1.031 1.228 1.209 1.049 1.218 1.220 1.033 1.201

6-Midpoint 1.222 1.027 1.224 1.207 1.049 1.219 1.208 1.029 1.192

7-Midpoint 1.224 1.030 1.227 1.207 1.047 1.217 1.221 1.038 1.203

8-Midpoint 1.230 1.033 1.230 1.206 1.048 1.217 1.224 1.041 1.209

9-Midpoint 1.224 1.028 1.229 1.208 1.048 1.220 1.220 1.038 1.202

10-Midpoint 1.223 1.026 1.226 1.212 1.053 1.220 1.221 1.037 1.205

11-Midpoint 1.222 1.030 1.226 1.209 1.050 1.219 1.223 1.041 1.202

12-Midpoint 1.227 1.033 1.230 1.207 1.050 1.216 1.216 1.036 1.205

9-Top 1.210 1.025 1.176 1.168 1.043 1.139 1.211 1.035 1.192

11-Top 1.215 1.027 1.185 1.171 1.046 1.142 1.222 1.038 1.196

9-Bottom 1.255 1.036 1.282 1.263 1.064 1.274 1.248 1.041 1.284

11-Bottom 1.246 1.034 1.276 1.261 1.062 1.273 1.252 1.043 1.288

H2O 1.000 1.001 1.001 1.002 0.999 1.000 1.000 0.999 1.001

3.4.3 Test Series #3 (Load Profile EBR-5 and EBR-3)

The third series of tests was a combination of two load profiles. The first run using the C&D 

battery modified the load profile identified by NEI as EBR-5 by initiating the load shedding at 60 

minutes versus the original specified time of 30 minutes. The second and third runs using the 

GNB and Enersys battery strings respectively were based on the load profile designated by NEI 

as EBR-3. This load profile represents a PWR in which automatic tripping of the 4kV and 600 V 

breakers occur during the first minute. Normal SBO loading is assumed for the first 90.2 minutes 

until load shedding is started and only critical safe shutdown loads are supplied. 

For EBR-3, the test parameters provided by NEI were based on a profile for a C&D model LCY39 battery which has a 2400 Ah 8-hour rating. The load profile was applied to the GNB and 
47

Enersys battery strings at BNL with scaling for the sizes (amp-hour ratings) of the batteries 

being tested at BNL as compared to the LCY-39 battery. The ampere loading and load shed 

times for these test runs are summarized in Table 3-11.

Table 3-11 Test Series #3 Loading (Rated Current and Time)

Battery Type Scaling 

Factor

Step 1 Step 2 Step 3 Step 4 (to 21.0 

Volts)

C&D LCR-29 (NEI) (EBR-5) ----- 450 amps 291 amps 103 amps 75 amps

C&D LCR-33 (test #10) 1.143 514 amps

(1 minute)

333 amps (60 

minutes)

118 amps (60 

minutes) 86 amps

C&D LCY-39 (NEI) (EBR-3) ---- 256 amps 62 amps N/A N/A

GNB NCN-21 (test #11) 0.623 160 amps (90.2 

minutes)

39 amps (10.5 

hours)

5-1 minute loads of 160 amps at 2 

hour intervals then 39 amps to the end

Enersys 2GN-23 (test #12) 0.750 192 amps (90.2 

minutes)

47 amps (10.5 

hours)

5-1 minute loads of 192 amps at 2 

hour intervals then 47 amps to the end

The results of the tests are in line with the estimates that had been made based on a battery 

sizing calculation derived from IEEE Standard 485-2010 and one that used a curve fit derived 

from battery vendor supplied battery specifications supplemented by BNL test data. This 

information is illustrated in Table 3-12.

Table 3-12 Test Series #3 Comparison of estimated to actual battery availability

Test Run # Estimate 1 Estimate 2 Actual

10 (C&D) 28.5 hours 30.4 hours 30.6 hours

11 (GNB) 42.8 hours 36-40 hours 45.2 hours

12 (Enersys) 42.7 hours 36-40 hours 41.2 hours

Estimate 1 – Estimated test duration based on curve fit derived from battery vendor and BNL test curves

Estimate 2 – Estimated test duration based on 14-step battery sizing spread sheet derived from IEEE 485

The Alber capacity test set was programmed to provide the loads to the battery at the 

prescribed time, and to automatically shut down the test when the overall voltage reached 21.0 

volts. The output from the Alber test reports is provided as figure 3-7, 3-8, and 3-9. These 

figures show the overall voltage on the left vertical axis and the load current on the right axis. 

