Confirmatory Battery Testing: 

The Use of Float Current 

Monitoring to Determine 

Battery State-of-Charge

Office of Nuclear Regulatory Research

NUREG/CR-7148

BNL-NUREG-98563-2012
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Confirmatory Battery Testing: 

The Use of Float Current 

Monitoring to Determine

Battery State-of-Charge

Manuscript Completed: October 2012

Date Published: November 2012

Prepared by:

W. Gunther, G. Greene, M. Villaran, Y. Celebi, 

and J. Higgins

Brookhaven National Laboratory 

Nuclear Science and Technology Department

Systems Engineering Group

Upton, NY 11973-5000

Liliana Ramadan, NRC Project Manager

NRC Job Code N6542

Office of Nuclear Regulatory Research

NUREG/CR-7148

BNL-NUREG-98563-2012

iii

ABSTRACT

In February 2007, the U.S. Nuclear Regulatory Commission (NRC) issued Regulatory 

Guide (RG) 1.129 Rev. 2, “Maintenance, Testing, and Replacement of Vented Lead-Acid 

Storage Batteries for Nuclear Power Plants." In this RG, the NRC staff endorsed the Institute of 

Electrical and Electronics Engineers (IEEE) Standard 450-2002, “Recommended Practice for 

Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary 

Applications." This standard provides the recommended practices, test schedules, and testing 

procedures including recommended methods for determining a battery’s state-of-charge to 

maintain permanently installed vented lead-acid storage batteries (typically of the lead-calcium 

type) for their standby power applications. Previous versions of this standard suggested that 

either float current or specific gravity could be used for determining the battery’s state-of-charge. 

The NRC sponsored the research project described herein to validate the use of float charging 

current as a measure of a battery’s state-of-charge for batteries that are used in the nuclear 

industry. This report describes the approach taken, the specific activities performed to achieve 

the objectives of this research effort, and the results achieved. It provides analysis of the data 

and offers observations and recommendations for use by the NRC and its licensees.

v

TABLE OF CONTENTS

Section Page

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

TABLE OF CONTENTS..............................................................................................................v

LIST OF FIGURES ................................................................................................................... vii

LIST OF TABLES.......................................................................................................................ix

EXECUTIVE SUMMARY ...........................................................................................................xi

ACKNOWLEDGMENTS........................................................................................................... xiii

1. Introduction .................................................................................................................... 1

1.1. Background......................................................................................................... 1

1.2. Program Objectives ............................................................................................ 1

1.3. Research Approach ............................................................................................ 1

2. Testing Protocols............................................................................................................ 7

2.1. Introduction......................................................................................................... 7

2.2. Battery String Configuration ................................................................................ 8

2.3. Test Equipment Used ......................................................................................... 9

2.4. Test Cycle Description .......................................................................................10

2.5. Testing Process: Evaluation of Charging Current as a Monitoring Technique ...12

3. Test Results ..................................................................................................................15

3.1. Performance Tests by Battery (Cycles 1-10)......................................................15

3.2. Float Current Responses on Recharge..............................................................23

3.3. Specific Gravity Measurements and Response..................................................30

3.4. Comparison of Float Current and Specific Gravity Responses...........................39

3.5. Conductance and Connection Resistance Data .................................................40

3.6. Temperature Monitoring Data ............................................................................42

3.7. Summary ...........................................................................................................45

4. Analysis of Test Results ................................................................................................47

4.1. An Assessment of Float Current as a Measure of State-of-Charge ....................47

4.2. Specific Gravity as a Measure of State-of-Charge..............................................50

4.3. Analysis of Battery Capacity Trends and Capacity Recovery (Test Cycle 11) ....52

4.4. Use of Conductance for Monitoring Battery Condition........................................57

4.5. Analysis of Temperature Measurements............................................................59

4.6. “Return-to-service” Tests (Test Cycles 12 to 14)................................................60

5. Conclusions...................................................................................................................71

6. References....................................................................................................................73

vii

LIST OF FIGURES

Figure Page

1-1 Battery test laboratory at BNL......................................................................................... 2

1-2 Battery capacity test set (Alber BCT-128TM) ................................................................... 3

1-3 Specific gravity readings using a digital hydrometer ....................................................... 4

1-4 Conductance readings in progress ................................................................................. 4

2-1 Battery discharge (performance) test.............................................................................. 7

2-2 Battery recharge activities .............................................................................................. 8

2-3 Circuit Using Calibrated 200 amp - 50 millivolt shunt.....................................................10

2-4 600 amp capacity resistive load bank ............................................................................10

3-1 Enersys cycle 1 discharge voltage profile......................................................................15

3-2 Enersys- cycle 1- cell 2 voltage response during discharge...........................................16

3-3 Enersys cycle 1 recharge voltage profile .......................................................................16

3-4 GNB cycle 1 discharge voltage profile ...........................................................................17

3-5 GNB cycle 1- cell 1 voltage response during discharge .................................................18

3-6 GNB cycle 1 recharge voltage profile ............................................................................18

3-7 GNB cycle 4 discharge voltage profile ...........................................................................19

3-8 GNB cycle 3 recharge voltage profile ............................................................................19

3-9 C&D cycle 6 discharge voltage profile ...........................................................................20

3-10 C&D cycle 6 recharge voltage profile.............................................................................21

3-11 Enersys battery capacity change over time ...................................................................22

3-12 GNB battery capacity change over time.........................................................................22

3-13 C&D battery capacity change over time.........................................................................22

3-14 Enersys cycle 5 recharge current ..................................................................................23

3-15 Enersys cycle 5 float current (late recharge)..................................................................24

3-16 GNB cycle 1 float current response at 28.0 volts ...........................................................25

3-17 GNB Cycle 1 float current response at 28.0 volts (late recharge cycle)..........................25

3-18 GNB cycle 3 float current response at 27.0 volts ...........................................................26

3-19 GNB cycle 3 float current response at 27.0 volts (late recharge cycle) ..........................26

3-20 C&D cycle 1 charging/float current response at 27.0 volts.............................................28

3-21 C&D cycle 1 float current response at 27.0 volts (late recharge cycle) ..........................28

3-22 C&D cycle 10 recharge/float current response at 28.0 V ...............................................29

3-23 C&D cycle 10 recharge/float current response at 28.0 V (late recharge cycle)...............29

3-24 Specific gravity sampling tube .......................................................................................31

3-25 Enersys cycle 5 early specific gravity response.............................................................32

3-26 Enersys cycle 5 full test cycle specific gravity response ................................................32

3-27 Enersys very long term specific gravity response (weekly readings)..............................33

3-28 GNB cycle 1 early specific gravity response ..................................................................33

3-29 GNB cycle 1 specific gravity full test cycle response......................................................34

3-30 GNB cycle 3 specific gravity early cycle response .........................................................34

3-31 GNB cycle 3 specific gravity full test cycle response......................................................35

3-32 GNB specific gravity very long term response (weekly readings)...................................35

3-33 C&D cycle 1 specific gravity early response ..................................................................36

3-34 C&D cycle 1 specific gravity full test cycle response......................................................36

3-35 C&D specific gravity very long term response (weekly readings)...................................37

3-36 Enersys cycle 2 midpoint specific gravity readings for all cells.......................................37

3-37 GNB cycle 8 midpoint specific gravity readings for all cells............................................38

3-38 C&D cycle 10 midpoint specific gravity readings for all cells ..........................................38

3-39 Enersys cycle 2 specific gravity vs. float current............................................................39
viii

3-40 GNB cycle 4 specific gravity vs. float current .................................................................39

3-41 C&D cycle 8 specific gravity vs. float current .................................................................40

3-42 Conductance measuring sequence for the C&D LCR-33 battery string..........................41

3-43 Enersys cycle 1 temperature response over time ..........................................................43

3-44 GNB cycle 5 temperature response over time ...............................................................43

3-45 C&D cycle 1 temperature response over time................................................................44

3-46 C&D cycle 1 cell and ambient temperatures ..................................................................44

3-47 Surface Mounted Thermocouple....................................................................................45

4-1 Enersys cycle 10 float current........................................................................................48

4-2 GNB cycle 9 float current (recharge at 27.0 volts) .........................................................48

4-3 GNB cycle 1 float current (recharge at 28.0 volts) .........................................................48

4-4 C&D cycle 2 float current (recharge at 27.0 volts)..........................................................49

4-5 C&D cycle 10 float current (recharge at 28.0 volts for 40 hours)....................................49

4-6 Enersys cycle 2 specific gravity vs. float current............................................................51

4-7 GNB cycle 4 specific gravity vs. float current .................................................................51

4-9 Specific gravity prior to the start of each test cycle - Enersys battery.............................53

4-10 Enersys capacity changes including cycle 11 ................................................................54

4-11 Specific gravity at the start of each test cycle - GNB......................................................54

4-12 GNB capacity changes including cycle 11 .....................................................................55

4-13 Specific gravity at the start of each cycle - C&D ............................................................56

4-14 C&D capacity changes including cycle 11 .....................................................................56

4-15 Sample cell conductance pre-test measurements - Enersys..........................................58

4-16 Sample cell conductance pre-test measurements - GNB...............................................58

4-17 Sample cell conductance pre-test measurements - C&D...............................................58

4-18 Analysis of temperature impact on capacity tests (C&D Cycle 1)...................................59

4-19 Enersys cycle 12 stable float current monitoring............................................................61

4-20 Enersys cycle 13 stable float current monitoring............................................................62

4-21 Enersys cycle 14 - float current to three time constants.................................................62

4-22 Enersys performance during first 10 cycles – ampere hours returned ...........................63

4-23 GNB Cycle 12 stable float current monitoring ................................................................64

4-24 GNB cycle 13 stable float current for three hours ..........................................................64

4-25 GNB cycle 14 float current to three time constants ........................................................65

4-26 GNB float current response during first 10 test cycles ...................................................66

4-27 C&D cycle 12 stable float current monitoring .................................................................67

4-28 C&D cycle 13 stable float current monitoring .................................................................67

4-29 C&D Cycle 14 float current response.............................................................................68

4-30 C&D float current response during first 10 test cycles....................................................69

A-1 ENERSYS cycle 14 - float current out to three time constants.....................................A-2

A-2 Normalized recharge current vs. time during the exponentially decreasing recharge 

current period for GNB cycle 8 ....................................................................................A-4

A-3 Normalized recharge current following current limit vs. time constant for ENERSYS 

cycles 1-10 ..................................................................................................................A-5

A-4 Percentage of battery capacity recharged following current limit vs. time constant for 

ENERSYS cycles 1-10 ................................................................................................A-6
ix

LIST OF TABLES

Table Page

1-1 Confirmatory Testing Research Project Schedule .......................................................... 5

2-1 Test Activities ................................................................................................................11

2-2 Recommended time correction factor (Table 1 of IEEE Std. 450-2002).........................13

2-3 Parameters monitored over time....................................................................................13

2-4 Summary of parameters measured ...............................................................................14

3-1 Summary of Enersys battery string voltage response....................................................17

3-2 Summary of GNB battery string voltage response .........................................................20

3-3 Summary of C&D battery string voltage response .........................................................21

3-4 Summary of Enersys recharge/float current characteristics ...........................................24

3-5 Summary of GNB recharge/float current characteristics ................................................27

3-6 Summary of C&D recharge/float current characteristics ................................................30

3-7 Conductance measurements – compared to battery capacity for all 30 cycles ..............42

4-1 Percent ampere-hours returned and recharge time .......................................................50

at float current of two amps ...........................................................................................50

4-2 Summary of “return-to-service” tests .............................................................................69

4-3 Actual float current value (Amps) at three time constants ..............................................70

A-1 Recharge data for the calculation of the exponential time constant. ............................A-3

xi

EXECUTIVE SUMMARY

To ensure that a battery has the capability to execute its safety function, it is necessary to 

confirm its fully charged condition and operational readiness. For the past three decades the 

typical nuclear power plant Technical Specifications required the measurement of specific 

gravity to determine the state-of-charge of the batteries. This requirement was based on RG 

1.129 Rev.1, “Maintenance, Testing, and Replacement of Large Lead Storage Batteries for 

Nuclear Power Plants,” which endorsed IEEE Std. 450-1975, IEEE Recommended Practice for 

Maintenance, Testing, and Replacement of Large Lead Storage Batteries for Generating 

Stations and Substations." The more recent version of this standard, IEEE Std. 450-2002, that 

was endorsed by the NRC, suggested that either float current or specific gravity could be used

for determining a vented lead-calcium battery’s state-of-charge. This report describes the 

project to validate this approach on batteries that are used in the nuclear industry.

In conducting this study, three sets of nuclear qualified batteries were procured from three 

different battery vendors. Each battery set consisted of 12 battery cells. These cells are the 

same models that are typically used in a Class IE dc system application. Two suitably sized 

battery chargers and a load bank were also obtained; the second battery charger was used to 

maintain the batteries not being tested on a continuous float charge. The test setup was as 

close as reasonably practicable (not including seismic battery racks) to a typical nuclear power 

station’s Class 1E battery design. Once the battery was fully charged and stabilized, a 4-hour 

discharge test was performed based on the battery vendor’s specifications. After the discharge 

test, the float current was continuously recorded while the battery was recharged. During the 

recharge, periodic specific gravity measurements were taken at the vendor-specified location

(about 1/3 down the length of the cell) for all of the cells, and at the top and bottom as well for 

two of the cells. The three measurements taken on two of the cells allowed us to obtain a 

vertical profile representation of the electrolyte’s distribution within the cell during the entire 

discharge-recharge cycle. Discharge test current and specific gravity readings were 

compensated for temperature as discussed in IEEE Std. 450-2002.

The major findings that were derived from more than thirty cycles of deep discharge testing are:

1) Both float current and specific gravity provide adequate means to determine battery 

state-of-charge. Float current has an advantage in that it provides an indicator of the 

entire battery string, while specific gravity is measured on a cell by cell basis.

2) Both float current and specific gravity have similar response times when the battery is 

recharged. Generally speaking, 100% of the ampere-hours discharged are returned to 

the battery within 24 hours of the start of the recharge cycle.

3) The amount of electrolyte stratification is significant following a performance test and it 

takes months before equilibrium is reached within the cells. Therefore, it is critical to 

measure specific gravity at the correct point as indicated by the battery vendor’s manual 

and supported by IEEE Std. 450-2002. 

4) The use of pilot cells to ascertain specific gravity is supported by the consistent 

response observed among all cells during both discharge and recharge.

5) Measuring float current through the use of a simple shunt connected to a data 

acquisition system provides accurate and repeatable measurements. We used a 200 

amp (A), 50 millivolt (mV) shunt placed in series with the output of the battery charger. 

We found that a more sophisticated device based on the principles of the Hall Effect 
xii

(similar to a clamp-on ammeter) is also effective, but it is less accurate at the low ends of 

the float current range (< 2 amps).

Other observations derived from the extensive testing that was performed are: 

Using a standard length of tubing to draw the electrolyte from the same point resulted in 

consistent “trendable” specific gravity data.

• Temperature compensation for capacity testing, specific gravity readings and conductance

readings is important. If not performed properly, the data will be skewed.

• Float current response will vary based on the recharge voltage applied to the battery. 

However, regardless of the voltage applied during recharge, the float current of a nearly fully 

charged battery becomes stable at less than two amps. 

• Calculations of the ampere-hours returned to the battery during recharge can be used to 

verify the battery’s state-of-charge. The majority (>60%) of the ampere-hours returned to the 

battery occurs while the battery charger is still in a current limit mode.

IEEE Std. 450-2002 contains the following criterion related to return to service for a battery: 

“When the charging current has stabilized at the charging voltage for three consecutive hourly 

measurements, the battery is near full charge.” Our test program also verified the point where 

the battery can be safely returned to service. In a series of six additional tests (two tests per 

battery string), the battery strings were able to meet their capacity and capability requirements 

at the point where the float current was stable for three hours. Thus the criterion used in IEEE 

Std. 450-2002 was found to be an acceptable practice for ensuring the capacity and capability 

requirements of the battery were met before returning it to service.

