Penni Dalton

Glenn Research Center, Cleveland, Ohio

Fred Cohen

The Boeing Company, Canoga Park, California

International Space Station Nickel-Hydrogen

Battery On-Orbit Performance

NASA/TM—2002-211721

July 2002

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Penni Dalton

Glenn Research Center, Cleveland, Ohio

Fred Cohen

The Boeing Company, Canoga Park, California

International Space Station Nickel-Hydrogen

Battery On-Orbit Performance

NASA/TM—2002-211721

July 2002

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

37th Intersociety Energy Conversion Engineering Conference

sponsored by the Institute of Electrical and Electronics

Engineers, Electron Devices Society

Washington, DC, July 28–August 2, 2002

IECEC–2002–20091
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NASA/TM2002-211721 1 

IECEC2002-20091 

INTERNATIONAL SPACE STATION NICKEL-HYDROGEN BATTERY 

ON-ORBIT PERFORMANCE 

Penni Dalton 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

216–433–5223 

penni.j.dalton@grc.nasa.gov

Fred Cohen 

The Boeing Company 

Rocketdyne Propulsion and Power 

Canoga Park, California 91309

818–586–3206 

fred.cohen@west.boeing.com 

ABSTRACT 

International Space Station (ISS) Electric Power 

System (EPS) utilizes Nickel-Hydrogen (Ni-H2) 

batteries as part of its power system to store 

electrical energy. The batteries are charged during 

insolation and discharged during eclipse. The 

batteries are designed to operate at a 35% depth of 

discharge (DOD) maximum during normal operation. 

Thirty-eight individual pressure vessel (IPV) NiH2 battery cells are series-connected and packaged 

in an Orbital Replacement Unit (ORU). Two ORUs 

are series-connected utilizing a total of 76 cells, to 

form one battery. The ISS is the first application for 

low earth orbit (LEO) cycling of this quantity of seriesconnected cells. 

The P6 (Port) Integrated Equipment Assembly 

(IEA) containing the initial ISS high-power 

components was successfully launched on November 

30, 2000. The IEA contains 12 Battery Subassembly 

ORUs (6 batteries) that provide station power during 

eclipse periods. This paper will discuss the battery 

performance data after eighteen months of cycling. 

INTRODUCTION 

At Assembly Complete (AC), the ISS Electric 

Power System (EPS) will be powered by 24 batteries 

during eclipse and extended operation periods. The 

batteries are evenly divided between four Integrated 

Equipment Assemblies (IEA), as part of the four 

photovoltaic modules (PVM). The fourth and final 

PVM will complete the EPS and is currently 

scheduled for launch in January, 2004. The battery 

ORU (see Fig. 1) is designed to operate for 6.5 years, 

with a mean-time-between-failure (MTBF) of 5 years, 

when run in the reference design 35% Depth of 

Discharge (DOD) low earth orbit (LEO) regime. 

Typical expected discharge currents can range from 

<25 Amps in a low-demand orbit to as high as ~75 

Amps to meet short peaking load requirements at a 

battery operating voltage range of 76 to 123 Vdc. 

The ORUs are individually fused to protect the ISS 

EPS from fault propagation that could result from a 

cell-to-EPS ground event. Primary charge control is 

accomplished by a pressure temperature algorithm, 

which incorporates acceptance test data in order to 

initialize basic reference parameters. 

FIGURE 1. ISS BATTERY SUBASSEMBLY ORU
NASA/TM2002-211721 2 

The ISS power system is the first on-orbit use of 

such a large quantity of series-connected individual 

pressure vessel (IPV) Ni-H2 battery cells, in an 

ORU/Battery (38/76 cells) configuration. Previous 

ground testing had been performed on 22 IPV NiH2

cells in series (Lowery, et al., 1990). Therefore, 

during the ISS program development stage, it was 

important to demonstrate that the “as-designed”

battery could be successfully run. This was 

accomplished at the Power Systems Facility (PSF) 

Laboratory at NASA Glenn (then NASA Lewis) 

Research Center in Cleveland, Ohio in 1992. Two 

Engineering Model ORUs in series were subjected to 

3,000 LEO peaking cycles at 35% DOD. The test 

demonstrated that the ORUs exceeded the ISS 

design requirements for electrical performance, heat 

generation, thermal uniformity, and charge 

management (Cohen and Dalton, 1994). 

ORU DESIGN CONSIDERATIONS 

Remembering that the original ISS battery design 

effort began in 1988, a long-life, high-performance 

battery was needed. Therefore, state-of-the-art Ni-H2

IPV chemistry was chosen at that time, and designed 

to meet the following ORU requirements: 

• 6.5-year design life 

• 81-Amp-hr nameplate capacity to limit the 

maximum reference DOD to less than 35% 

• Contingency orbit capability consisting of 

one additional orbit at reduced power after a 

35% DOD without recharge 

• 5-year MTBF 

• Easy on-orbit replacement utilizing the 

robotic arm. 