Note that these figures skew somewhat the voltage response in the sense that it is depicted as 

a series of step changes rather than a relatively smooth linear decrease when steady state 

loads are applied as illustrated in figure 3-10. 
48

Figure 3-7 Test Run #10 (C&D LCR-33 Battery)

Figure 3-8 Test Run #11 (GNB NCN-21 Battery)
49

Figure 3-9 Test Run #12 (Enersys 2GN-23 Battery)

Voltage Response: Figure 3-10 illustrates the voltage response for Enersys cell #6 during the 

conduct of test run #12. This curve is representative of the response from the other cells in that 

the voltage decreases fairly linearly until the battery approaches ~1.85 volts where the voltage 

decreases more rapidly with time. 

As noted in the previous tests, it is expected that this type of curve can be used to determine the 

battery availability for any desired end voltage at or higher than 1.75 volts per cell. The end 

voltage for determining battery availability is a plant-specific determination based on the 

supplied equipment voltage requirements and the lengths of the cable runs.

Figure 3-10 Voltage Change over Time (Enersys cell #6- test run #12)

While the end point for each of the tests is when the overall battery voltage reaches 21.0 volts, it 

should be noted that the individual cell voltage characteristics vary among the three battery 
50

types. Table 3-13 shows the end voltages for each of the three battery strings highlighting the 

highest and lowest along with the end voltage spans in the right column.

Table 3-13 Third End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)

Battery (Run #) Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10 Cell 11 Cell 12

End 

Voltage 

Span

C&D ( #10) 1.775 1.709 1.748 1.756 1.780 1.731 1.767 1.784 1.768 1.734 1.773 1.784 .075

GNB (#11) 1.748 1.785 1.771 1.771 1.748 1.755 1.731 1.749 1.748 1.785 1.766 1.767 .054

Enersys (#12) 1.739 1.685 1.749 1.761 1.758 1.722 1.785 1.791 1.778 1.779 1.798 1.772 .113

Short-term impact of intermittent loads: What’s not shown on figures 3-8 to 3-10 but was 

observed by reviewing the detailed battery string voltages over time was the brief but 

measurable decrease in overall string voltage when the intermittent loads were applied during 

test runs 11 and 12. These intermittent loads were applied to simulate valve and pump 

operations that may be required to achieve a cold shutdown condition. This loading will vary 

from plant to plant based on the size of the motors and their starting or inrush current. 

For the GNB battery (test run #11), when the five-one minute loads of 160 amps were applied, 

the overall string voltage decreased by 0.4 to 0.5 volts and then recovered to the original 

voltage level within 0.1 volts when the intermittent load was removed and the load returned to a 

steady state value of 39 amps. For the Enersys battery (test run #12), the overall string voltage 

decrease was 0.3 to 0.5 volts when the five one-minute loads of 192 amps were applied. The 

voltage level recovered within 0.1 volt of the original level when the load returned to a steady 

state value of 47 amps except after the 5th applied load when the voltage was 0.2 volts less than 

the battery voltage before the load was applied. These data are illustrated in Table 3-14.

Table 3-14 Voltage changes with application of intermittent loads

Battery/

Intermittent 

Load #

Intermittent Load 

#1

Intermittent Load 

#2

Intermittent Load 

#3

Intermittent Load 

#4

Intermittent Load 

#5

Pre Load Post Pre Load Post Pre Load Post Pre Load Post Pre Load Post

GNB- Test 

Run #11

23.7 23.3 23.7 23.7 23.3 23.6 23.6 23.2 23.6 23.5 23.1 23.5 23.5 23.0 23.5

EnersysTest Run 

#12

23.8 23.4 23.7 23.7 23.3 23.6 23.6 23.2 23.5 23.5 23.2 23.5 23.5 23.0 23.3

While the application of intermittent loads about midway through the run did not appear to 

impact the overall battery availability since not many ampere-hours are expended, the voltage 

dip when the load is applied could impact the power supplies and inverters that are being 

powered by the battery especially as the overall string voltage gets closer to its minimum value. 