Similarly, three cycles of tests were performed in which each battery was returned to service 

when the float current reached the value equivalent to three time constants on the recharge/float 

current curve. This occurred within about twelve hours and at a higher current than the 

previously described return to service tests. In each case, the battery was also able to meet its 

capacity and capability requirements. This calculated float current value obtained from the 

battery-specific recharge/float current curve may be a more practical method for returning the 

battery to service at the point where it is capable of meeting its capacity and capability 

requirements.
xiii

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 Thomas Koshy and Matthew McConnell for their insights and helpful comments on 

the test program. We appreciate the support provided by Robert Lofaro and Edward Grove of 

BNL in the development of the project management and quality assurance plans, respectively. 

We also wish to thank William Maloney, formerly of BNL, for his input during the preparation of 

the testing program and Paul Giannotti of BNL, for his technical expertise in developing some of 

the support instrumentation. We acknowledge and appreciate the administrative support from 

Maryann Julian in producing this document.

1

1. INTRODUCTION

1.1. Background

In February 2007, the U.S. Nuclear Regulatory Commission (NRC) issued Regulatory Guide 

(RG) 1.129 Rev. 2, “Maintenance, Testing, and Replacement of Vented Lead-Acid Storage 

Batteries for Nuclear Power Plants." In this RG, the NRC staff endorsed the Institute of 

Electrical and Electronics Engineers (IEEE) Standard 450-2002, “Recommended Practice for 

Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary 

Applications." This standard provides the recommended practices, test schedules, and testing 

procedures including recommended methods for determining a battery’s state-of-charge to 

maintain permanently installed vented lead-acid storage batteries (typically of the lead-calcium 

type) for their standby power applications. 

To ensure that a battery has the capacity and capability to execute its safety function, it is 

necessary to confirm its fully charged condition and operational readiness. For the past three 

decades, the typical nuclear power plant Technical Specifications required the measurement of 

specific gravity to determine the state-of-charge of the batteries. This requirement was based 

on Regulatory Guide (RG) 1.129 Rev.1, “Maintenance, Testing, and Replacement of Large 

Lead Storage Batteries for Nuclear Power Plants,” which endorsed IEEE Std. 450-1975, “IEEE 

Recommended Practice for Maintenance, Testing, and Replacement of Large Lead Storage 

Batteries for Generating Stations and Substations." A more recent version of this standard, 

IEEE Std. 450-2002, that was endorsed by the NRC in Rev. 2 of RG 1.129, suggested that 

either float charging current or specific gravity could be used for determining a vented leadcalcium battery’s state-of-charge. The testing program approved by the NRC using a series of 

4-hour performance tests was implemented to validate this approach on batteries that are 

commonly used in the nuclear industry. Comparisons were made of the recharge/float current 

and the specific gravity responses as the cells were charged following the four hour 

performance test. Note that these test results are only applicable to vented lead-calcium 

batteries.

1.2. Program Objectives

The primary objective of this research project was to determine whether the float charging 

current can be a useful indicator for determining a vented lead-calcium battery's state-of-charge 

over the life of the battery. This project evaluated the acceptability of using float charging 

current as a means of monitoring battery state of charge for lead-acid calcium batteries from 

three vendors. A secondary objective was to evaluate the point at which a battery could be 

returned to service and meet its performance requirements.

1.3. Research Approach

The approach taken in this research project involved testing of Class 1E batteries representative 

of those used in commercial nuclear power plants. These batteries were installed in a 

configuration similar to that used in the nuclear power plants and were subjected to full 

discharge and recharge cycling equivalent to what would be experienced over its nominal 20-

year life. The number of cells used in the testing (12-cells per battery string) is smaller than 

what is typically used in a nuclear power plant, where 60 or 120 cell batteries are employed. 

However, the test results of the 12-cell string are directly applicable to the larger batteries since 

they are carrying the same current and the cell voltages are the same. The scaling of the 

overall battery voltage is linear to the number of cells in the string. Note that this testing did not
2

age the batteries by subjecting the batteries to elevated temperatures. Charging (float) current 

and specific gravity were measured during each test cycle in accordance with IEEE Std. 450-

2002, along with other battery parameters, to monitor the status of the batteries. Testing was

performed in accordance with a Quality Assurance Plan developed specifically to meet the 

needs of this project that ensured an acceptable level of quality for the test results. 

1.3.1. Establish 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 met BNL 

safety procedures. Facility attributes included area temperature and humidity control and

monitoring, electrolyte spill control measures, and adequate ventilation to prevent hydrogen 

accumulation. Security measures were established so that access to the testing area was

limited to those directly involved with the battery confirmatory testing program. Data acquisition 

equipment was installed to acquire and store the measured parameters during testing.

1.3.2. 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 in the test area.

1.3.3. Battery Test Setup

Figure 1-1 Battery test laboratory at BNL

Battery racks were used that replicate a nuclear power plant installation with the exception that 

the racks were not seismically qualified. There are three racks of 12-cells each representing the 

three battery vendors. Adequate spacing between the racks was provided to ensure that test 
3

leads could be attached and specific gravity and conductance measurements could be safely 

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

1.3.4. Data Acquisition Equipment

Nuclear power plants employ test equipment for the performance tests required by their 

technical specifications. One of the most common units used is the BCT-128TM manufactured 

by Alber Corp (Figure 1-2). This test equipment monitors and displays cell voltages and can be 

programmed to discharge a battery under constant power, constant current, or variable current. 

For the purposes of our testing, we employed the test set in conjunction with a load bank, to 

discharge the battery at a constant current for a four hour performance test. The unit 

automatically disconnected the load when the overall string voltage reached 21.0 volts. It was 

also configured to shut down the test if any one cell reached 1.6 volts; however, that never 

occurred.

Figure 1-2 Battery capacity test set (Alber BCT-128TM)

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

several manual operations were required periodically. These included specific gravity readings, 

intercell resistance readings, and conductance readings. The intercell resistance readings 

ensured the integrity of the connections; the conductance readings were taken to determine if 

they could provide an indicator of cell state-of-charge.

Test equipment traceable to the National Institutes of Science and Technology (NIST) was used 

to acquire these data. Figure 1-3 illustrates the use of a Storage Battery Systems SBS-2500 

digital hydrometer for the required specific gravity readings, and Figure 1-4 shows the use of the 

Midtronics Celltron Ultra CTU-6000 universal stationary battery analyzer to obtain conductance 

measurements.
4

Figure 1-3 Specific gravity readings using a digital hydrometer

Figure 1-4 Conductance readings in progress

1.3.5. Research Project Management

Discharge (performance) and recharge testing were performed on only one battery string at a 

time to ensure that proper controls and monitoring were focused on that activity. We 

established a test schedule that resulted in one test cycle being completed per week. This 

allowed the battery string to be in a float mode with stable float current for at least 120 hours 

before subjecting the battery to another performance test. 

The batteries that were not actively undergoing testing were maintained on float charge using a 

separate battery charger. Periodic inspection and maintenance of the batteries not under active 

testing was performed to maintain them in good condition and a fully charged state.

Table 1-1 shows the activities performed along with the scheduled milestones. 
5

Table 1-1 Confirmatory Testing Research Project Schedule.

Activity FY09 FY10 FY11 FY12

Qtr. 4 Qtr. 

1

Qtr. 

2

Qtr. 

3

Qtr. 

4

Qtr. 

1

Qtr. 

2

Qtr. 

3

Qtr. 

4

Qtr. 

1-2

Develop Plans X

Program Test Plan X

Quality Plan X

Procurement Plan X

Milestone: Plans Complete 

(Draft)

X

Milestone: Plans Complete 

(Final)

X

Establish Test Facility X X X

Identify suitable space for 

testing

X

Set up laboratory space X X

Procure and install data 

acquisition equipment

X X X

Perform safety review and 

obtain approval to test

X

Milestone: Test Facility Ready X

Acquire Test Specimens X X X X X

Order battery, charger, rack, 

and load bank- all vendors

X

Receive and install chargers, 

battery racks, & load bank

X

Vendor 1 installs batteries X

Vendor 2 installs batteries X

Vendor 3 installs batteries X

Milestone: All Test Specimens 

Acquired and Installed

X

Perform Battery Float Current 

Tests

X X X

Milestone: Complete Vendor 1 

Testing – 10 cycles

X

Milestone: Complete Vendor 2 

Testing – 10 cycles

X

Milestone: Complete Vendor 3 

Testing – 10 cycles

X

Perform Return to Service 

Battery Tests

X X

Complete stable float current 

testing

X

Complete 3 time constant 

testing

X

Milestone: Complete Return to 

Service Testing

X

7

2. TESTING PROTOCOLS

2.1. Introduction

Testing of the first battery string, the Enersys Model 2GN-23, was initiated on September 21, 

2010 with a four-hour performance test (discharge test). Following successful completion of the 

performance test, the battery was recharged for 120 hours. Prior to the start of the discharge 

test, baseline readings of specific gravity, cell conductance, and cell temperature were taken. 

During the recharge cycle, measurements of specific gravity, cell voltage, cell temperature, cell 

conductance and float current were completed. 

The Alber Battery Capacity Test Set was used to control and monitor the performance test. It 

provided data on the discharge current, individual cell voltages, and a real-time calculation of 

battery capacity. An Omega temperature data acquisition system was used to monitor cell 

temperatures over the entire cycle. A second Omega module was used to record the voltage 

across a calibrated shunt installed in series with the output of the battery charger. This shunt 

provided the float charging current output. A second means of measuring float charging current 

employed the Polytronics float current monitor. This instrument provided continuous readings of 

float current from the battery charger during the entire recharge cycle as well as individual cell 

voltages. Manual readings of specific gravity using an SBS-2500 digital hydrometer were taken 

periodically during the discharge-recharge cycle along with cell conductance readings using a 

Midtronics CTU-6000 conductance meter. The overall test process is illustrated in Figures 2-1 

(discharge mode) and 2-2 (recharge mode). 

Figure 2-1 Battery discharge (performance) test

The same process was used for each of ten discharge/recharge cycles for the Enersys battery 

string, and then continued for the Exide GNB and C&D battery strings over 30 weeks of testing. 

Weekly, a summary level data report was provided to the NRC project manager followed up 

periodically with conference calls to discuss the data and its interpretation.
8

Figure 2-2 Battery recharge activities

A test plan was developed by BNL and reviewed and approved by the NRC project manager 

prior to initiation of the testing. This overarching plan included the process used for the 

procurement of the batteries, the management of the project, a quality assurance plan, and the 

testing protocol to be followed.

2.2. Battery String Configuration

Each battery string was charged to their rated conditions to account for any self-discharge that 

occurred during shipping and installation in accordance with IEEE Std. 450-2002 and the vendor 

recommendations. While recharging the battery, we monitored and recorded the charging 

current and specific gravity to ensure the data acquisition equipment was working properly. 

Constant voltage was used, and the appropriate temperature correction factors were applied to 

the specific gravity readings taken. We consulted with the battery manufacturers for the proper 

recharge voltage, temperature, and the expected duration of recharge.

Two battery chargers were employed for the testing program. A 200 amp capacity charger was 

used for recharging the battery string under test. This charger, manufactured by Enersys 

(Model AT30-24-200), has an output voltage regulation of 0.25% and an adjustable current limit 

mode that was set to 90% of rating or 180 amps. A second battery charger (100 amp capacity) 

was used to maintain the other two battery strings in a fully charged condition (float voltage of 

27.0 volts). For the performance test, the charger was disconnected from the battery and a fullcapacity discharge of the battery performed using the supplied load bank and the respective 
9

manufacturer’s specifications for a four-hour test. The discharge was automatically terminated 

when the battery string of 12-cells reached 21.0 volts (average of 1.75 volts per cell). A second 

automatic shutdown was programmed into the test unit if any cell reached a terminal voltage of 

1.60 volts. This latter condition was never experienced during the testing program. Test 

parameters monitored during the deep discharge included cell temperatures, ambient area 

temperature, discharge current, terminal voltages, and periodic specific gravity and conductance 

readings.

The charger was reconnected and placed in a constant voltage mode (current limiting) to 

recharge the battery bank following completion of the performance test. The charging current 

was recorded at a high resolution during this iteration (milliamp level). Periodic midpoint specific 

gravity measurements were taken on all of the cells and specific gravity profiles (top, bottom 

and midpoint) were taken on two cells during discharge and recharge. As a matter of record, 

recharging of the battery commenced within about an hour following the discharge cycle in 

accordance with the vendor’s recommendations. The midpoint reading was typically about 1/3 

the distance from the top of the cell as specified by the battery vendor and supported by IEEE 

Std. 450-2002.

Ten charge and discharge cycles were conducted for each battery string. For the Enersys 

battery string, recharge was conducted at the high end of the float voltage range (27.0 volts) as 

recommended by the manufacturer. For the GNB and C&D batteries, recharge was conducted 

at the equalizing voltage of 28.0 volts on several cycles to compare the float current 

characteristics with the standard recharge voltage of 27.0 volts. This approach was approved 

by representatives from these battery vendors.

2.3. Test Equipment Used

Nuclear power plants employ test equipment for the performance tests required by their 

technical specifications. One of the most common units used is the BCT-128TM manufactured 

by Alber Corp (Figure 1-2). This test equipment monitors and displays cell voltages and can be 

programmed to discharge a battery under constant power, constant current, or variable current. 

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

several manual operations were required periodically. These included specific gravity readings, 

intercell resistance readings, and conductance readings. 

Test equipment calibrated to standards traceable to NIST was used to acquire these data so 

that they can be correlated to changes in battery capacity. A float current monitoring device 

was used to delineate the charging current measured during testing of the first battery string and 

then was used as a backup for the remainder of the testing program. Figure 2-3 is a 

representation of the circuit containing the 200 amp, 50 mV calibrated current shunt that was 

used to determine the float current on the GNB and C&D battery strings and for all three battery 

strings during the “return-to-service” testing. Figure 2-4 is the 600 amp, 24 volts capacity load 

bank used in the testing. 
10

Figure 2-3 Circuit Using Calibrated 200 amp - 50 millivolt shunt

Figure 2-4 600 amp capacity resistive load bank

2.4. Test Cycle Description

The NRC approved test plan ensured that each battery string was tested in accordance with the 

manufacturer’s requirements and that the testing was carried out consistently throughout the 

testing program. The plan also addressed the monitoring techniques to be evaluated and 

provided guidance on the steps to take if a failure occurred. Specific BNL procedures were 

used to augment the test plan. They provided precautions and prerequisites for conducting the 

test as well as detailed information about the data to be acquired.

The tests are summarized as follows:
11

Table 2-1 Test Activities.

Test Activity Specimens 

Tested

Objective of Test

Vendor Acceptance Test

(performed by vendor prior to 

shipment or at BNL following 

installation).

ALL As part of the nuclear safety Class 1E 

procurement and installation, each 

battery will pass an acceptance test as 

required by IEEE Std. 450-2002.

Capacity Test-Time adjusted 

method

ALL-multiple 

times

This testing was conducted in 

accordance with section 7.4 of IEEE 

Std. 450-2002. The vendor’s battery 

ratings were used to determine the 

loading to be applied. Parameters 

were monitored to determine changes 

that occur during deep discharges.

Charging ALL-multiple 

times

Constant voltage, current- limited 

recharging of the battery string will be 

applied as soon as possible following 

the capacity test. Charging current 

was closely monitored and was used 

to determine when the battery string 

had reached full capacity. This is the 

key activity where specific gravity and 

charging current are compared to 

determine how accurately they reflect 

battery state-of-charge.

Cycle 11 Capacity Test was 

conducted after the electrolyte 

had reached an equilibrium 

state (specific gravity uniform 

in pilot cells).

All-one cycle 

each

Determine the impact of the stratified 

electrolyte on battery capacity.

Return-to-Service Testing: 

this series of tests were 

conducted several months 

after the completion of the first 

portion of the test to address 

a question regarding the point 

at which a battery can be 

returned to service following 

maintenance or discharge 

testing.

All-three cycles 

each (cycles 12-

14 for each 

battery string)

Determine the point at which the 

battery can be returned to service with 

assurance that it can meet its capacity 

and capability objectives. Two 

methods tested-stable float current for 

three hours and three time constants.

2.4.1. Three Types of Batteries in this Program

The specific batteries to be tested were selected by the NRC and are representative of the 

batteries used in most U.S. nuclear power plants. The specific lead-calcium batteries used 

were:
12

• Enersys 2GN-23 cells with a nominal capacity (8-hour rating) of 1800 Amp-hours,

• Exide GNB NCN-21 cells with a nominal capacity (8-hour rating) of 1496 Amp-hours, and

• C&D Technologies LCR-33 cells with a nominal capacity (8-hour rating) of 2320 Amp-hours.

The Exide GNB specimens were supplied by Nuclear Logistics Inc., the sole supplier of nuclear 

grade batteries for Exide. 

2.4.2. Installation Processes

Qualified vendors were contracted to install the batteries at BNL. A baseline record of the 

mechanical and electrical installation parameters was documented. This included open circuit 

voltage, initial charge, and charging current readings. Connection resistances were measured

to ensure the integrity of the connections.

2.5. Testing Process: Evaluation of Charging Current as a 

Monitoring Technique

The testing was conducted as delineated in Section 7.4 of IEEE Std. 450-2002 as follows:

• Set up the load and the necessary instrumentation to maintain the test discharge rate for 

battery string being tested. The discharge rate is based on the 4-hour battery rating.

• Disconnect the charging source, connect the load bank to the battery, start the timing, and 

continue to maintain the selected discharge rate. (This was accomplished using the Alber 

BCT-128 Capacity Test Set.)

• Maintain the discharge rate until the battery terminal voltage decreases to a value equal to 

the minimum average voltage per cell as specified by the design of the installation times the 

number of cells. For acceptance and performance tests as an example, a 12-cell battery 

with a minimum design voltage of 1.75 volts per cell, then the minimum battery voltage for 

the test is 12 × 1.75 or 21 volts.

• Read and record the individual cell voltages and the battery terminal voltage. These 

measurements were taken continuously by the data acquisition system. Other parameters 

such as specific gravity, intercell resistance, and conductance were taken while the load is 

applied at the beginning of the test, at regular intervals, and at the completion of the test. 

The process for taking and recording the parameter values are addressed in separate BNL 

procedures that follow good testing practices and vendor recommendations.

Note: For safety reasons (concern for cell voltage reversal), the capacity test set was programmed to 

automatically terminate the test if any one cell reached 1.6 volts.

• Observe the battery for any abnormal conditions especially any intercell connector heating 

that could result in damage to the cell.

• At the conclusion of the discharge, the Alber battery capacity test set calculates the battery 

capacity in accordance with Section 7.3 of IEEE Std. 450-2002 applying the temperature 

corrections presented in Table 1 of IEEE Std. 450-2002 (see Table 2-2 in this document). 

• The load bank is then turned off and the battery realigned to the battery charger. (Before 

energizing the charger, we conducted a series of specific gravity and conductance 

readings.)

While recharging the battery, we monitored and recorded the charging current and specific 

gravity. Constant voltage was applied, and the appropriate temperature correction factors 
13

made. Little change in electrolyte level occurred during the testing program so no electrolyte or 

distilled water needed to be added over the 10 cycles. 

Table 2-3 summarizes the parameters that were measured during the conduct of the discharge 

and recharge cycles. Table 2-4 identifies the primary instrument used to measure the 

parameter and its precision.

Table 2-2 Recommended time correction factor (Table 1 of IEEE Std. 450-2002).

Table 2-3 Parameters monitored over time.

Cycle # Charging 

Current

Specific 

Gravity

Conductance Intercell 

Resistance

Cell Voltage Cell 

Condition

Discharge 

Cycle 

Charge 

Cycle

• N/A

• Charger 

output to 

.01A

All cells at 

the midpoint 

level and 

profiles (top, 

midpoint, 

and bottom) 

of 2 cells at 

regular 

intervals

All cells prior to 

charge and 

discharge 

cycles

All cells 

prior to 

charge and 

discharge 

cycles 

All cells 

continuously

Visual 

assessment 

Recorded 

following 

each cycle

Repeat 

~10X
14

Table 2-4 Summary of parameters measured.

Parameter Test Equipment Precision

Cell Voltage Alber BCT-128 .01V

Inter-cell voltage Midtronics CTU-6000 .001V

String volts & amps Alber BCT-128 0.2V/0.1%A

Electrolyte 

Temperature

SBS-2500 Digital hydrometer 0.2 ̊ F

Float Current Polytronics (for Enersys Battery)

Calibrated Shunt for GNB and C&D

10 mA

1 mA

Cell Conductance Midtronics CTU-6000 2%

Specific Gravity SBS-2500 Digital hydrometer 0.0001 g/cm3

Ambient Temperature Omega thermocouple 0.5 ̊F

Cell Surface 

Temperature

Omega thermocouple 0.5 ̊F
15

3. TEST RESULTS

This section of the report discusses some of the data that were acquired using examples of 

representative test cycles. A complete set of data for every test cycle on each battery string is 

on file and maintained at BNL. Since the data taken on all three battery strings are fairly 

uniform, in some cases only data for one or two of the battery vendors are presented to illustrate 

the types of measurements taken and the responses obtained.

3.1. Performance Tests by Battery (Cycles 1-10)

3.1.1. Enersys Battery String 

Cycle 1 Discharge Voltage Profile (Figure 3-1): Note that the starting voltage is 27.0 volts with 

the battery charger secured. When the discharge test is initiated, the battery voltage drops 

precipitously from 27.0 volts to about 23 volts. This same general behavior was experienced on 

all three batteries during each of the discharge tests. The rated load current for the Enersys 

battery is 385 amps corrected to 376 amps in this test to account for the electrolyte temperature 

of 73 ̊ F. (Performance test results are referenced to 77  ̊F per IEEE Std. 450-2002)

A resistive load bank was used and was controlled by the Alber Capacity Test Set at a steady 

current over the nominal four-hour test. The performance test was automatically terminated 

when the battery string voltage reached 21.0 volts. Note that “Battery OV” represents the 

overall battery string voltage (left axis), and “Battery Load” represents the discharge current to 

the load bank in amps (right axis). For this test, the automatic shutdown occurred in four hours 

and four minutes which translated to a capacity of 101.7%. 

Figure 3-1 Enersys cycle 1 discharge voltage profile

The individual cell voltages were also monitored during the performance test and were observed 

to behave uniformly over the 10 test cycles within tenths of a volt. An example of one cell’s 

voltage response during the cycle 1 performance test is provided in Figure 3-2. Similar voltage 

data for each individual cell were recorded that manifested themselves as a proportional 
16

reflection of the overall battery string voltage versus time during the discharge and recharge 

cycles. During the conduct of the test, the 12-cell voltages were displayed on the computer 

monitor allowing us to compare and measure individual cell voltages in real time.

Figure 3-2 Enersys- cycle 1- cell 2 voltage response during discharge

Enersys Cycle 1 Recharge Voltage Profile: Once the load is removed from the battery string, 

the overall voltage increases to about 24.0 volts. The battery charger was reconnected and 

recharged at a float voltage of 27.0 volts for all 10 cycles of the Enersys testing as per the 

manufacturer’s recommendations. Figure 3-3 illustrates the steady increase in voltage that 

occurs during recharge, reaching the terminal float voltage of 27.0 volts in about 11 hours for 

Enersys cycle 1.

Figure 3-3 Enersys cycle 1 recharge voltage profile
17

The overall battery voltage response for the 10 Enersys test cycles is summarized in Table 3-1.

Table 3-1 Summary of Enersys battery string voltage response.

Cycle #

Starting/Recharge

Voltage

Initial 

Voltage

Drop Value

Time to Reach 

Recharge Voltage 

(Hours)

1 27 23.0 10.5

2 27 23.2 8.5

3 27 23.2 8.5

4 27 23.3 8.7

5 27 23.3 16.5

6 27 23.3 7.0

7 27 23.3 17.2

8 27 23.3 10.7

9 27 23.4 15.0

10 27 23.4 6.0

3.1.2. GNB Battery String

Cycles 1 and 2 of the 10 test cycles of the GNB battery string were conducted at a recharge 

voltage of 28.0 volts. The remaining cycles were conducted at 27.0 volts. This vendor permits 

recharge to be conducted at the equalizing voltage which is the approach taken by some 

nuclear power plants. 

GNB Cycle 1 Discharge Voltage Profile (28.0 volts): As illustrated in Figure 3-4, when the load 

is applied to the battery, the overall voltage decreases to about 23.2 volts. In this first cycle, the 

battery string voltage reached the automatic shutdown point of 21.0 volts in nearly four hours 

and six minutes which translates to a battery capacity of 102.4%.

Figure 3-4 GNB cycle 1 discharge voltage profile
18

Figure 3-5 illustrates an individual cell’s voltage response during the discharge iteration. Note 

the initial dip in voltage when the load is applied and the recovery as a stable current is 

established (Coup de Fouet effect).

Figure 3-5 GNB cycle 1- cell 1 voltage response during discharge

GNB Cycle 1 Recharge Voltage Profile (28.0 volts): During recharge at an equalizing voltage of 

28.0 volts, the battery charger stays in current limit mode longer and the battery string reaches 

the charger terminal voltage in approximately 8.5 hours. This is illustrated in Figure 3-6.

Figure 3-6 GNB cycle 1 recharge voltage profile
19

GNB Cycle 4 Discharge Voltage Profile (27.0 volts): In this cycle (Figure 3-7), the battery string 

voltage response to the automatic shutdown voltage of 21.0 volts does not vary significantly 

from the discharge response with the battery string starting at 28.0 volts.

Figure 3-7 GNB cycle 4 discharge voltage profile

GNB Cycle 3 Recharge Voltage Profile (27.0 volts): As illustrated in Figure 3-8, recharging at 

27.0 volts resulted in a significantly longer time to achieve the battery rated voltage (~13.5 hours 

vs. 8.5 hours that is illustrated in Figure 3-6). However, in later test cycles where the recharge 

was conducted at 27.0 volts, the battery string reached its terminal voltage of 27 volts within 8.0 

hours. Table 3-2 summarizes the overall battery voltage response for the 10 cycles of testing of 

the GNB battery.

Figure 3-8 GNB cycle 3 recharge voltage profile
20

Table 3-2 Summary of GNB battery string voltage response.

Cycle # Starting/Recharge 

Voltage

Initial 

Voltage 

Drop Value

Time to Reach 

Recharge

Voltage (Hours)

1 28 23.0 8.4

2 28 23.1 8.1

3 27 23.1 13.5

4 27 23.2 10.5

5 27 23.2 8.0

6 27 23.2 12.5

7 27 23.2 6.6

8 27 23.2 8.5

9 27 23.3 6.5

10 27 23.3 8.7

3.1.3. C&D Battery String

As with the GNB battery string, the C&D cycle testing was conducted at both a float voltage 

level of 27.0 volts and an equalizing voltage level of 28.0 volts. In cycle 6, conducted at 27.0 

volts, Figure 3-9 illustrates the steady voltage decrease that occurred towards the automatic 

shutoff voltage of 21.0 volts. Figure 3-10 depicts the recharge voltage profile. Note that it takes 

approximately 17 hours for the battery string to reach its required terminal voltage of 27.0 volts. 

Table 3-3 summarizes the battery string voltage response for the 10 C&D cycles.

Figure 3-9 C&D cycle 6 discharge voltage profile
21

Figure 3-10 C&D cycle 6 recharge voltage profile

Table 3-3 Summary of C&D battery string voltage response.

Cycle # Starting/Recharge 

Voltage

Initial 

Voltage 

Drop Value

Time to Reach 

Recharge

Voltage (Hours)

1 27 23.1 14.25

2 27 23.2 12.25

3 28 23.2 14.75

4 28 23.3 11.25

5 27 23.3 18.5

6 27 23.3 16.5

7 27 23.3 11.0

8 27 23.4 10.5

9 28 23.3 15.5

10 28 23.4 11.0

3.1.4. Battery Capacity Performance Tests

The battery capacity test set compares the time that it takes for the battery to reach 21.0 volts 

(1.75 volts/cell) to the rated (100%) performance time as the measure of battery capacity. This 

was calculated for every test cycle that was conducted. A trend of the capacity values for the 10 

cycles conducted on each of the battery strings is portrayed in Figures 3-11, 3-12, and 3-13. 

Note that in all cases, the battery achieved greater than 100% capacity performance in the first 

cycle, with the capacity decreasing by 5-10% by the 10th cycle. One of the battery vendors 

stated that the battery should be able to be cycled 50-100 times before reaching an end of life 

state. While the cycling was not intended to age the battery, it is indicative of the number of 

performance tests that would be experienced by a typical battery over its 20-year life.
22

Figure 3-11 Enersys battery capacity change over time

Figure 3-12 GNB battery capacity change over time

Figure 3-13 C&D battery capacity change over time
23

3.2. Float Current Responses on Recharge

Monitoring float current response during the recharge of a battery following a performance test 

was a key objective of this test program. Our initial testing set up for the first battery employed 

a float current monitoring device manufactured by Polytronics. For the second and third battery 

strings, we used a calibrated shunt as the primary current data and the Polytronics unit as a 

backup. An offset was detected in the Polytronics unit that required correction of the Enersys 

float current data. The data included in this section reflect the correction. 

As illustrated in this section, the response varies somewhat from cycle to cycle and certainly 

between battery types. What is common is the general shape of the float current response 

curve and the approximate time at which a stable float current is reached, although the latter is 

sensitive to the voltage applied during recharge. More detailed analysis of the data represented 

by these curves and tables is provided in Section 4. This section contains observations of the 

float current curve characteristics as measured by the calculated curve time constant as well as 

the calculation of amp-hours returned to the battery. 

Enersys Float Current Response 

In Figure 3-14, the float current is shown to be constant at the current limit setting of the battery 

charger (180 amps) for approximately six hours at which time it starts decreasing in an 

exponential-like manner. The latter part of the curve in Figure 3-14 is expanded in Figure 3-15 

to show that at about 24-hours a stable float current is achieved at about 0.5 amps. Slight 

variations were observed for the Enersys battery over the 10 test cycles; these data are 

summarized in Table 3-4. Note that all recharge cycles for the Enersys battery string were 

performed at the high end of the specified float voltage (27.0 volts) as recommended by the 

vendor.

Figure 3-14 Enersys cycle 5 recharge current
24

Figure 3-15 Enersys cycle 5 float current (late recharge)

Table 3-4 Summary of Enersys recharge/float current characteristics.

Cycle 

Number

Ah 

Discharged

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 % Recharged 

at ∆tCL + 5*tC

1

(73°F)

1530 Ah 11.2 11.9 101% 1.98 6.50 16.4 101%

(1 amp)

2

(70°F)

1480 Ah 11.9 13.6 101% 2.27 6.00 17.4 102%

(1 amp)

3

(68°F)

1486 Ah 13.8 14.8 101% 2.34 5.76 17.5 102%

(3 amp)

4

(69°F)

1468 Ah 13.6 19.9 103% 2.38 5.66 17.6 102%

(4 amp)

5

(74°F)

1516 Ah 14.0 20.6 103% 2.26 6.06 17.4 102%

(5 amp)

6

(73°F)

1466 Ah 13.7 22.2 104% 2.18 5.61 16.5 102%

(7 amp)

7

(73°F)

1496 Ah 15.8 26.0 103% 2.24 5.51 16.7 100%

(9 amp)

8

(74°F)

1440 Ah 15.8 26.3 104% 2.16 5.28 16.1 100%

(9 amp)

9

(73°F)

1429 Ah 18.3 26.8 102% 2.17 5.24 16.1 99%

(9 amp)

10

(70°F)

1395 Ah 17.2 28.6 104% 2.16 4.87 15.7 99%

(10 amp)

Rated discharge capacity at 385 amps for four hours at 77°F is 1540 Ah.

Recharge for all cycles was conducted at 27 volts.

Columns 3, 5, and 9 are based upon comparisons of the measured Ah discharged (col. 2) to the rated Ah capacity for a 4-hr 

discharge performance test from 2.25 V/cell to an end voltage of 1.75 V/cell.

In Table 3-4, it is worth noting that we somewhat arbitrarily selected 2 amps as a point of 

comparison among the three battery types. The ampere-hours returned to the battery was 

calculated from the float current response curve and compared to the actual amount of amperehours that were discharged as determined by the capacity test set data. Finally, the time 

This figure expands the previous graph to 

focus on latter part of the recharge cycle
25

constant of the float current response curve was calculated using a process described in more 

detail in Appendix A.

GNB Float Current Response

The Exide GNB nuclear grade batteries, provided by Nuclear Logistics, Inc., were recharged at 

an equalizing voltage of 28.0 volts for cycles 1 and 2 and at the float voltage level of 27.0 volts 

for the remaining eight cycles in order to assess the difference in the float current response. 