A 0.5 volt dip on the 12 cell string employed for this test translates to a 2.5 volt dip when this is 

extrapolated to a 60 cell string typically used in most nuclear plant applications. This one 

minute change in voltage could be sufficient to trip a power supply or inverter if the voltage is 

close to its low voltage protection set point. 

Specific Gravity Results: As noted earlier, specific gravity measurements were taken on each 

cell prior to the discharge test, following the discharge, and following the 100-hour equalizing 
51

charge. The measurements were taken at the midpoint location in a consistent manner. A 

summary of the specific gravity measurements are provided in table 3-15. The distilled water 

measurement was taken after each set of readings as a calibration check on the hydrometer.

Table 3-15 Specific Gravity Summaries for test series #3

Battery C&D (Test Run #10) GNB (Test Run #11) Enersys (Test Run #12)

Cell/Position PreTest

Post 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge

1-Midpoint 1.225 1.043 1.228 1.207 1.030 1.211 1.211 1.019 1.195

2-Midpoint 1.222 1.037 1.225 1.211 1.036 1.217 1.211 1.017 1.191

3-Midpoint 1.224 1.041 1.227 1.210 1.032 1.216 1.207 1.019 1.189

4-Midpoint 1.226 1.042 1.224 1.209 1.033 1.217 1.214 1.021 1.185

5-Midpoint 1.226 1.043 1.227 1.209 1.031 1.217 1.219 1.019 1.201

6-Midpoint 1.223 1.039 1.223 1.208 1.032 1.219 1.210 1.017 1.192

7-Midpoint 1.225 1.042 1.225 1.208 1.029 1.216 1.220 1.024 1.200

8-Midpoint 1.230 1.045 1.228 1.207 1.030 1.215 1.223 1.026 1.204

9-Midpoint 1.223 1.040 1.227 1.210 1.030 1.219 1.219 1.024 1.200

10-Midpoint 1.223 1.038 1.224 1.212 1.035 1.219 1.221 1.023 1.202

11-Midpoint 1.223 1.041 1.222 1.209 1.032 1.216 1.221 1.027 1.200

12-Midpoint 1.227 1.044 1.226 1.207 1.032 1.213 1.218 1.024 1.205

9-Top 1.199 1.034 1.171 1.164 1.025 1.132 1.217 1.021 1.186

11-Top 1.207 1.037 1.183 1.166 1.028 1.136 1.219 1.024 1.190

9-Bottom 1.267 1.045 1.293 1.265 1.050 1.279 1.257 1.027 1.290

11-Bottom 1.258 1.044 1.280 1.263 1.049 1.279 1.261 1.031 1.294

H2O 1.001 0.999 1.002 1.000 1.001 1.000 1.001 1.001 1.001

3.4.4 Test Series #4 (Load Profiles EBR-5 and EBR-2)

The fourth test series completed was a combination of two load profiles. The first run using the 

C&D battery modified the load profile identified by NEI as EBR-5 by initiating the load shedding 

at 30 minutes versus the previously tested load shed times of 120 and 60 minutes. The second 

and third runs using the GNB and Enersys battery strings respectively were based on the load 

profile designated by NEI as EBR-2. For this load profile, normal SBO loading is assumed for

the first two hours with load shedding taking place at the end of two hours. Critical momentary

(1 minute) loads are supplied by the Train A battery for this PWR unit. Loads shed include some 

non-1E loads and other non-essential loads in accordance with extended SBO operating 

procedures.
52

For EBR-2, the test parameters provided by NEI were based on a profile for a GNB model NCN33 battery which has a 2416 Ah 8-hour rating. The load profile was tested on the GNB and 

Enersys battery strings with scaling for the sizes (amp-hour ratings) of the batteries as 

compared to the GNB-33 battery. Scaling the load profile to the ampere-hour rating ratio of the 

BNL batteries versus the NEI load profile battery allows a direct comparison of the test duration. 

The ampere loading and load shed times for these test runs are summarized in table 3-16. 

Note that a one minute loading was added to test run #15 to confirm that the impact that this 

demand has on the overall battery performance is small.

Table 3-16 Test Series #4 Loading (Rated Current and Time)

Battery Type Scaling 

Factor

Step 1 Step 2 Step 3 Step 4 (to 21.0 

Volts)

C&D LCR-29 (NEI) (EBR-5) ----- 450 amps 291 amps 103 amps 75 amps

C&D LCR-33 (test #13) 1.143 514 amps 

(1 minute)

333 amps 

(30 minutes)

118 amps 

(30 minutes) 

86 amps

GNB NCN-33 (NEI) (EBR2)---- 181 amps 61 amps N/A N/A

GNB NCN-21 (test #14) 0.623 112 amps (120 

minutes)

38 amps (to 

21.0 volts)

N/A

Enersys 2GN-23 (test 

#15)

0.750 469 amps 

(1 minute)

135 amps 

(120 minutes)

45 amps (to 

21.0 volts)

N/A

The results of the tests were in line with the estimates that had been made based on a battery 

sizing calculation derived from IEEE 485 and one that used a curve fit derived from battery 

vendor supplied battery specifications supplemented by BNL test data. This information is 

illustrated in Table 3-17.

Table 3-17 Test Series #4 Comparison of estimated to actual battery availability

Test Run # Estimate 11 Estimate 22 Actual

13 (C&D) 30.1 hours 32.0 hours 32.2 hours

14 (GNB) 44.9 hours 42.0 hours 47.7 hours

15 (Enersys) 45.7 hours 43.8 hours 44.2 hours

1 – Estimated test duration based on curve fit derived from battery vendor and BNL test curves

2 – Estimated test duration based on 14-step battery sizing spread sheet derived from IEEE 485

Table 3-18 illustrates the second goal of this load profile- evaluating the impact of the load shed 

time on the same battery type. As noted in Table 3-18, the battery availability time was 4 hours 

longer when the initiation of load shedding occurred an hour earlier for each of the two load 

shed steps. In test run #13, the same profile was applied assuming that load shedding occurs in

1⁄2 hour for each of the two load shed steps. This increases the battery availability time by 

another 1.6 hours. The three tests (runs #4, #10, and #13) provide a spectrum of battery 

availability times based on the time to initiate load shedding on the same battery and identical 

load profiles.
53

Table 3-18 Impact of load shedding time on battery availability

Test Run # Load Shed Time Estimated (Avg.) Measured

4 (C&D) 2 hours 26.2 hours 26.6 hours

10 (C&D) 1 hour 29.5 hours 30.6 hours

13 (C&D) 1⁄2 hour 31.1 hours 32.2 hours

The Alber capacity test set was programmed to provide the loads to the battery at the 

prescribed time, and to automatically shut down the test when the overall voltage reached 21.0 

volts. The output from the Alber test reports is provided as figure 3-11, 3-12, and 3-13. These 

figures show the overall voltage on the left vertical axis and the load current on the right axis.

Figure 3-11 Test Run #13 (C&D LCR-33 Battery)

Figure 3-12 Test Run #14 (GNB NCN-21 Battery)
54

Figure 3-13 Test Run #15 (Enersys 2GN-23 Battery)

Voltage Response: Figure 3-14 illustrates the voltage response for C&D cell #6 during the 

conduct of test run #13. This curve is representative of the response from the other cells in that 

the voltage decreases fairly linearly until the battery approaches ~1.85 volts where the voltage 

decrease more rapidly with time. 

As noted in the previous tests, it is expected that this type of curve can be used to determine the 

battery availability for any desired end voltage at or higher than 1.75 volts per cell. The end 

voltage for determining battery availability is a plant-specific determination based on the 

supplied equipment voltage requirements and the lengths of the cable runs.