Feedback obtained from the battery vendors indicated that some of their customers (the nuclear 

power plants) used an equalizing charge following a discharge test to speed up the electrolyte 

mixing process. We therefore wanted to determine how the float current responses would differ 

under these two recharge conditions. This section illustrates the float current response for 

representative cycles of recharge at a float voltage of 27.0 volts and an equalizing charge of 

28.0 volts. All 10 cycles of recharge data are then summarized in Table 3-5.

Figure 3-16 illustrates the recharge/float current response with the battery charger set at its 

equalizing voltage of 28.0 volts. The battery stays in a current limit mode (180 amps in this 

case) for more than six hours. Once the battery approaches 28.0 volts, the current from the 

charger decreases rapidly towards an asymptotic float current value of less than 0.5 amps, as 

shown in Figure 3-17.

Figure 3-16 GNB cycle 1 float current response at 28.0 volts

Figure 3-17 GNB Cycle 1 float current response at 28.0 volts (late recharge cycle)

This figure expands the previous 

graph to focus on latter part of 

the recharge cycle
26

Figures 3-18 and 3-19 depict the recharge/float current response with the battery charger set at 

27.0 volts. The current limit mode is maintained for a shorter time (less than four hours) but the 

recharge/float current response is more gradual than the response at 28.0 volts. In both cases, 

the same stable float current is ultimately obtained.

Figure 3-18 GNB cycle 3 float current response at 27.0 volts

Figure 3-19 GNB cycle 3 float current response at 27.0 volts (late recharge cycle)

This figure expands the previous graph to 

focus on latter part of the recharge cycle
27

It was apparent from performing the recharge at an equalizing charge (28.0 volts) vs. a float 

charge (27.0 volts) that the characteristic curve was significantly different as follows:

1. The time the charger stays in current limit is longer for an equalizing charge

2. The time constant is shorter for an equalizing charge

3. The steady-state float current is reached sooner for an equalizing charge

The 10 cycles of the GNB float current data are summarized in Table 3-5: 

Table 3-5 Summary of GNB recharge/float current characteristics.

Cycle 

Number

Ah 

Discharged

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 % Recharged 

at ∆tCL + 5*tC

1

(72°F)

1278 8.5 8.5 100% 0.97 6.34 11.2 100%

(1 amp)

2

(65°F)

1191 7.3 8.8 102% 1.22 5.67 11.8 102%

(1 amp)

3

(58°F)

1157 10.3 14.8 103% 2.90 3.92 18.4 103%

(<1 amp)

4

(67°F)

1201 12.0 15.8 101% 2.51 4.25 16.8 101%

(1 amp)

5

(64°F)

1190 13.3 16.6 101% 2.51 4.15 16.7 101%

(2 amp)

6

(66°F)

1184 Detailed data are not available due to an interruption of power to the data acquisition systems.

7

(64°F)

1168 15.0 18.8 101% 2.49 3.93 16.4 100%

(4 amp)

8

(69°F)

1178 14.6 19.7 102% 2.30 4.15 15.7 100%

(4 amp)

9

(71°F)

1151 15.8 17.9 101% 2.28 4.03 15.4 100%

(4 amp)

10

(70°F)

1184 14.9 19.8 101% 2.24 4.25 15.5 100%

(4 amp)

Rated discharge capacity at 317 amps for four hours at 77°F is 1268 Ah.

Recharge for Cycles 1 and 2 were conducted at 28.0 volts or 2.33 volts per cell (highlighted data).

Columns 3, 5, and 9 are based upon comparisons of the measured Ah discharged (col. 2) to the rated Ah capacity for a 4-hr 

discharge performance test from 2.25 V/cell to an end voltage of 1.75 V/cell.

As noted above, detailed data are not available for GNB cycle 6 due to an interruption of power 

to the data acquisition systems. However, the four-hour discharge at 296 amps and the 

subsequent recharge to 27 volts for seven days was unaffected by the electrical problem.

C&D Battery Float Current Response

The C&D LCR-33 battery is the largest in capacity of the three battery types tested. As a result, 

the recharge times were longer than the other two battery strings. Recharge of the C&D 

batteries occurred at both 27.0 volts and 28.0 volts to assess float current response in both 

cases. These voltages were chosen based on the vendor recommendations for float and 

equalize voltage values. The recharge for cycle 1 conducted at 27 volts is illustrated in Figures 

3-20 and 3-21:
28

Figure 3-20 C&D cycle 1 charging/float current response at 27.0 volts

Figure 3-21 C&D cycle 1 float current response at 27.0 volts (late recharge cycle)

Figures 3-22 and 3-23 show the response with a recharge voltage of 28.0 volts up to 40 hours 

at which time the charger automatically switched to an output voltage of 27.0 volts. The float 

current dropped from about 2 amps to 0.5 amps when the charger output switched from 28.0 to 

27.0 volts.

This figure expands the previous graph 

to focus on latter part of the recharge 

cycle

Suspected test equipment 

or connection anomaly
29

Figure 3-22 C&D cycle 10 recharge/float current response at 28.0 V

Figure 3-23 C&D cycle 10 recharge/float current response at 28.0 V (late recharge cycle)

The recharge/float current data for all 10 cycles is summarized in Table 3-6. The highlighted 

rows indicate where recharge was conducted at 28.0 volts; otherwise 27.0 volts was used.

This figure expands the previous graph 

to focus on latter part of the recharge 

cycle

Charger output voltage 

change from 28 to 27 

volts at 40 hours
30

Table 3-6 Summary of C&D recharge/float current characteristics

Cycle 

Number

Ah 

Discharged

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 % Recharged at 

∆tCL + 5*tC

1

(70°F)

2050 14.6 20.6 102% 3.03 8.87 24.0 102%

(2 amp)

2

(72°F)

2007 14.8 21.3 102% 3.05 8.52 23.8 103%

(2 amp)

3

(73°F)

2000 12.1 24.0 105% 1.78 9.85 18.8 104%

(5 amp)

4

(72°F)

1956 11.9 24.0 105% 1.80 9.57 18.6 104%

(4 amp)

5

(72°F)

1944

(97.2%)

14.9 25.5 105% 3.07 7.82 23.2 105%

(5 amp)

6

(71°F)

1936

(96.8%)

18.3 28.6 103% 3.04 7.72 22.9 102%

(5 amp)

7

(72°F)

1929

(96.5%)

18.7 30.2 103% 2.98 7.70 22.6 102%

(5 amp)

8

(72°F)

1920

(96.0%)

19.4 31.3 103% 2.98 7.60 22.5 101%

(6 amp)

9

(72°F)

1900 12.5 40.0 107% 2.00 8.90 18.9 104%

(5 amp)

10

(72°F)

1901 12.1 38.7 107% 1.94 9.05 18.8 104%

(4 amp)

Rated discharge capacity at 500 amps for four hours at 77°F is 2000 Ah.

Recharge for cycles 1-2 and 5-8 was conducted at 27 volts.

Recharge for cycles 3-4 was conducted at 28 volts for 24 hours, then reduced to 27 volts.

Recharge for cycles 9-10 was conducted at 28 volts for 40 hours, then reduced to 27 volts.

Recharge current data for cycle 5 are from the Polytronics unit.

Note: Highlighted data are associated with recharge at an equalizing voltage of 28.0 volts (2.33 volts per cell)

As can be observed by looking at Figures 3-20 to 3-23 and Table 3-6, the time needed to return 

100% of the discharged amp-hours to the battery is less when recharge is conducted at the 

equalizing voltage of 28.0 volts than when recharged at the nominal float voltage of 27.0 volts. 

This is also reflected by the shorter time constant for the equalizing voltage (~2 hours) versus 

the float voltage time constant (~3 hours). However, it should also be noted that the time to 

reach a stabilized float current (2 amps) takes longer when the battery is recharged at an 

equalizing voltage. This is described more completely in Section 4.1.

3.3. Specific Gravity Measurements and Response

The nominal specific gravity for all three battery types is 1.215. Thousands of specific gravity 

measurements were taken over the course of this test program. The most common 

measurement was the one recommended by the battery vendors, which is approximately 1/3 of 

the distance from the top of the plates; we refer to this measurement as the midpoint reading. 

The battery vendors provide a tube that allows the instrument probe to reach this point on a 

consistent basis (see Figure 3-24). BNL also measured specific gravity on two cells on each 

battery string near the top of the cell and close to the bottom of the jar. We refer to the three 

readings on these two cells as a profile of the electrolyte acid concentration. These profile 

readings were repeated weekly on the two cells even after the 10 cycles of testing were 

completed for that string. In fact, eight months of data on the Enersys battery revealed that it 

takes a very long time for equilibrium to be reached following a series of performance tests. 

Weekly specific gravity profile testing on the GNB (seven months) and C&D (four months) 

battery strings also supported this observation. The impact of stratification on battery capacity 

and capability is measureable but not operationally significant as described further in Section 

4.3.
31

Also provided in this section are data that clearly reveal the consistency among cells as the 

discharge and recharge conditions occur. These data support the use of pilot cells in a battery 

string as a reasonable method of determining the state of battery specific gravity for the string.

Figure 3-24 Specific gravity sampling tube

Enersys Battery String:

Measurements of specific gravity were taken during the discharge of the battery (four hour 

performance test) and regularly during the recharge of the battery. Figure 3-25 illustrates the 

specific gravity response over 25 hours for a typical discharge/recharge cycle on the Enersys 

battery. Figure 3-26 depicts the response of the specific gravity for a typical weekly test cycle. 

The last set of readings of the battery on one test cycle became the starting point for the next 

test cycle, with one performance test performed per week. Furthermore, even after the testing 

was completed on a battery string, specific gravity profile measurements continued to be taken 

weekly. For the Enersys battery string, which was the first battery tested, the readings 

continued for more than eight months while the battery remained on a float charge. It took that 

long for uniformity to be reached in the specific gravity measurements at the top, middle, and 

bottom of the cell. The eight months needed for the electrolyte in the Enersys battery to reach 

equilibrium is illustrated in Figure 3-27.

Specific Gravity 

Sampling Tube
32

Figure 3-25 Enersys cycle 5 early specific gravity response

Figure 3-26 Enersys cycle 5 full test cycle specific gravity response
33

Figure 3-27 Enersys very long term specific gravity response (weekly readings)

GNB Battery String: 

The GNB battery, supplied by Nuclear Logistics Inc., was the second battery string to be tested. 

As indicated in the following figures (Figures 3-28 to 3-31), the specific gravity response during 

the performance test and recharge is very similar to what was seen with the Enersys battery.

Similarly, Figure 3-32 shows that this battery took about seven months on a float charge for the 

electrolyte to reach equilibrium. The GNB cycle 1 testing is used as an example and is 

compared with GNB cycle 3. For cycle 1 (Figures 3-28 and 3-29), an equalizing voltage of 28.0 

volts was used during the recharge cycle. For cycle 3 (Figures 3-30 and 3-31), a float voltage of 

27.0 volts was used during recharge. 

Figure 3-28 GNB cycle 1 early specific gravity response

~ 8 months for 

equilibrium to be reached
34

Figure 3-29 GNB cycle 1 specific gravity full test cycle response

In cycle 1, note that the midpoint specific gravity reading is restored to its initial value in about 

15 hours and is stable within 24 hours. In cycle 3 (Figures 3-30 and 3-31), with a recharge 

voltage of 27.0 volts, it was observed that the specific gravity response was virtually identical.

Figure 3-30 GNB cycle 3 specific gravity early cycle response
35

Figure 3-31 GNB cycle 3 specific gravity full test cycle response

Figure 3-32 GNB specific gravity very long term response (weekly readings)

~ 7 months for equilibrium 

to be reached
36

C&D Battery String:

The third battery tested was the C&D battery. Figure 3-33 illustrates the short term response 

during the performance test and about 20 hours afterwards. Note that the midpoint specific 

gravity is restored to its initial value during recharge within about 15 hours and is at its stable 

value within 24 hours. The specific gravity in the C&D batteries reached an equilibrium state 

within four months following the completion of the 10 cycles of performance testing as illustrated 

in Figure 3-35.

Figure 3-33 C&D cycle 1 specific gravity early response

Figure 3-34 C&D cycle 1 specific gravity full test cycle response
37

Figure 3-35 C&D specific gravity very long term response (weekly readings)

Another aspect of specific gravity is the response in each cell. This is important to know since 

nuclear power plants rely on using data from pilot cells to ascertain the condition for the entire 

battery string. Figures 3-36 to 3-38 provide examples of the cell midpoint specific gravity 

readings that indicate that the response among the cells is uniform and that the use of a pilot 

cell is appropriate.

Figure 3-36 Enersys cycle 2 midpoint specific gravity readings for all cells

~ 4 months for equilibrium 

to be reached
38

Figure 3-37 GNB cycle 8 midpoint specific gravity readings for all cells

Figure 3-38 C&D cycle 10 midpoint specific gravity readings for all cells
39

3.4. Comparison of Float Current and Specific Gravity Responses

This section presents specific gravity and float current response data in a common graph to 

illustrate their response over time during the discharge and recharge cycles. The first example 

(Figure 3-39) shows the dramatic decrease in specific gravity (blue line) as measured at the 

midpoint of the cell followed by a steady increase to its original value during recharge. 

Figure 3-39 Enersys cycle 2 specific gravity vs. float current

The recharge current is depicted by the red line in Figure 3-39. With the battery charger

disconnected during the performance test, the recharge current is zero. At approximately five 

hours following the initiation of the performance test, the recharge of the battery is initiated. The 

battery charger goes into a current limit mode (capped for this testing at 180 amps) and then 

decreases in an exponential manner towards a stable value of about 0.5 amps. Note the close 

relationship in time when both the specific gravity and the float current stabilize (approximately 

20 hours).

Figure 3-40 GNB cycle 4 specific gravity vs. float current
40

Figure 3-40 for the GNB battery is similar in overall appearance but reaches a stable value for 

both specific gravity and float current slightly sooner than the Enersys battery shown in 

Figure 3-39. Likewise, the largest of the three batteries, the C&D LCR-33 reaches stable 

specific gravity and float current values at approximately 25 hours as shown in Figure 3-41.

Figure 3-41 C&D cycle 8 specific gravity vs. float current

3.5. Conductance and Connection Resistance Data

In addition to the float current and specific gravity measurements discussed in the previous 

sections, individual battery cell conductance measurements were taken at key hold points 

during the performance of the battery discharge and recharge testing cycles. This section of the 

report describes the conductance and connection resistance data gathered during the battery 

performance testing cycles.

3.5.1. Measurements

Conductance measurements for this testing program were obtained using the Midtronics 

Celltron Ultra CTU-6000 universal stationary battery analyzer. The conductance test is 

performed by connecting the two test instrument leads to the positive and negative terminal 

posts of the individual cells of the battery under test. If there is more than one positive and 

negative terminal post per cell and if multiple cells are connected in series by conductive straps 

to create a battery string or bank, as in this test program, a sequence of conductance 

measurements is performed. The electrical resistance of each of the cell-to-cell connecting 

straps is also measured as part of the conductance measurement process and is presented on 

the CTU-6000 LCD visual display along with the conductance values for the each of the battery 

cells in the string being evaluated. The temperature of the battery electrolyte must be measured 

and programmed into the instrument to correct the measured conductance values for thermal 

effects.

The measurement sequence for each specific battery installation can be preprogrammed into 

the instrument so that the operator can apply the probes to each of the cells in the batteryunder-test, one-by-one, in the sequence required. The instrument LCD visual display will then 
41

cue the operator to the next measurement in the sequence. When the measurement probes are 

applied to each of the testing points, a low-voltage, low-frequency ac signal is applied to the 

battery-under-test, and the instrument will measure the resulting ac current that flows in the test 

circuit. After the test circuit has stabilized (3-8 seconds was our experience during this test 

program), an audible tone sounds to alert the operator that the measurement step is complete 

and the next testing point in the sequence can begin; the results of the just-completed 

measurement step are presented on the LCD display along with a cue for the location of the 

next measurement point in the test sequence. A different audio tone is generated if a problem 

occurs during the performance of a conductance measurement step and information on the 

problem is shown on the instrument display. The technician can then make the necessary 

adjustments or corrections and then retry the measurement.

Figure 3-42 Conductance measuring sequence for the C&D LCR-33 battery string

Figure 3-42 shows the sequence of measurements required to measure the conductance for the 

C&D Model LCR-33, 12-cell, 4-posts-per-jar, single-tier rack-mounted Class 1E battery tested in 

this program. The results of the analysis provide the individual cell voltage and conductance, as 

well as the electrical resistance of the cell-to-cell connection straps, for the battery-under-test.