Figure 3-14 Voltage Change over Time (C&D cell #6- test run #13)

While the end point for each of the tests is when the overall battery voltage reaches 21.0 volts, 

the individual cell voltages were also measured during the test. A cell voltage of 1.3 volts would 

also terminate the test. Table 3-19 shows the cell end voltages for each of the three battery 

strings, highlighting the highest and lowest along with the end voltage spans in the right column.
55

Table 3-19 Fourth End Voltages of Each Cell (When Overall Voltage Reached 21.0 volts)

Battery (Run 

#) Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10

Cell 

11 Cell 12

End 

Voltage 

Span

C&D ( #13) 1.791 1.727 1.762 1.768 1.787 1.714 1.773 1.800 1.763 1.679 1.768 1.784 .121

GNB (#14) 1.753 1.791 1.772 1.767 1.749 1.761 1.728 1.748 1.738 1.786 1.766 1.765 .063

Enersys (#15) 1.748 1.683 1.746 1.759 1.743 1.720 1.785 1.792 1.785 1.780 1.804 1.778 .121

Specific Gravity Results: A summary of the specific gravity measurements for this fourth load 

sequence is illustrated in table 3-20. It shows the values prior to initiation of the discharge, 

following the discharge, and following the 100-hour equalizing charge. A calibration check was 

performed as well.

Table 3-20 Specific Gravity readings for Test Series #4

Battery C&D (Test Run #13) GNB (Test run #14) Enersys (Test Run #15)

Cell/Position PreTest

Post 

Discharge

Post 

Recharge

PreTestPost 

Discharge

Post 

Recharge Pre-Test Post Discharge

Post 

Recharge

1-Midpoint 1.224 1.032 1.222 1.203 1.030 1.208 1.212 1.014 1.191

2-Midpoint 1.220 1.025 1.221 1.208 1.036 1.213 1.215 1.012 1.193

3-Midpoint 1.224 1.029 1.223 1.206 1.033 1.214 1.214 1.015 1.190

4-Midpoint 1.226 1.030 1.221 1.206 1.033 1.213 1.218 1.016 1.194

5-Midpoint 1.227 1.031 1.223 1.206 1.031 1.213 1.223 1.016 1.201

6-Midpoint 1.222 1.027 1.219 1.205 1.032 1.214 1.214 1.013 1.188

7-Midpoint 1.225 1.030 1.221 1.204 1.030 1.211 1.225 1.020 1.202

8-Midpoint 1.232 1.033 1.225 1.203 1.029 1.212 1.228 1.023 1.205

9-Midpoint 1.221 1.028 1.223 1.205 1.030 1.214 1.225 1.021 1.199

10-Midpoint 1.223 1.026 1.220 1.208 1.035 1.217 1.227 1.020 1.205

11-Midpoint 1.222 1.030 1.218 1.205 1.033 1.213 1.227 1.024 1.200

12-Midpoint 1.227 1.032 1.223 1.204 1.032 1.212 1.222 1.020 1.200

9-Top 1.207 1.026 1.176 1.174 1.025 1.130 1.224 1.019 1.194

11-Top 1.216 1.029 1.186 1.177 1.028 1.136 1.227 1.022 1.197

9-Bottom 1.264 1.038 1.291 1.263 1.047 1.283 1.248 1.024 1.292

11-Bottom 1.250 1.035 1.283 1.261 1.045 1.281 1.251 1.027 1.296

H2O 1.001 0.999 1.000 1.002 0.999 1.001 1.001 1.000 1.001
56

3.5 Recharge Characteristics of Load-Profile Tests

The primary difference between the Sequence 1 and Sequence 2 tests was that Sequence 1 

testing was conducted at constant loads while Sequence 2 tests contained several load 

changes over the course of the test period, including intermittent (1-minute) loads to simulate 

MOV and/or pump operations. In addition, Sequence 1 testing was conducted out to 72 hours 

while the longest Sequence 2 test duration was 47.7 hours. The major difference between 

Sequence 1 and Sequence 2 associated with the recharge of the batteries was that in 

Sequence 1, recharge was conducted at a float voltage of 2.25 volts per cell (27 volts) while in 

Sequence 2, the recharge was conducted at an equalizing voltage of 2.33 volts per cell (28 

volts). 

This section addresses the recharge characteristics that were experienced following the load 

profile tests conducted in Sequence 2. As illustrated in Figure 3-15, when recharging the 

battery at an equalizing voltage, the percentage of ampere-hours returned to the battery is 

between 85 and 90% when the charger came out of a current limit mode regardless of the 

battery discharge time (8 hours to 44.2 hours). As an example, Figure 3-15 contains the 

recharge curves for the Enersys battery. For EBR-5, the test duration was 27.7 hours, for EBR4 it was 26.3 hours, for EBR-3 it was 41.2 hours, and for EBR-2 the test duration was 44.2 

hours. The pre and post 8-hour baseline test recharge curves are also included. The form of 

the graphs once the charger comes out of current limit mode is very similar. The time to return 

100% of the ampere-hours expended during the test is also nearly identical (slightly greater than 

1 time constant).

Figure 3-15 Enersys Battery Recharge Characteristics during Load Profile Testing

Table 3-21 is a compilation of the recharge data for each of the three batteries showing the total 

ampere-hours discharged in each test, how long the battery charger was on current limit, the 

calculated time constant in hours, and the percentage of ampere-hours returned to the battery in 

three time constants. For all three batteries, more than 100% of the ampere-hours discharged 

were returned to the battery within three time constants. The similarities of the data among the 

three batteries are reflected in the close approximation of the curves of which Figure 3-15 is an 

example.

80

85

90

95

100

105

110

0 1 2 3 4 5 6

percentage of amp-hrs recharged

current limit + number of time constants

8-hour pre test

EBR-5 (27.7 hours)

EBR-4 (26.3 hours)

EBR-3 (41.2 hours)

EBR-2 (44.2 hours)

8-hour post test
57

Table 3-21 Recharge Characteristics Observed/Calculated During Sequence 2 Testing

ENERSYS: percent recharge vs. time constant

Load Profile Duration (Hours) 8-hour 

(pre) 27.7 26.3 41.2 44.2 8-hour (post)

amp-hr discharged 1714 2080 2063 2166 2176 1716

current limit (CL, hr) 8.22 9.97 9.95 10.42 10.47 8.35

time constant (tc, hr) 1.69 2.1 1.99 2.28 2.28 1.57

percentage of amp-hr recharged

at current limit 86.3 86.3 86.8 86.5 87.0 87.9

CL + 3 tc 102.0 103.1 102.7 103.8 104.4 102.3

GNB percent recharge vs. time constant

Load Profile Duration (Hours) 8-hour (pre) 22.2 45.2 47.7 8-hour 

(post) amp-hr discharged 1429 1811 2000 1963 1446

current limit (CL, hr) 6.67 8.35 8.99 8.94 6.78

time constant (tc, hr) 1.57 1.97 2.47 2.70 1.60

percentage of amp-hr recharged

at current limit 84.0 83.0 80.9 82.3 84.8

CL + 3 tc 101.7 101.0 101.5 105.1 102.5

C&D percent recharge vs. time constant

Load Profile Duration (Hours) 8-hour 

(pre) 26.6 39.9 30.6 32.2 8-hour (post)

amp-hr discharged 2289 2854 2978 2919 2915 2339

current limit (CL, hr) 11.34 13.95 14.65 14.12 14.37 11.59

time constant (tc, hr) 2.00 2.50 2.59 2.77 2.62 1.94

percentage of amp-hr recharged

at current limit 89.1 88.0 88.5 87.0 89.1 89.5

CL + 3 tc 103.7 103.0 103.3 103.1 104.5 103.1

3.6 Summary of the Load Profile Testing (Sequence 2)

In Sequence 2, ELAP conditions were simulated using 4 load profiles that were provided by the 

nuclear industry as being representative of the load shedding schemes used by NPPs to extend 

the battery availability while maintaining power to critical plant loads. The four load profiles 

were from four different NPPs (3 PWRs and 1 BWR) who had analyzed the equipment they 

need to assure core cooling while providing the critical instrumentation required by plant 

operators and decision makers. 