3.5.2. Conductance and Connection Resistance Test Results

Conductance measurements were performed at three ‘hold’ points for each cell in a battery 

string as the battery was subjected to the 10 test cycles. The measurements were made as 

follows: 1) prior to the start of the initial four hour discharge, 2) after completion of the four-hour 

discharge but prior to start of the recharge, and 3) approximately 24 hours into the recharge. A 

fourth measurement was taken included at 168 hours after the start of the discharge test which, 

in fact, corresponded to the first measurement prior to the start of the initial four-hour discharge 

for the next test cycle.

Table 3-7 presents the averaged conductance measurements for the 12-cells in each battery 

string. As can be seen from the data, the conductance measured reflects the battery capacity at 

the time of the measurement: conductance measurements are highest in the fully charged state 

and lowest immediately at the end of the four-hour discharge segment of the performance test. 

The corresponding plots of the average conductance of the battery string show that the battery 

capacity is very nearly restored at the end of each step in the testing cycle.
42

Table 3-7 Conductance measurements – compared to battery capacity for all 30 

cycles

There is a trend of a slight decrease in the average fully-charged conductance for the Enersys 

GN-23 battery and the C&D LCR-33 battery over the course of the 10 testing cycles, but it is 

minimal. This corresponds to a similar indication of a trend showing slightly decreasing battery 

capacity as indicated by the last column in each table: battery capacity as measured by the 

Alber software. The GNB-Exide NCN-21 battery actually showed a trend to slightly increase its 

average fully-charged conductance measurement over the 10 test cycles. Battery capacity for 

the GNB-Exide NCN-21 as measured by the Alber software indicated a slightly decreasing 

battery capacity over the 10 cycles.

3.6. Temperature Monitoring Data

This section provides a summary of the temperature monitoring data accumulated during the 

discharge-recharge cycles. The significance of the temperature readings is that the 

performance test is temperature compensated based on the temperature at the start of the test. 

As can be seen in Figure 3-43, the average temperature of the cells from the start of the test to 

the completion of the test varies by approximately 8-10°F. We used the initial temperature only 

for setting the current to be used in the discharge test in accordance with IEEE Std. 450-2002. 

Slightly different capacity measurements would have resulted if we were to use an average 

temperature experienced over the course of the test.
43

While it is not practical to continuously compensate the data for temperature during the test, it is 

important to note that there is some impact to the battery capacity outcome based on the 

temperature compensation so the rationale employed for testing should be consistently applied. 

Figure 3-43 Enersys cycle 1 temperature response over time

This kind of response was also observed in the GNB and C&D battery strings as illustrated in 

Figure 3-44 (GNB cycle 5) and Figure 3-45 (C&D cycle 1). So despite the size of the battery jar 

or its ampere-hour rating, the surface temperature on the jar fluctuates in the same manner as 

the battery is discharged. It slowly returns to its initial condition as the battery is recharged. 

Ambient temperature was also monitored during the cycle testing. An example of the ambient 

change along with the comparable temperature changes for two of the cells is illustrated in 

Figure 3-46. 

Figure 3-44 GNB cycle 5 temperature response over time
44

Figure 3-45 C&D cycle 1 temperature response over time

Figure 3-46 C&D cycle 1 cell and ambient temperatures

The slight ambient temperature fluctuation observed during the four hour performance testing 

was deemed insignificant due to the large mass of the cells that resulted in a several hour time 

delay between a change in ambient temperature to an observed change in electrolyte 

temperature. IEEE Std. 450-2002, Section 7.3.2.2 states that the temperature correction should 

be applied to a time based performance test in accordance with the formula, C= {Xa x KC /Xt} x 
45

100 where C is the % capacity at 25°C; Xa is the actual rate used for the test; Xt is the 

published rating for the time to reach a specified terminal voltage; and KC is the correction factor 

for the cell temperature before the start of the test as provided in Table 2-2 of IEEE Std. 450-

2002. For example, referring again to Figure 3-46, the rated four hour performance test for the 

C&D battery for cycle 1 was automatically corrected by the Alber Capacity test set so that the 

rating of 500 amps was corrected to 481 amps since our starting electrolyte temperature was 

70 ̊F (KC = 1.04). In the actual case of cycle 1 for the C&D battery, it took four hours and 15 

minutes to reach an overall string voltage of 21.0 volts. This yielded a capacity of 106.5% for 

cycle 1.

Figure 3-47 illustrates the location of the surface mounted thermocouple for one of the batteries.

Figure 3-47 Surface Mounted Thermocouple

3.7. Summary

In this section, we have summarized the extensive amount of data that were acquired during the 

30 cycles of discharge and recharge testing. Specific gravity, charging (float) current, 

conductance, and temperature were frequently measured during the cycles. Calibration checks 

were performed on the equipment periodically to ensure the quality of the data. Regular 

inspections of the batteries, including electrolyte level and connection resistance, ensured that 

the batteries were tested within their normal operating limits. 

Both recharge/float current and specific gravity responded similarly during the charging cycle 

indicating that either can provide a measure of a battery’s state-of-charge. The three battery 

types responded very similarly during the discharge and recharge cycles lending a level of 

confidence to generalizations made that can be used in regulatory guidance. The analyses and 

conclusions are discussed further in Sections 4 and 5, respectively.

47

4. ANALYSIS OF TEST RESULTS

In this section of the report, further analyses of the data are discussed in the following areas: 

• Float current as a measure of “state-of-charge”

• Specific gravity as a measure of “state-of-charge”

• Capacity testing trends and their meaning, 

• Electrolyte stratification effects, and

• “Return-to-service” test results.

4.1. An Assessment of Float Current as a Measure of State-of-Charge

Specific gravity has been used for many years as a means of determining a battery’s state-ofcharge. For the lead-calcium type of vented lead acid battery used in nuclear power plants, 

there are often technical specification requirements associated with specific gravity, such as 

frequency of checking, the expected values to be obtained, and the corrective action, or limiting 

conditions for operation, that are imposed if specific gravity readings are lower than required. In 

addition, battery manufacturers include specific gravity measurements as a standard 

maintenance check for their batteries.

One of the changes made over the years and supported in IEEE Standard 450-2002, is the use 

of charging/float current as a means to monitor the state-of-charge of lead acid batteries. 

Revision 2 of Regulatory Guide 1.129, issued in February 2007, endorsed IEEE Std. 450-2002 

with certain clarifying regulatory positions that are described in Section C of the Regulatory 

Guide.

In this testing program, we calculated the ampere-hours that are restored to the battery as it 

was recharged as one means to determine that the battery had been recharged. In many test 

cycles, the total number of ampere-hours returned to the battery exceeded the number of 

ampere-hours discharged before

Note that the largest of the cells, the C&D LCR-33 model, took a longer time to reach a stable 

float current due to the higher number of ampere-hours that needed to be returned to the battery 

following the performance test. The battery charger that was used in the project was limited to 

180 amps (current limit setting). As shown in Figure 4-5, the charger output switched from 28 

volts to 27 volts at 40 hours, thereby resulting in a corresponding decrease in the float current at 

that point.

the float current reached a steady-state level. For the lead acid 

batteries that we tested, the stable float current achieved was in the 0.5 to 2.0 amp range. At 

that point in the current vs. time curve, only small numbers of ampere-hours are being returned 

to the cell. This is illustrated in Figures 4-1 to 4-5 for the three different battery strings. In 

Figure 4-1, the float current response below 10 amps is shown for the cycle 10 test of the 

Enersys battery. Figures 4-2 and 4-3 depict the float current response for the GNB battery 

when the recharge was conducted at 27.0 and 28.0 volts, respectively. Similarly, Figures 4-4 

and 4-5 illustrate the response for the C&D battery with the recharge conducted at 27.0 and 

28.0 volts, respectively.
48

Figure 4-1 Enersys cycle 10 float current

Figure 4-2 GNB cycle 9 float current (recharge at 27.0 volts)

Figure 4-3 GNB cycle 1 float current (recharge at 28.0 volts)

When the float current reached 2 

amps, 101% of the discharged 

ampere-hours had been restored.

When the float current reached 2 

amps, 104% of the discharged 

ampere-hours had been restored.

When the float current reached 2 

amps, 100% of the discharged 

ampere-hours had been restored.
49

Figure 4-4 C&D cycle 2 float current (recharge at 27.0 volts) 

Figure 4-5 C&D cycle 10 float current (recharge at 28.0 volts for 40 hours)

The float current responses on the three battery strings support the following conclusions:

1. The battery’s state-of-charge as measured by the return of the discharged ampere-hours to 

the battery can be correlated to the stabilizing of the float current (generally about 0.5 to 2.0

amps). The stable current is slightly higher when the applied voltage (equalizing charge) is 

higher.

2. Once the float current reaches an asymptotic level, the ampere-hours being returned to the 

battery are minimal. This observation relates to the statement in section C.3 (d) of 

Regulatory Guide 1.129, Revision 2 that recommends a stable float current reading be 

achieved for three hours before the battery is considered to be near full charge. Testing to 

confirm that this approach is valid is discussed further in Section 4.6, “Return-to-Service”

Tests.

When the float current reached 2 

amps, 102% of the discharged 

ampere-hours had been restored.

When the float current reached 2 amps, 107% of 

the discharged ampere-hours had been restored
50

3. Recharging at an equalizing voltage rather than a float level voltage results in returning 

ampere hours to the battery more quickly, but does not consistently improve the time to 

reach a stable float current (see Table 4-1). The literature suggests that the condition of the 

battery over its life could be detrimentally affected by recharging for extended times at an 

equalizing voltage. We did not observe such effects during this testing program under 

controlled equalizing voltages (not exceeding 40 hours). 

Table 4-1 summarizes the amount of ampere-hours returned to the battery when the float 

current reached a level of two amps. A two amps float current generally represents the point 

where the amount of ampere-hours being returned to the battery is minimal.

Table 4-1 Percent ampere-hours returned and recharge time

 at float current of two amps

Battery 

Type 

Cycle 

1

Cycle 

2

Cycle 

3

Cycle 

4

Cycle 

5

Cycle 

6

Cycle 

7

Cycle 

8

Cycle 

9

Cycle 

10

Enersys 101%

13.0 h

102%

15.9 h

102%

18.9 h

103%

19.9 h

103%

20.6 h

104%

22.2 h

104%

26.0 h

104%

26.3 h

103%

26.8 h

104%

28.6 h

GNB 100%

8.5 h

102%

8.8 h

103%

14.8 h

101%

15.8 h

101%

16.6 h *

101%

19.0 h

102%

19.7 h

101%

19.7 h

101%

19.8 h

C&D 102%

20.6 h

102%

21.3 h

105%

24.0 h

105%

24.0 h

105%

25.5 h

103%

28.6 h

103%

30.2 h

103%

31.3 h

107%

40.0 h

107%

38.7 h

*data unavailable due to power interruption

Note: shaded cycles indicate that recharge was conducted at 28.0 volts; all others at 27.0 volts

While each battery type exhibits a slightly different charge/float current, their fundamental 

performance overall is similar. However, the differences between battery types need to be 

reconciled at the plant level based on the model of the battery used, its age, and its 

manufacturer. The type of float current testing that was performed in the laboratory can be 

conducted in a power plant environment. The use of a calibrated shunt and standard data 

acquisition equipment would make this possible.

4.2. Specific Gravity as a Measure of State-of-Charge

As discussed in Section 3, midpoint specific gravity responds very similarly to charging/float 

current during battery recharge. That is, it reaches a stable value in the same manner as the 

charging/float current. Figures 4-6 to 4-8 illustrate this relationship for each battery type.
51

Figure 4-6 Enersys cycle 2 specific gravity vs. float current

Figure 4-7 GNB cycle 4 specific gravity vs. float current
52

Figure 4-8 C&D cycle 8 specific gravity vs. float current

The analyses of these responses indicate that both specific gravity and float current are equally 

responsive to the recharge of the battery. Float current has a slight advantage in that it provides 

the state of charge of the entire battery string, while specific gravity must be measured for a 

number of cells to conclude that the battery string has achieved the necessary state-of-charge. 

When using just the midpoint readings, the specific gravity must be taken at the correct depth 

within the cell. Measuring the specific gravity too far down into the cell will yield an artificially 

high value that will not be representative of the vendor’s specifications for full charge (typically 

1.215). This would give a false indication of a fully charged battery.

4.3. Analysis of Battery Capacity Trends and Capacity Recovery

(Test Cycle 11)

A decreasing trend of the capacity values for the 10 test cycles conducted on each of the 

battery strings was previously portrayed in Figures 3-11, 3-12, and 3-13. Note that in all cases, 

the battery achieved greater than 100% capacity performance in the first cycle, with the capacity 

decreasing by 5-10% by the 10th cycle. While the cycling of the battery was not intended to age 

it, the 10 test cycles represents the approximate number of performance tests that would be 

experienced by a typical battery over its 20-year life in a nuclear power plant.

Initially, the change in capacity over time was assumed to be due to battery depletion as it was 

exercised during the deep cycle testing. However, it was also noticed that the amount of 

electrolyte stratification increased over the 10 cycles. The question was raised whether this 

increase in stratification could be the cause of the capacity decrease, mainly due to the low 

specific gravity measured at the top of the cell. The following paragraphs provide analysis of

the change in specific gravity and the corresponding change in capacity that occurred over the 

first 10 cycles of testing, and describes the results of the performance test that was performed 

(cycle 11) for each battery string several months after the electrolyte had reached an equilibrium 

condition.
53

For the Enersys battery, the capacity changed from 101.7% to 94.3%, a decrease of 7.4% over 

the 10 cycles. As shown in Figure 4-9, the specific gravity prior to the start of the cycle for the 

midpoint reading changed from 1.215 in cycle 1 to 1.185 at the beginning of cycle 10, a 2.5% 

change, while the top reading changed from 1.217 prior to the beginning of cycle 1 to 1.113 

prior to the beginning of cycle 10, an 8.5% decrease. The bottom reading changed from 1.218 

to 1.31, an increase of 9.2%. 

To help determine the impact of the electrolyte stratification on battery capacity, the NRC project 

manager approved the running of an eleventh four hour performance test after this battery had 

been on a float charge for 10 months. The results were that the capacity recorded was 104.7%,

even higher than the when the battery string was received. The capacity for the Enersys battery 

over the 11 test cycles is illustrated in Figure 4-10.

Figure 4-9 Specific gravity prior to the start of each test cycle - Enersys battery
54

Figure 4-10 Enersys capacity changes including cycle 11

For the GNB battery, the capacity changed from 102.4% to 97.1%, a decrease of 5.3% over the 

10 cycles. As shown in Figure 4-11, the specific gravity at the start of the cycle for the midpoint 

reading changed from 1.209 in cycle 1 to 1.206 at the beginning of cycle 10, virtually no change, 

while the top reading changed from 1.217 at the beginning of cycle 1 to 1.126 at the beginning 

of cycle 10, a 7.5% decrease. The bottom reading changed from 1.212 to 1.283, an increase of 

5.9%. The smaller change in capacity as compared to the change in the top specific gravity 

readings again warranted conducting another capacity test once the specific gravity had 

reached equilibrium.

Cycle 11 of the test program was conducted on the GNB battery approximately eight months 

following cycle 10. During this time, the battery was maintained on a 27.0 volt float voltage 

(2.25 volts per cell). Specific gravity reached equilibrium as determined by the weekly profile 

readings taken on cells 9 and 11. The result of the capacity test was that 101.8% capacity was 

obtained, nearly the same as when the battery string was fresh. The 11 cycles of testing are 

depicted in Figure 4-12.

Figure 4-11 Specific gravity at the start of each test cycle - GNB
55

Figure 4-12 GNB capacity changes including cycle 11 
56

For the C&D battery, the capacity changed from 106.5% in cycle 1 to 96.6% in cycle 10, a 

decrease of 9.9%. As shown in Figure 4-13, the specific gravity at the start of the cycle for the 

midpoint reading changed from 1.218 in cycle 1 to 1.22 at the beginning of cycle 10, virtually no 

change, while the top reading changed from 1.209 at the beginning of cycle 1 to 1.126 at the 

beginning of cycle 10, a 6.8% decrease. The bottom reading changed from 1.212 to 1.283, an 

increase of 5.8%. These smaller changes in the profile readings and, therefore, lesser 

stratification, should not result in a nearly 10% change in capacity. 