Table 3-22 summarizes the battery availability times for all four load profiles. In general it can 

be seen that there is a consistently close relationship between the estimated times that were 

predicted for battery availability versus the measured times from the testing of the four load 

profiles. The table provides columns for the estimated times using two different methods as well 

as the measured test time. The table shows that the measured battery availability met or 

exceeded the estimates in 7 of the 12 tests. The temperature of the battery at the start of the 

test is also included. Recall that IEEE capacity tests are based on a temperature of 77°F. Our 

goal was to be as close to that number as possible at the start of the test in order to minimize 

the need to apply correction factors.
58

Table 3-22 Summary of Estimated To Actual Battery Availability Times (Hours)

TEST # BATTERY Estimate 

IEEE 485

Estimate 

BNL DATA MEASURED TEMP (°F)

Test Series #1

4 C&D 27.0 25.3 26.6 77

5 GNB 29.2 30.4 32.61 78

6 Enersys 29.2 28.4 27.7 78

Test Series #2

7 C&D 40.6 37.7 39.9 77

8 GNB 20.3 21.2 22.21 77

9 Enersys 26.3 27.2 26.3 77

Test Series #3

10 C&D 30.4 28.5 30.61 77

11 GNB 36-40 42.8 45.21 78

12 Enersys 36-40 42.7 41.2 78

Test Series #4

13 C&D 32.0 30.1 32.21 78

14 GNB 42.0 44.9 47.71 78

15 Enersys 43.8 45.7 44.2 77

1 Measured battery availability was greater than both estimates

Employing the load shedding schemes that have been put forward by the nuclear industry 

through NEI, the measured durations from the batteries varied from 22 to 48 hours, much 

greater than the current NPP specific SBO battery coping times of 4 to 8 hours.

Voltage Response: As noted in the sequence 1 testing, the voltage response of the battery 

during long-term constant load discharges is nearly linear until the cell voltage nears 1.85 volts 

at which time the voltage decrease accelerates. The other voltage response characteristic 

noted in the load profile testing was that as the load experiences a step decrease, the voltage 

slightly increases for a short time and then reverts to a near-linear decrease [5]. This 

characteristic can be seen in figure 3-16, in which there is an immediate drop in voltage when 

the 1 minute load is applied; the voltage increases when that load is replaced by a lower two 

hour loading and then increases again after those 2 hours when a lower steady state load is 

maintained on the battery string until the overall voltage decreases to 21.0 volts. 

When intermittent higher loads of one-minute durations were applied during the load profile 

tests #11 and #12, it was observed that the overall battery string voltage decreased by as much 

as 0.5 volts while the higher load was applied. For a 60-cell battery string typically employed at 

NPPs, this voltage drop would be equivalent to about 2.5 volts, a fairly significant change that 

could impact power supplies and inverters powered from the battery. The NPP operator needs 

to be aware of the voltage change when performing these necessary intermittent operations.
59

Figure 3-16 GNB Voltage Response to Load Profile #4 (EBR-2)

Specific Gravity Results: Specific gravity measurements indicated that the electrolyte is quite 

depleted following the extended battery operations associated with each load profile. Figure 3-

17 is a plot of the post discharge average midpoint specific gravity readings for the three battery 

types. Note the significant decrease in specific gravity as the test duration is increased. For 

instance, the 8-hour baseline performance tests had an average midpoint specific gravity that 

ranged from 1.08 to 1.10 for the 3 battery types, while the value decreased to between 1.02 and 

1.04 when the test duration exceeded 36 hours. While this did not directly affect the battery’s 

ability to provide load, it could impact its ability to be recharged especially if there is a delay in 

the time between discharge and initiation of the recharge.

Figure 3-17 Average “end-of-test” midpoint specific gravity readings for Sequence 2 tests

21

22

23

24

25

26

27

0 10 20 30 40 50

overall voltage (V)

Test Duration (hours)

Momentary increases in voltage as load is 

decreased followed by linear decrease during 

steady state load to about 1.85V/cell or 22.2 

volts for the 12 cell string

1.083

1.035

1.018

1.099

1.05

1.032

1.09

1.041

1.03

1.00

1.02

1.04

1.06

1.08

1.10

1.12

0 10 20 30 40 50

Specific gravity

Test duration (Hours)

Enersys 2GN-23

Exide/GNB NCN-21

C&D LCR-33

61

4 SUMMARY AND CONCLUSIONS

The Japanese earthquake and tsunami event on March 11, 2011, illustrated the fact that 

restoration of AC power at NPPs can be significantly impacted by external events and can take 

a long time to recover under certain scenarios. Extending the availability of safety-related Class 

1E batteries can be accomplished through the manual shedding of non-essential loads thereby 

reducing the demand on the battery. The testing described in this report showed how typical 

nuclear-grade batteries perform when operated for extended periods under these postulated 

scenarios.

BNL used the three sets of nuclear qualified batteries with each battery string consisting of 12 

cells. The batteries were tested in a manner consistent with how they would be operated in a 

typical nuclear power station’s Class 1E DC power system using a resistive load bank. Two test 

sequences were performed. The first applied lower loading on the battery strings to assess 

their operation at regular time intervals out to 72 hours. The second test sequence applied 

varying load conditions on the battery to simulate NPP load shedding responses to an ELAP.

Overall the batteries tested at BNL met their performance objectives during the conduct of 

extended battery operation. While the number of discharge/recharge cycles that these batteries 

have been subjected to during the Phase 1 testing described in NUREG/CR-7148 [1] and the 

testing described in this report is more than what is expected to be experienced during their 

nominal 20 year life, there was no apparent degradation to them or reduction in their capacity. 

However, these batteries were neither artificially aged nor subjected to the same aging criteria 

that would be expected in their 20-year life in a NPP environment. Therefore, it cannot be 

concluded that the batteries in operation near the end of their design life would perform in the 

same manner as the ones that were tested in this research program. This fact represents the 

largest uncertainty associated with the test results. 

The significant observations made from this testing that are described in this report are:

1. Battery performance at various time intervals out to 72 hours was demonstrated. The 

efficiency of the battery at lower loadings increases by approximately 30% thereby 

increasing the available battery amp-hours during the extended operation. 

2. For the load profiles provided by industry, the measured durations from the batteries 

varied from 22 to 48 hours, much greater than the current plant SBO coping times of 4 to 

8 hours. This increased availability of the batteries to supply critical equipment and 

instrumentation is accomplished through an aggressive load shedding process. 

Information associated with the load profiles provided by NEI is included in this report. 

Test results showed that the time to initiate load shedding is important, but even when 

that time is 2 hours (as compared to 1⁄2 hr), battery availability beyond 24 hours can still 

be achieved. 

3. The projected availability of a battery can be accurately calculated using IEEE Standard 

485-2010 and battery vendor information. This includes the impact of the time to initiate 

and complete load shed actions. Procedures can be put in place at the NPPs to identify 

the critical loads that need to be maintained as long as possible as well as the process 

to eliminate non-essential loads in a timely manner. Knowing the ampere requirements 

for these critical loads and the loads that will be shed permits the availability of the 

battery to be calculated using IEEE Standard 485-2010, including accepted factors to 

address battery aging, design margin and the plant-specific battery end-voltage. 
62

4. The battery voltage response during discharge is generally linear over the discharge but 

decreases rapidly at about 1.85V. The momentary voltage drop from intermittent 

operation of equipment during load shedding can impact the operation of power supplies 

and inverters as the battery supplied voltage gets close to their low voltage protection 

setpoints. NPP operators should be cognizant of the battery voltage response and the 

critical equipment that could be adversely affected as the battery voltage decreases. The 

specific equipment that could be affected by a low voltage battery condition should be 

identified in emergency operating procedures to allow the operators to take mitigating 

actions before the supplied equipment is de-energized. 

5. There is significant depletion of the electrolyte during long duration discharges due to the 

higher efficiency of the battery under lower current discharge conditions. This could 

affect the ability to recharge the battery. Severe accident management guidance that 

provides information on how to slowly recharge a battery that has been depleted for 

more than 8 hours would allow the operators to safely return the battery to service when 

AC power becomes available. 

6. No new battery failure mechanisms were observed during extended operational testing; 

however, as noted above, the significant depletion of the electrolyte during very long 

discharges (>36 hours) could impact the ability to recharge the battery under some 

conditions (e.g., a delay between the time of the battery depletion and initiation of 

recharge)
63

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NUREG/CR-7188 Testing to Evaluate Extended Battery Operation in Nuclear Power Plants