Figure 4-13 Specific gravity at the start of each cycle - C&D

Cycle 11 testing was performed approximately 5.5 months after the completion of cycle 10. The 

electrolyte stratification observed at the time of cycle 10 had slowly over time disappeared while 

the battery was maintained on a 27.0 volt float voltage (2.25 volts per cell). With the electrolyte 

at equilibrium, the capacity returned to 102.3% as indicated by Figure 4-14. 

Figure 4-14 C&D capacity changes including cycle 11
57

The above data suggest that electrolyte stratification has an impact on battery capacity. This is 

probably true to some extent, but other factors must be considered as well. The first is that the 

scheduling of weekly capacity tests on the battery string did not allow the full recovery of battery 

capacity, a phenomenon sometimes called “capacity walkdown.” The second is that by 

maintaining the battery on float for several months following the deep-cycle testing, we were 

slowly but steadily putting ampere-hours back into the cells while at the same time allowing the 

electrolyte to equilibrate. 

We note that one of the battery vendors (C&D) states in their operating manual (RS-1476, 

Section 4) that typically a standby battery like the one we tested “will not be subjected to more 

than one test discharge each year” and “Additional deep and/or frequent discharges can shorten 

service life, even with proper maintenance and operation”. It is therefore unlikely that nuclear 

power plants will experience as significant a degree of electrolyte stratification as we did as a

result of the ten consecutive weeks of discharge tests. However, even after a single discharge 

test, nuclear power plants will experience some electrolyte stratification, and must therefore be 

cognizant of how specific gravity measurements are taken to ensure a representative value is 

obtained.

4.4. Use of Conductance for Monitoring Battery Condition

There is a trend of a slight decrease in the average fully-charged conductance for the Enersys 

GN-23 battery and the C&D LCR-33 battery over the course of the 10 test cycles, but it is only 

minimal. This corresponds to a similar indication of a trend showing slightly decreasing battery 

capacity as indicated by the last column in each table: battery capacity as measured by the 

Alber software. The GNB-Exide NCN-21 battery actually showed a trend to slightly increase its 

average fully-charged conductance measurement over the 10 test cycles. Battery capacity for 

the GNB-Exide NCN-21 as measured by the Alber software indicated a slightly decreasing 

battery capacity over the 10 cycles.

Figures 4-15 to 4-17 illustrate the conductance trends for sample cells in each of the three 

battery types. The Enersys cell conductance appears to follow a similar trend to the measured 

battery capacity. Conductance would be a useful measure if in fact this correlation was 

consistent and true for all battery types. The samples chosen from the GNB and C&D battery 

strings do not show a pronounced change over the 10 cycles of performance testing. While the 

absolute values of conductance vary for the GNB and C&D batteries, they both show quite a 

large variation among the cells (~1000 mhos); however, the cells operated uniformly by all other 

measures. We, therefore, cannot draw any conclusions about the overall usefulness of 

conductance from these series of tests.
58

Figure 4-15 Sample cell conductance pre-test measurements - Enersys 

Figure 4-16 Sample cell conductance pre-test measurements - GNB

Figure 4-17 Sample cell conductance pre-test measurements - C&D
59

While these data might be useful in determining if a particular cell is experiencing degradation, 

we found it less useful for ascertaining the overall state-of-charge of the battery string. As 

stated in Section 3, the conductance meter we used also had the ability to measure connection 

resistance which we found useful as part of our battery operability checks. Annual checks of 

connection resistance (cell-to-cell and terminal) are recommended by IEEE Std. 450-2002, 

Section 5.2.3.

4.5. Analysis of Temperature Measurements

IEEE Std. 450-2002 devotes a fair amount of discussion to the impact of temperature on battery 

performance, including the following:

• Monthly checks of ambient temperature; quarterly checks of the temperature of 10% of the 

battery cells.

• For selecting the discharge rate for a capacity test, the Standard states that for the time 

adjusted method, the temperature correction should be applied to the capacity calculations. 

Table 1 in the Standard provides the recommended temperature correction factor.

• In Annex B.2, the Standard states that the specific gravity values are based on a 

temperature of 77°F, and should be corrected for the actual electrolyte temperature.

• Annex C.4 states that the internal resistance decreases and the electrochemical reaction 

rates increase as the temperature of the electrolyte increases. This section of the Standard 

refers to Annex D.3 which states that large cell temperature deviations are sometimes 

caused by shorting conditions, which are also evident by abnormal cell voltage and/or

increasing float current. This same section indicates that operation of the battery at elevated 

temperatures will reduce life expectancy but will not adversely affect battery capacity. 

Annex H describes this effect in more detail.

Figure 4-18 is provided as an example of the temperature response observed during the testing 

as measured by surface-mounted thermocouples. In this example the temperature change in 

some cells ranges from about 68 ̊F to 77 ̊F during the course of the 4-hour performance test. 

The load to be applied for the performance test is temperature compensated based only on the 

initial temperature of the electrolyte. It is not practical to change the temperature compensation 

dynamically during the conduct of the test, however, the cell temperature response is worthy of 

note if only to demonstrate that during high load conditions like a 4-hour performance test, a 

substantive increase in electrolyte temperature should be expected.

Figure 4-18 Analysis of temperature impact on capacity tests (C&D Cycle 1)
60

4.6. “Return-to-service” Tests (Test Cycles 12 to 14)

The purpose of this testing was to evaluate the ability of a battery to meet its capacity and 

capability requirements when it has met the “return-to-service” criteria. The “return-to-service”

terminology is common in nuclear power plant technical specifications when referring to 

equipment operability, but it is also inferred in IEEE Std. 450-2002 (Annex A.2) in the discussion 

associated with determining the fully charged condition of a battery. In this latter context, IEEE 

Std. 450-2002 states that “When the charging current has stabilized at the charging voltage for 

three consecutive hourly measurements, the battery is near full charge.” The nuclear industry 

and the U.S. NRC, through Technical Specification Task Force (TSTF) - 500, have been 

working to revise the technical specifications to reflect the use of float current as a means of 

determining battery operability. It is expected that the results of this testing can be used to help 

demonstrate the point at which a battery can be returned to service and be expected to fulfill its 

safety function. 

In this testing program, the first criterion used to determine the “return-to-service” time was 

when a stabilized recharge float current was achieved for three-hours. When the battery had

been recharged to that point, a second performance test was conducted to determine that the 

battery could meet its design function (>80% capacity). Following this second performance test, 

the battery string was recharged for at least 72 hours before the test was repeated. 

The second criterion used to determine the “return-to-service” time was when the float current

reached a value equivalent to three time constants on the exponential shaped recharge/float 

current curve. For our battery strings, this occurred when the float current reached 9 amps, as 

compared to between 0.5 and 2 amps for achieving a stable float current. At that point, the 

recharge was terminated and a second performance test was conducted. In both cases, the 

battery strings were successful in meeting their design function as described in this section.

The utility of the recharge time constant concept in evaluating the state-of-charge and the 

“return-to-service” limits for a battery is evident from the testing performed. Each battery type 

has its own unique characteristics and the limits for evaluating the state-of-charge and the 

“return-to-service” limits for a battery would have to be established on a case-by-case basis that 

includes knowing the battery charger recharge parameters such as its current limit setting and 

its recharge voltage value. However, as discussed later in this section, the recharge time 

constant concept provides an opportunity to reconsider the protocols whereby decisions are 

made concerning the operability status of batteries when returning them to service. A detailed 

explanation of the exponential time constant and its derivation is contained in Appendix A.

4.6.1. Enersys “Return-to-service” Tests

“Return-to-service” Testing of the Enersys Model 2GN-23 Batteries was conducted in 

accordance with an approved test plan. A four-hour performance test (discharge test) was 

conducted followed by a recharge at a float voltage of 27.0 volts. When the battery reached the 

point where a stable float current was achieved for three hours, a second four hour performance 

test was conducted. Following completion of the second performance test, the battery was 

again recharged at a float voltage of 27.0 volts for five days. The process was then repeated a 

second time. In both tests, the battery was able to meet the performance requirements (>80% 

of rated capacity). The results of the two “return-to-service” tests are summarized below. 
61

Enersys Cycle 12: The initial performance test for cycle 12 resulted in a capacity of 101.0%. 

The recharge of the battery was initiated and reached a float current of 1.1 amps where it 

remained stable (within 0.5 amps) for the next three hours. At this time the charger output was 

secured and a series of specific gravity measurements were taken. The second performance 

test was initiated approximately 24 hours from the start of the recharge. The results from this 

second capacity test revealed a battery capacity of 98.6%. Following a series of specific gravity 

measurements, the battery was placed on a 27.0 volt float charge. Figure 4-19 illustrates the 

float current response at the point of current stabilization for cycle 12. 

Enersys Cycle 13: The initial performance test for cycle 13 resulted in a capacity of 96.9%. 

The recharge of the battery was initiated and reached a float current 1.0 amp where it remained 

stable (within 0.5 amps) for the next three hours. At this time the charger output breaker was 

opened and a series of specific gravity measurements were taken. The second performance 

test was initiated approximately 26 hours from the start of the recharge. The results from this 

second capacity test revealed a battery capacity of 94.8%. Following a series of specific gravity 

measurements, the battery was placed on a 27.0 volt float charge. Figure 4-20 illustrates the 

float current response at the point of current stabilization for cycle 13.

Analysis of Enersys “Return-to-service” Testing – Stable float current: When the Enersys 

battery was returned to service following a four-hour performance test, it was successful in two 

tests (Enersys cycles 12 and 13) in demonstrating that it could meet its performance 

requirements when recharged to the point where the float current was stable for three hours. 

For stable current, we used the criteria that the float current remained the same (within 0.5 

amps) for three consecutive hours. In both cases this occurred when the float current was 

about 1.0 amp.

Figure 4-19 Enersys cycle 12 stable float current monitoring
62

Figure 4-20 Enersys cycle 13 stable float current monitoring

Enersys Cycle 14: An initial performance test for cycle 14 was conducted as in cycles 12 and 

13. The recharge of the battery was initiated following the completion of a set of specific gravity 

readings. The battery reached a float current of 9 amps (three time constants) at approximately 

12.5 hours from the start of the recharge (see Figure 4-21). At this time the charger output was 

secured and a series of specific gravity measurements were taken. The second performance 

test achieved a battery capacity of 93.3%. Following a series of specific gravity measurements, 

the battery was placed on a 27.0 volt float charge.

Analysis of Enersys “Return-to-service” Testing - Three Time Constants: During the first 

10 cycles of testing, the calculation of the ampere-hours returned to the battery during recharge 

indicated that greater than 90% of the ampere hours were returned by the time the float current 

reached three time constants (see Figure 4-22). Note that the recharge was conducted at a 

float voltage of 27.0 volts (2.25 volts per cell) for all 10 cycles. In Enersys Cycle 14, the four 

hour performance test conducted at the point where the three time constants had been obtained 

confirmed these results through the use of a four hour performance test in which the battery 

achieved a 93.3% capacity, indicative of the substantial number of ampere-hours returned to the 

battery at that point. 

Figure 4-21 Enersys cycle 14 - float current to three time constants
63

Figure 4-22 Enersys performance during first 10 cycles – ampere hours returned

4.6.2. GNB “Return-to-service” Tests

“Return-to-service” Testing of the GNB Model NCN-21Batteries was conducted in accordance 

with the approved test plan. A four hour performance test (discharge test) was conducted 

followed by a recharge at a float voltage of 27.0 volts. When the battery reached the point 

where a stable float current was achieved for three hours, a second four hour performance test 

was conducted. Following completion of the second performance test, the battery was again 

recharged at a float voltage of 27.0 volts for five days. The process was then repeated a 

second time. In both tests, the battery was able to meet the performance requirements (>80% 

of rated capacity). The results of the two “return-to-service” tests are summarized below. 

As with the Enersys battery testing, the Alber Battery Capacity Test Set was used to control and 

monitor the four hour performance tests. It provided data on the discharge current, individual 

cell voltages, and a real time calculation of battery capacity. The Omega data acquisition 

system was used to monitor cell temperatures over the entire cycle and charging/float current 

during the recharge phases of the tests. The Polytronics float current monitor was also used to 

provide continuous readings of the charging/float current from the battery charger during the 

recharge cycles. Manual readings of specific gravity were taken periodically during the 

discharge-recharge cycles. 

GNB Cycle 12: The initial performance test for cycle 12 achieved a capacity of 97.0%. The 

recharge of the battery was initiated and reached a stable float current below 1.0 amps where it 

remained stable (within 0.5 amps) for the next three hours. At this time the charger output was 

secured and a series of specific gravity measurements were taken. The second performance 

test was initiated approximately 22 hours from the start of the recharge. The results from this 

second capacity test revealed a battery capacity of 95.1%. Following a series of specific gravity 

measurements, the battery was placed on a 27.0 volt float charge for the remainder of the week. 

Figure 4-23 illustrates the float current response at the point of stabilization.
64

Figure 4-23 GNB Cycle 12 stable float current monitoring

GNB Cycle 13: The initial performance test for cycle 13 achieved a capacity of 95.0%. The 

recharge of the battery was initiated and reached a float current of about 1.0 amp where it 

remained stable (within 0.5 amps) for the next three hours. At this time the charger output 

breaker was opened and a series of specific gravity measurements were taken. The second 

performance test was initiated approximately 20 hours from the start of the recharge. The 

results from this second capacity test revealed a battery capacity of 94.4%. Following a series of 

specific gravity measurements, the battery was placed on a 27.0 volt float charge. Figure 4-24

illustrates the float current curve for GNB cycle 13 used to determine the “return-to-service” 

point.

Analysis of GNB “return-to-service” testing-stable float current: When the GNB battery 

was returned to service following a four hour performance test. It was successful in 

demonstrating that it could meet its performance requirements when recharged to the point 

where the float current was stable for three hours (GNB cycles 12 and 13). For stable current, 

we used the criteria that the float current remained the same (within 0.5 amps) for three 

consecutive hours. In both cases this occurred when the float current was below 1.0 amp. The 

“Return-to Service” battery capacity achieved in both tests was approximately the same (95.1% 

in cycle 12 and 94.4% in cycle 13. 

Figure 4-24 GNB cycle 13 stable float current for three hours
65

GNB Cycle 14: “Return-to-service” Testing of the GNB Model NCN-21 battery using a three

Time Constant criterion was conducted in accordance with the approved test procedure. The 

purpose of this test was to confirm that the battery could meet its performance requirements 

when the float current had reached a point on the exponential float current curve equivalent to 

three time constants. The float current value at that point was calculated to be 9 amps. To 

accomplish this, the battery was discharged to a point where the overall string voltage was 21.0 

volts. The battery was then recharged at a float voltage of 27.0 volts. When the battery 

reached the point where the float current was 9 amps, the battery charger was secured and a 

four hour battery performance test was conducted. 

The initial performance test for cycle 14 resulted in a capacity of 94.1%. The recharge of the 

battery was initiated following the completion of a set of specific gravity readings. The battery 

reached a float current of 9 amps at approximately 12 hours from the start of the recharge (see 

Figure 4-25). At this time, the charger output was secured and a series of specific gravity 

measurements were taken. The second performance test revealed a battery capacity of 94.5%. 

Following a series of specific gravity measurements, the battery was placed on a 27.0 volt float 

charge.

Analysis of GNB “Return-to-service” Testing - Three Time Constants: During the first 10 

cycles of testing, the calculation of the ampere-hours returned to the battery during recharge 

indicated that greater than 90% of the ampere hours were returned by the time the float current 

reached three time constants. Figure 4-26 illustrates this response. Note that cycles 1 and 2, 

where the recharge was conducted at an equalizing voltage of 28.0 volts (2.33 volts per cell), 

achieve the 90% level even sooner since the charger stays in a current limit mode longer than 

when recharge is conducted at a float voltage of 27.0 volts (2.25 volts per cell). In Cycle 14, the 

four hour performance test conducted at the point where the 3 time constants had been 

obtained confirmed these results through the use of a performance test which achieved a 94.5% 

battery capacity, indicative of the substantial ampere-hours returned to the battery at that point 

during recharge of the battery. 

Figure 4-25 illustrates the float current curve following the first discharge test. Highlighted on 

the curve are the equivalent points for one, two, and three time constants. The charger was 

secured when the float current reached a value of 9 amps, approximating the three time 

constant point on the exponential curve. It can therefore be concluded that the battery has the 

necessary capacity to carry out its design function by the time it is returned to service at the 

equivalent value of three time constants (9 amps in this case). 

Figure 4-25 GNB cycle 14 float current to three time constants
66

Figure 4-26 GNB float current response during first 10 test cycles

4.6.3. C&D “Return-to-service” Tests

“Return-to-service” Testing of the C&D Model LCR-33 Batteries was conducted in accordance 

with the approved test plan. The purpose of this testing was to confirm that the battery could 

meet its performance requirements when a stable float current was achieved for three hours 

during recharging of the battery. To accomplish this, a four-hour performance test (discharge 

test) was conducted followed by a recharge at a float voltage of 27.0 volts. When the battery 

reached the point where a stable float current was achieved for three hours, a second four hour 

performance test was conducted. Following completion of the second performance test, the 

battery was again recharged at a float voltage of 27.0 volts for five days. The process was then 

repeated a second time (cycle 13). In both tests, the battery was able to meet the performance 

requirements (>80% of rated capacity). The results of the two “return-to-service tests” are 

summarized below. 

C&D Cycle 12: The initial performance test for cycle 12 resulted in a capacity of 98.3%. The 

recharge of the battery was initiated and reached a stable float current below 2.0 amps where it 

remained stable (within 0.5 amps) for the next three hours. At that time, the charger output was 

secured and a series of specific gravity measurements were taken. The second performance 

test was initiated at approximately 26 hours from the start of the recharge. The results from this 

second capacity test revealed a battery capacity of 97.9%. Following a series of specific gravity 

measurements, the battery was placed on a 27.0 volt float charge for the remainder of the week.

C&D Cycle 13: The initial performance test for cycle 13 resulted in a capacity of 96.6%. The 

recharge of the battery was initiated and reached a float current of about 2.0 amps where it 

remained stable (within 0.5 amps) for the next three hours. At this time the charger output 

breaker was opened and a series of specific gravity measurements were taken. The second 

performance test was initiated approximately 29 hours from the start of the recharge. The 

results from this second capacity test revealed a battery capacity of 95.7%. Following a series 

of specific gravity measurements, the battery was placed on a 27.0 volt float charge.

Analysis of C&D “Return-to-service” Testing - Stable Float Current: When the C&D battery 

was returned to service following a four-hour performance test, it was successful in 
67

demonstrating that it could meet its performance requirements when recharged to the point 

where the float current was stable for three hours (C&D cycles 12 and 13). For stable current, 

we used the criterion that the float current remained the same (within 1.0 amp) for three 

consecutive hours. In both cases this occurred when the float current was about 2.0 amps. 

Figures 4-27 and 4-28 are the float current curves for C&D cycles 12 and 13 illustrating the

“return-to-service” points.

Figure 4-27 C&D cycle 12 stable float current monitoring

Figure 4-28 C&D cycle 13 stable float current monitoring

C&D Cycle 14: “Return-to-service” Testing of the C&D Model LCR-33 Batteries was conducted 

in accordance with the approved test procedure. The purpose of this testing was to confirm that 

the battery could meet its performance requirements when the float current had reached a point 

on the float current curve equivalent to three time constants. The float current value at that 
68

point on the exponential curve is 9 amps. To accomplish this, the battery was discharged to a 

point where the overall string voltage was 21.0 volts. The battery was then recharged at a float 

voltage of 27.0 volts. When the battery reached the point where the float current was 9 amps, 

the battery charger was secured and a four hour battery performance test was conducted. 

Following completion of the second discharge, the battery was again recharged at a float 

voltage of 27.0 volts. In this test, the battery was able to meet the performance requirements 

(>80% of rated capacity). The results of the “return-to-service” test at three time constants” are 

summarized below. 

The initial performance test for cycle 14 resulted in a capacity of 97.9%. The recharge of the 

battery was initiated the same day following the completion of a set of specific gravity readings. 

The battery reached a float current of 9 amps approximately 18 hours from the start of the 

recharge (see Figure 4-29). At this time the charger output was secured and a series of specific 

gravity measurements were taken. The second performance test resulted in a battery capacity 

of 96.0%. Following a series of specific gravity measurements, the battery was placed on a 

27.0 volt float charge.

Analysis of C&D “Return-to-service” Testing - Three Time Constants: During the first 10 

cycles of testing, the calculation of the ampere-hours returned to the battery during recharge 

indicated that greater than 90% were returned by the time the float current reached 3 time 

constants. Figure 4-30 illustrates this response. For the C&D battery, recharge was conducted 

at 28.0 volts (2.33 volts per cell) four times while for the remaining six cycles recharge was 

conducted at 27.0 volts (2.25 volts per cell). For the four cycles where recharge occurred at 

2.33 volts per cell, the charger stayed in current limit for a longer time, and the recharge/float 

current curve was steeper (shorter time constant) resulting in a faster restoration of amperehours. In Cycle 14, the four-hour performance test conducted at the point where the three time 

constants had been obtained achieved a 96.0% capacity confirming the observation made in 

Figure 4-30. 

Figure 4-29 illustrates the float current curve following the first discharge test. Highlighted on 

the curve are the equivalent points for one, two, and three time constants. The charger was 

secured when the float current reached a value of 9 amps, approximating the three time 

constant point on the exponential curve. It can, therefore, be concluded that the battery has the 

necessary capacity to carry out its design function when it is returned to service at the 

equivalent value of three time constants (9 amps in this case).

Figure 4-29 C&D Cycle 14 float current response
69

Figure 4-30 C&D float current response during first 10 test cycles

Summary of “Return-to-service” Testing: Table 4-2 summarizes the results of the capacity 

tests performed for cycles 12-14 for each battery string. The results of this testing indicate that 

the battery “return-to-service” criteria using stable float current and a float current equivalent to 

three time constants are both valid means to ensure that a battery can meet its design criteria 

following its full discharge.

Table 4-2: Summary of “return-to-service” tests

As noted previously, once the float current reaches an asymptotic level, the ampere-hours being 

returned to the battery are minimal. This observation relates to the statement in Section C.3 (d) 

of Regulatory Guide 1.129, Revision 2 that recommends a stable float current reading be 

achieved for three hours before the battery is considered to be near full charge. The testing 

performed that is described in Section 4.6 clearly indicates that the battery can meet its 

performance requirements when a stable float current has been achieved for three hours. In 

fact, the testing showed that the battery can meet its performance requirements before stable 

float is achieved (i.e. at the three time constant point on the recharge/float current curve).
70

The data also showed that for the three battery types tested in this program, the process of 

selecting a float current value at which the battery can meet its design objectives can be 

achieved through a process that combines empirical data with the observation that the float 

current response curve following a battery discharge reasonably approximates an exponential 

function. This allows the operator to calculate the point (float current value) at which an 

adequate amount of ampere-hours (e.g., >80% of the design rating) have been returned to the 

battery to ensure that the battery can meet its design function when returned to service.

For the “return-to-service” testing of the three battery types at BNL, the battery charger current 

limit was set at 180 amps and recharge was conducted at a design voltage of 2.25 volts per cell 

or 27.0 volts for the battery string. Recharging at a higher voltage, as was demonstrated in 

several recharge cycles for the GNB and C&D battery strings, alters the shape of the float 

current curve (steeper curve/shorter time constant) but the overall form of the curve can still be 

approximated by an exponential function. At one time constant, the value of the normalized 

recharge current is approximately 0.368 (1/e), at two time constants the normalized recharge 

current is approximately 0.135 (1/e2

), and at three time constants the normalized recharge 

current is approximately 0.050 (1/e3

). Multiplying the current limit value by 0.050 (three time 

constants) should yield a current value on the float current curve that represents the point where 

greater than 80% of the design ampere-hours have been restored to the battery, thereby 

providing adequate assurance that it can meet its design function assuming that the battery has 

been properly sized in accordance with IEEE Standard 485-1987. 

Because the shape of the recharge curve is not exactly exponential, there will be some variation 

among battery strings that warrants establishing a battery/battery charger specific recharge 

current profile to ensure that the approximation described above is reasonable. Table 4.3 

illustrates the recharge/float current that actually existed over the 10 cycles of testing. For the 

Enersys battery, the 9 amp float current value selected using the ideal exponential curve 

approximates the actual values, ranging from 4.6 to 18.4 amps. For the GNB battery, cycles 1 

and 2 were conducted at 28.0 volts so that there is a steeper decline of current with time 

resulting in values of 1.6 and 1.3 amps, respectively at three time constants. Note that for the 

remainder of the cycles, the current at 3 time constants ranged from 6.0 to 12.5 amps, a 

relatively good approximation to the ideal value of 9 amps. Finally, for the C&D battery, the 

range of currents at 3 time constants is from 3.4 amps to 15.2 amps. 

Table 4-3: Actual float current value (Amps) at three time constants

Battery 

Type

Cycle 

1

Cycle 

2

Cycle 

3

Cycle 

4

Cycle 

5

Cycle 

6

Cycle 

7

Cycle 

8

Cycle 

9

Cycle 

10

Enersys 4.6 10.5 14.9 13.7 14.4 15.5 16.6 16.9 16.9 18.4

GNB 1.6* 1.3* 6.0 8.5 10.2 Not 

obtained

12.3 12.5 12.4 12.5

C&D 3.4 7.1 8.2* 7.4* 13.3 13.3 13.9 15.2 12.8* 9.1*

*Recharge conducted at an equalizing voltage of 28.0 volts (2.33 volts per cell)

Using a current shunt similar to the one shown schematically in Figure 2-5 is a convenient and 

repeatable way to monitor float current during all phases of battery and battery charger 

operation. While we chose to have the output connected permanently to a data acquisition 

system, using a calibrated portable digital voltmeter is also acceptable. When performing 

periodic performance tests, one can not only assess the battery capacity but also monitor the 

recharge/float current to verify that the float current response curve continues to conform to the 

general exponential function thereby supporting the use of the three time constant “return-toservice” value. Thus, we conclude that use of the three time constant method is more practical 

because it provides a current value that can be directly measured.
71

5. CONCLUSIONS 

The primary objective of this research project was to determine whether float current monitoring 

is a useful indicator for determining a vented lead-calcium battery’s state-of-charge. A 

secondary objective was to evaluate the criteria for selecting the point when a battery can be 

returned to service and meet its design requirements. In conducting this study, we procured 

three sets of nuclear qualified batteries from three battery vendors (Enersys, Exide/GNB from 

NLI, and C&D Technologies) that manufacture nuclear qualified batteries. Each battery set 

consisted of 12 battery cells. These cells are the same models that are typically used in a Class 

IE dc system application. Two suitably sized battery chargers and a load bank were also 

obtained; the second battery charger was used to maintain the batteries not being tested on a 

continuous float charge.

The test setup was as close as reasonably practicable (not including seismic battery racks) to a 

typical nuclear power station’s Class 1E battery design. Specific gravity measurements were 

taken in accordance with the battery manufacturer recommendations and IEEE Std. 450-2002. 

Once the battery was charged and stabilized in accordance with the recommendations in IEEE 

Std. 450-2002, a series of 4-hour performance discharge tests were performed based on the 

battery vendor’s specifications. After each performance test, the battery was charged and the 

charging/float current continuously recorded while periodic specific gravity measurements were 

taken. The specific gravity measurements were taken at the vendor specified location (termed 

the midpoint) and at the top, midpoint, and bottom of the jar for two of the cells in order to obtain 

a profile of the electrolyte distribution within the cell. Discharge test current and specific gravity 

readings were compensated for temperature as discussed in IEEE Std. 450-2002.

From the 30 cycles of testing used to compare specific gravity and float current, we conclude 

the following with regard to the primary objective of this test program:

1) Both float current and specific gravity provide adequate means to determine battery state-ofcharge. Float current has an advantage in that it provides an indicator of the entire battery 

string, while specific gravity is measured on a cell by cell basis.

2) Both float current and specific gravity have similar response times when the battery is 

recharged. Generally speaking, 100% of the ampere-hours discharged are returned to the 

battery within 24 hours of the start of the recharge cycle. Float current response will vary 

based on the recharge voltage applied to the battery. However, regardless of the voltage 

applied during recharge, the float current of a nearly fully charged battery becomes stable at 

less than two amps. 

3) The use of pilot cells to ascertain specific gravity is supported by the consistent response 

observed among all cells during both discharge and recharge.

4) The amount of electrolyte stratification is significant following a performance test and it can 

take several months before equilibrium is reached again within the cells. Therefore, it is 

critical to measure specific gravity at the correct distance from the top of the cell as indicated 

by the battery vendor’s manual and supported by IEEE Std. 450-2002. The electrolyte 

stratification, by itself, does not appear to impact the ability of the battery to meet its capacity 

and capability requirements.

5) Measuring float current through the use of a simple shunt connected to a data acquisition 

system provides continuous, accurate and repeatable measurements. A more sophisticated 

device based on the principles of the Hall Effect (similar to a clamp-on ammeter) was also 

effective but was less accurate at the low ends of the float current range (< two amps).
72

6) Recharging at an equalizing voltage rather than a float level voltage results in returning 

ampere hours to the battery more quickly, but does not consistently improve the time to 

reach a stable float current (see Table 4-1). The literature suggests that the conditioning of 

the battery over its life could be detrimentally affected by recharging for extended times at 

an equalizing voltage. However, no deleterious effects were observed during this testing 

program under controlled equalizing voltages (not exceeding 40 hours). 

Other observations related to the testing of lead calcium batteries used in nuclear power plant 

applications resulting from this test program are:

Temperature compensation for capacity testing, specific gravity readings, and conductance 

readings is important and has a measureable impact on the data, 

The location of the sampling point (distance from the top of the cell) for the specific gravity 

measurement is critical due to the significant amount of stratification that occurs following a 

performance test. Using a standard length of tubing to draw the electrolyte from the same point 

resulted in consistent “trendable” data, and

Calculations of the ampere-hours returned to the battery during recharge can be used to verify 

the battery’s state-of-charge. The majority (>60%) of the ampere-hours were returned to the 

battery while the battery charger was in a current limit mode. 

IEEE Std. 450-2002 contains the following criterion related to return to service for a battery: 

“When the charging current has stabilized at the charging voltage for three consecutive hourly 

measurements, the battery is near full charge.” Our test program also verified the point where 

the battery can be safely returned to service. In a series of six additional tests (two tests per 

battery string), the battery strings were able to meet their capacity and capability requirements 

at the point where the float current was stable for three hours. Thus the criterion used in IEEE 

Std. 450-2002 was found to be an acceptable practice for ensuring the capacity and capability 

requirements of the battery were met before returning it to service.

Similarly, three cycles of tests were performed in which each battery was returned to service 

when the float current reached the value equivalent to three time constants on the recharge/float 

current curve. This occurred within about twelve hours and at a higher current than the 

previously described return to service tests. In each case, the battery was also able to meet its 

capacity and capability requirements. This calculated float current value obtained from the 

battery-specific recharge/float current curve may be a more practical method for returning the 

battery to service at the point where it is capable of meeting its capacity and capability 

requirements.
73

6. REFERENCES

Alber BCT-2000 Software and BCT-128 User’s Guide, 4200-003R4.1.1, 

Alber, G. and Nispel, M., Thermal Runaway in Flooded Lead Calcium Batteries, Tech Note, 

Alber Corp. 

Anderson, James W., Testing of Large Lead Stationary Batteries, IEEE Transaction on Energy 

Conversion, Vol. EC-1, No. 3, September 1986.

C&D Technologies, Inc.; Standby Battery Vented Cell Installation and Operating Instructions, 

RS-1476, Section 12-800, August 2008.

Clark, Steve, State-of-Charge: Specific Gravity versus Battery Charging Current, Battcon 2010 

Conference Paper.

Determining [conductance] Reference Values,” Midtronics, Inc. Website: 

http://www.midtronics.com/stationary-reference-values-technology/stationary-referencevalue-development

DOE Standard 3003-2000, Backup Power Sources for DOE Facilities, January 2000.

Enersys Publication No. US-FL-IOM-002, Safety, Storage, Installation, Operation & 

Maintenance Manual, January 2007.

Exide Technologies- GNB Flooded Classic; Installation and Operating Instructions, Section 

93.10, March 2010.

Feder, D.O., Croda, T.G., Champlin, K.S., McShane, S.J., and Hlavac, M.J., “Conductance 

Testing Compared to Traditional Methods of Evaluating the Capacity of Valve-Regulated 

Lead-Acid Batteries and Predicting State-of-Health,” paper presented at the International 

Lead Zinc Research Organization (ILZRO) Meeting, Nice, France, May 15, 1992.

Feder, D.O., Hlavac, M.J., and Koster, W., “Evaluating the State-of-Health of Lead Acid Flooded 

and Valve-Regulated Batteries: A Comparison of Conductance Testing vs. Traditional 

Methods,” paper presented at the LABAT ’93 International Conference on Lead-Acid 

Batteries, Varna, Bulgaria, June 7-11, 1993.

Feder, D.O., Hlavac, M.J., and McShane, S.J., “Updated Status of Conductance/Capacity 

Correlation Studies to Determine the State-of-Health of Automotive SLI and Stand-By Lead 

Acid Batteries,” paper presented at the Zinc and Lead Asian Service (ZALAS) 5th Asian 

Battery conference 1993, Bali Indonesia, September 1-3, 1993.

Floyd, Kyle et al, Assessment of Lead-Acid Battery State-of-Charge by Monitoring Float 

Charging Current, IEEE Explore, 1994.

IEEE Stds. 450-2002 and 2010, IEEE Recommended Practice for Maintenance, Testing, and 

Replacement of Vented Lead-Acid Batteries for Stationary Applications.

Jones, B, “Conductance Monitoring of Recombination Lead Acid Batteries,” paper presented at 

the eleventh International Lead Conference, Venice Italy, May24-27, 1993.

Kniveton, M.W., “Field Experience of Testing VRLA Batteries by Measuring Conductance, paper 

presented at the 8th Battery Conference and Exhibition, Solihull, Westmidlands, UK, May 11, 

1994.
74

McShane, S.J., and Hlavac, M.J., “Conductance Testing of Standby Batteries in Signaling and 

Communications Applications for the Purpose of Evaluating Battery State-of-Health,” paper 

presented at the Association of American Railroads’ 1993 Eastern Regional Meeting, Lake 

Buena Vista, Florida, April 29, 1993.

Midtronics Celltron universal stationary battery analyzer, Instruction Manual, 168-641F, April 

2006.

Omega Data Acquisition System User’s Guide, OMB-DAQ-54/55/56 Data Acquisition Modules, 

OMB-491-0901, rev 6.1.

Polytronics Engineering Ltd. BtmGlobal System Controller User Manual, Version 3.1.6, 2006.

Regulatory Guide 1.129, Maintenance, Testing, and Replacement of Vented Lead-Acid Storage 

Batteries for Nuclear Power Plants, Rev. 2, 2007.

SBS-2500 Digital Hydrometer, Operating Instructions, November, 2009.

Soileau, Robert D., A Diagnostic Testing Program for Large Lead Acid Storage Battery Banks, 

IEEE Transactions on Industry Applications, Vol. 30, N0. 1, January/February 1994.

Stationary Battery Guide: Design, Application, and Maintenance, EPRI TR- 100248, Rev. 2, 

August 2002.

Thaller, L. and Zimmerman A., Understanding and managing capacity walkdown in nickelhydrogen cells and batteries, Energy Conversion Engineering Conference, 2002. 
A-1

APPENDIX A. THE EXPONENTIAL TIME CONSTANT DURING THE 

RECHARGE OF A VENTED LEAD-ACID BATTERY

A.1. DISCUSSION

The state-of-charge of a vented lead-acid storage battery has traditionally been monitored by 

conducting labor-intensive measurements of the specific gravity of the electrolyte in specified 

pilot cells, nominally sulfuric acid with a specific gravity of 1.215. However, measurements of 

the specific gravity of pilot cells during deep discharge and subsequent recharge performance 

testing have demonstrated that the specific gravity distribution in a battery cell can remain 

stratified for a long time following the recharge to the fully-charged condition. Therefore, 

considerable care must be taken in performing specific gravity measurements to ensure that 

they are always conducted at the same depth into the electrolyte of the pilot cells to ensure 

consistency between the measurements.

To alleviate the difficulties associated with manual measurements of the specific gravity of 

selected pilot cells, the IEEE has recently proposed that a battery can be declared fully-charged 

when the float current during recharge has become stable (not changing with time) for three 

consecutive hours in recognition of the electrolyte stratification condition, especially following a 

recharge, an equalizing charge, or addition of water to the cells. The IEEE Standard 450-2002 

(p. 7) states that the following may be used as indicators of return to a fully-charged condition 

following a discharge:

• Stabilized charging current when measured at the manufacturer’s recommended voltage 

and temperature,

• Assurance that the amp-hours returned to the battery are greater than the amp-hours 

removed plus the charging losses.

Furthermore, IEEE Standard 450-2002 (p. 9) states that, if following a modified performance 

test the battery delivers a tested capacity of 80% or more (assuming that the battery has been 

sized in accordance with IEEE Standard 485), the battery is acceptable for service.

IEEE Standard 450-2002 (p. 17) states that “the pattern of charging current delivered by a 

conventional voltage-regulated charger after a discharge is the most accurate method for 

determining the state of charge” and, by inference, the most reliable method for determining 

when the battery can be returned to service.

As a battery is being recharged following a deep discharge, the recharge current will initially be 

limited to a maximum value set by the charger which we will refer to as the current limit of the 

charger. As the cells approach full charge, the battery demand will begin to fall below the 

current limit setting of the charger and will continue to decrease as the voltage of the battery 

approaches the charger output voltage. The charging current will continue to decrease as the 

battery capacity (amp-hours) is restored, eventually reaching an asymptotic lower limit at which 

it will stabilize; at this point, the discharged amp-hours will have been fully restored and the 

stabilized float current will continue at a very low current to replace the charging losses. A 

typical example of the current profile vs. time during a recharge cycle following a deepdischarge performance test for a typical battery tested in this program is shown in Figure A-1.
A-2

Figure A-1 ENERSYS cycle 14 - float current out to three time constants

In Figure A-1, the time that the battery recharge is in current limit is shown to persist for 

approximately five hours at 180 amps, after which time the recharge current demand by the 

battery decreases as the discharged capacity (amp-hours) is restored by the charger. Although 

not depicted in this Figure (refer to Figure 3-17 for an example), the current would eventually 

asymptotically approach a lower, stable float current that would usually be in the range less than 

two amps. The region between the upper current limit and the lower stable float current limit is 

the region in which the decay of the recharge current vs. time closely resembles a negative 

exponential function of time. It is in this range that the characteristic exponential time constant 

for the battery recharge is defined.

The concept of the exponential time constant for the recharge current vs. time in the region 

between the current limit and the stable float current is a useful metric in considering how long it 

is necessary to recharge a battery before the battery can be returned to service, even before 

achieving the fully-charged status as indicated by the three-hour stable float current criterion 

espoused in IEEE Standard 450-2002 (p. 17).

The first step in calculating the time constant for the exponentially decreasing recharge current 

is to cull the recharge current data of all data for the current limit period and the stable float 

current period, so that the remaining data are on the exponentially decreasing portion of the 

recharge curve. The time axis for the data should be shifted so that the time of the first data 

point after current limit is set to zero, and the current data should all be normalized by the value 

of the first data point so that the normalized current data are bounded between unity and zero. 

The results for this procedure are shown below in Table A-1 for the exponentially decreasing 

recharge current data for GNB Cycle 8 for some of the data. The data in columns 1 and 2 are 

the time since start of recharge (which includes the time on current limit) and the measured 

recharge current in amps. The data in columns 3 and 4 are the time shifted to zero for the first 

data point after current limit and the recharge current normalized by the value of the first current 

data point after current limit.
A-3

Table A-1 Recharge data for the calculation of the exponential time constant.

Time Current Shifted time Normalized 

current

(-) (hr) (A) (hr)

4.17 177.4 0.00 1.00

4.18 176.8 0.02 1.00

4.20 175.2 0.03 0.99

4.22 173.8 0.05 0.98

4.23 175.1 0.07 0.97

4.25 170.6 0.08 0.96

4.27 169.9 0.10 0.96

4.28 168.4 0.12 0.95

4.30 167.7 0.13 0.95

4.32 167.8 0.15 0.95

4.33 166.2 0.17 0.94

4.35 164.0 0.18 0.92

4.37 163.3 0.20 0.92

4.38 162.1 0.22 0.91

4.40 162.0 0.23 0.91

4.42 160.1 0.25 0.90

4.43 159.6 0.27 0.90

4.45 158.2 0.28 0.89

4.47 156.8 0.30 0.88

4.48 154.6 0.32 0.87

In this table, only the first 20 data points are listed. In practice, one would use many more data

points as shown in Figure A-2 below where the graph includes 200 data points for the 

exponentially decreasing recharge current for GNB cycle 8.

Assuming that the functional form of the normalized recharge current vs. time data is given by 

I/I0 = K*exp(-bt) where K and b are constants, a regression analysis is performed which yields 

the equation shown in the figure and depicted by the solid line through the data. The constant K 

is not equal to 1.00 because the data are not perfectly exponential over the range chosen for 

the curve fit. When the time, t, is equal to one time constant, the value of I/I0 would be equal to 

1/e = 0.36788. Substituting 0.36788 into the equation in Figure A-2 for I/I0 and solving the 

equation for t yields the result for this example that tC = 2.30 hr (see Table 3-5). There are other 

mathematical approaches to solving for the exponential time constant; however, this example is 

sufficient to illustrate the underlying rationale.
A-4

Figure A-2 Normalized recharge current vs. time during the exponentially decreasing

recharge current period for GNB cycle 8

An alternative method to solve for the time constant is to realize that the recharge current would 

have decreased to approximately 0.36788 x 180 amps (the current limit) = 66.2 amps at one 

time constant. Searching the recorded data for the time after the end of current limit for the 

recharge current to decrease to 66.2 amps would have yielded a result for the time constant of 

2.27 hr, in agreement with the more rigorous approach followed in Table A-1 and Figure A-2.

The utility of the recharge current time constant can be illustrated by consideration of the 

percentage of the battery capacity that is recharged following a deep-discharge performance 

test for the three batteries that were tested during this program as shown below in Table A-2. 

The data for each battery are listed according to test cycle; for each cycle for each battery, the 

amp-hours discharged during the performance test are listed, followed by the time on current 

limit during recharge, and the time constant for the exponentially decreasing period of recharge 

for each cycle for each battery. From Table A-2, it is evident that for all the tests on each of the 

three batteries tested at least 60% of the battery’s capacity was restored during recharge during 

the time the recharge current was still on current limit. In addition, the data for all the tests 

indicate that the batteries were recharged to more than 80% of their capacities within one time 

constant following the departure from current limit, and were recharged to more than 90% of 

their capacities within two time constants following the departure from current limit.
A-5

Figure A-3 Normalized recharge current following current limit vs. time constant for 

ENERSYS cycles 1-10

One can get a better perspective of the dependence of the recharge current and the recharged 

capacity as a function of time after the current limit period by examining the data in graphical 

form. For this evaluation, the combined data for the first 10 cycles for the ENERSYS battery are 

presented below. In Figure A-3, the normalized recharge current is presented vs. time, while 

Figure A-4 presents the percentage of battery capacity recharged vs. time (both in units of time 

constants following the current limit period).

It is clear in Figure A-3 that the normalized recharge current closely follows an exponential 

function out to five time constants beyond the time on current limit. At one time constant, the 

value of the normalized recharge current should be approximately 0.368 (1/e), at two time 

constants the normalized recharge current should be approximately 0.135 (1/e2

), and at three 

time constants the normalized recharge current should be approximately 0.050 (1/e3

). It is 

expected that the data will deviate from a true negative exponential with increasing time 

constants, because the current is not asymptotically approaching zero as would a pure 

exponential function but is approaching the lower limit of stable float current.

Figure A-4 presents the same data as discussed in Table A-2 but out to five time constants 

beyond the time on current limit. Here it is evident that the battery capacity rises rapidly until 

about two time constants beyond the current limit, after which time the rate of recharge 

decreases, providing little additional capacity afterwards. The utility of the recharge time 

constant concept in evaluating the state-of-charge and the “return-to-service” limits for a battery 

is evident from these comparisons. Each battery would have its own unique characteristics and 

the limits for evaluating the state-of-charge and the “return-to-service” limits for a battery would 

have to be established on a case-by-case basis. However, the recharge time constant provides 

an opportunity to reconsider the protocols whereby decisions are made concerning the status of 

batteries for service.

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6

normalized recharge current

current limit + number of time constants

exp(-t/tc)

Cycle 1

Cycle 2

Cycle 3

Cycle 4

Cycle 5

Cycle 6

Cycle 7

Cycle 8

Cycle 9

Cycle 10
A-6

Figure A-4 Percentage of battery capacity recharged following current limit vs. time constant 

for ENERSYS cycles 1-10
A-7

Table A-2 Percentage of the discharged Amp-hours recharged vs. time.

ENERSYS: PERCENT RECHARGE vs. TIME CONSTANT

Cycle 1 2 3 4 5 6 7 8 9 10

amp-hr discharged 1530 1480 1486 1468 1516 1466 1496 1440 1429 1398

current limit (CL, hr) 6.50 6.00 5.76 5.66 6.06 5.61 5.51 5.28 5.24 4.87

time constant (tc, hr) 1.98 2.27 2.34 2.38 2.26 2.18 2.24 2.16 2.17 2.16

Percentage of amp-hr recharged that were discharged (all cycles were recharged at 27 volts)

at current limit 79.1 74.0 71.4 71.1 73.0 72.0 69.8 69.2 67.2 66.3

CL + 1 tc 93.9 91.7 88.7 89.0 89.5 88.3 86.2 85.4 84.4 83.2

CL + 2 tc 99.4 98.5 96.5 96.1 95.9 95.3 93.6 92.9 91.5 91.2

CL + 3 tc 100.7 100.8 99.2 99.3 98.8 98.5 96.9 96.4 95.0 94.8

GNB: PERCENT RECHARGE vs. TIME CONSTANT

Cycle 1* 2* 3 4 5 6 7 8 9 10

amp-hr discharged 1278 1191 1157 1201 1190 1168 1178 1151 1184

current limit (CL, hr) 6.34 5.67 3.92 4.25 4.15 3.93 4.15 4.03 4.25

time constant (tc, hr) 0.97 1.22 2.90 2.51 2.51 2.49 2.30 2.28 2.24

Percentage of amp-hr recharged that were discharged (*cycles 1 and 2 were recharged at 28 volts)

at current limit 91.3 96.7 62.1 64.8 63.0 60.8 63.6 63.1 64.8

CL + 1 tc 99.0 98.6 90.2 88.3 86.7 84.7 85.6 85.0 85.7

CL + 2 tc 100.0 101.5 99.3 96.8 95.5 93.6 93.9 93.3 93.6

CL + 3 tc 100.2 101.8 101.9 99.8 98.9 97.4 97.5 96.9 97.2

C&D: PERCENT RECHARGE vs. TIME CONSTANT

Cycle 1 2 3* 4* 5 6 7 8 9* 10*

amp-hr discharged 2050 2007 2000 1956 1944 1936 1929 1920 1900 1901

current limit (CL, hr) 8.87 8.52 9.85 9.57 7.82 7.72 7.70 7.60 8.90 9.05

time constant (tc, hr) 3.03 3.05 1.78 1.80 3.07 3.04 2.98 2.98 2.00 1.94

Percentage of amp-hr recharged that were discharged (*cycles 3, 4, 9 and 10 were recharged at 28 volts)

at current limit 77.9 76.4 88.5 87.8 73.6 71.8 71.9 71.4 84.3 85.5

CL + 1 tc 94.9 93.7 98.7 98.5 91.9 89.3 89.0 88.6 96.1 97.0

CL + 2 tc 100.3 99.8 102.0 102.0 98.9 96.1 95.7 95.2 100.7 101.3

CL + 3 tc 101.5 101.8 103.0 103.1 101.8 99.1 98.8 98.3 102.7 102.9




UNITED STATES 

NUCLEAR REGULATORY COMMISSION

WASHINGTON, DC 20555-0001

--------------------

OFFICIAL BUSINESS
NUREG/CR-7148 Confirmatory Battery Testing: The Use of Float Current Monitoring

to Determine Battery State-of-Charge