The cells selected for use in the Battery ORUs 

are manufactured by Eagle Picher Industries. The 

cells are RNH-81-5 EPI IPV NiH2, and utilize a backto-back plate configuration. They are activated with 

31% potassium hydroxide (KOH) electrolyte. The 

ORUs are assembled and acceptance tested by 

Space Systems/Loral. 

ISS BATTERY CONFIGURATION 

The Battery Subassembly ORU, as designed and 

built, is pictured below in Figs. 2 and 3. 

NiH2 cells for the current 12 ISS Battery ORUs 

were manufactured and activated 3.6 to 4.4 years 

prior the November 30, 2000 launch date. The flight 

ORUs were used for IEA systems ground testing and 

final checkout, but were stored open-circuit, 

discharged, and at –10 °C when they were not in use. 

Twelve Battery ORUs were integrated onto the 

P6 IEA in July 2000 at the Kennedy Space Center 

(KSC). These 12 ORUs comprise six separate 

batteries, with three batteries on each of two power 

channels. For the P6 IEA, these power channels are 

designated as 2B and 4B. During insolation, power is 

supplied to the source bus by solar arrays that meet 

the demands of user loads as well as battery 

recharging. The batteries interface through a Battery 

Charge/Discharge Unit (BCDU) and provide the 

power to the source bus for the ISS during eclipse 

periods. 

Each ORU is individually fused for fault 

propagation protection of the EPS in the event of a 

cell short. The 120-amp fuse block consists of two 

parallel fuse strings, one 60 ampere string on each 

power cable. The fuses are constructed using high 

voltage, high reliability space rated components.

Each battery ORU also contains a letdown resistor 

that connects across the power terminals and 

provides the capability to individually discharge the 

hardware at the ORU level. 

The battery ORU contains a Battery Signal 

Conditioning and Control Module (BSCCM). The 

BSCCM provides conditioned battery monitoring 

signals from the ORU to the Local Data Interface 

(LDI) located within the BCDU. Available data 

includes 38 cell voltages, four pressure (strain gauge) 

readings, six cell and three baseplate temperatures. 

This data is provided as an analog multiplexed 

voltage. A separate signal provides ORU total 

voltage output. The BSCCM also accepts and 

executes commands from the BCDU/LDI to control 

ORU cell heater and resistor letdown functions. 

FIGURE 2. ISS BATTERY SUBASSEMBLY ORU 

WITH MLI BLANKET 

For battery charging, the BCDU conditions power 

from the 160 V source bus and charges the battery at 

pre-determined currents that are calculated based on 

State of Charge (SOC). The charging algorithm, 

based on a temperature and pressure SOC 

calculation, was described in a previous paper 

(Cohen and Dalton, 2001). During periods of eclipse, 

the BCDU extracts power from the battery, conditions 

this power, and supplies power to the source bus. 
NASA/TM2002-211721 3 

FIGURE 3. ISS FLIGHT MODEL BATTERY 

SUBASSEMBLY ORU WITH COVER REMOVED 

The batteries are actively cooled using the ISS 

Thermal Control System (TCS). The battery cells are 

assembled in an ORU box, using a unique finned 

radiant heat exchanger baseplate. The ORU is then 

mounted on the IEA using ACME screws and mated 

to the TCS. The TCS was designed to maintain the 

Battery ORUs at a nominal operating temperature 

range of 5 ± 5°C (41 ± 9°F) with minimum heater 

operation when run at a 35% DOD LEO regime. 

ISS ON-ORBIT OPERATION 

The ISS main power system charge algorithm 

has pre-set parameters. Maximum charge rate is 

determined and set based on the on-orbit operation 

need. Currently, a 50-Amp maximum charge rate 

setpoint is employed due to operating scenarios that 

feather arrays to save fuel and/or reduce the 

possibility of charge build-up on the ISS structure 

during EVA activity. As such, it is necessary to 

replenish the battery energy used during eclipse as 

quickly as possible when it is available from the solar 

arrays. The taper charge profile is pre-programmed 

in a look-up table with the following parameters: 

SOC% 20 85 90 94 96 98 1.00 1.01 >1.05 

Chg Rate 

(Amps) 

 50 50 50 50 40 27 10 5 1 

The above table is on-orbit programmable and 

can be revised to allow optimal charge rates for 

changing operational scenarios, as well as for 

compensation of changing battery performance 

characteristics caused by aging. 

ISS ON-ORBIT DATA 

The ISS on-orbit data is telemetered to the 

ground, and is available real time through data 

screens on consoles located in the Engineering 

Support Rooms (ESRs) and the Mission Engineering 

Room (MER). Stored, long-term data can be 

accessed from the Orbiter Data Reduction Complex 

(ODRC) through the consoles. The on-orbit start-up 

procedures and the battery initial performance were 

reported by Cohen and Dalton, 2001. 

 Representative, current on-orbit data is shown 

following the text in Figs. 6 through 15. This data is 

for flight days 117 (April 27, 2002) for channel 2B and 

124 (May 4, 2002) for channel 4B. As of these dates, 

the batteries had completed approximately 8,200 and 

8,300 LEO cycles respectively. The data depicts one 

battery for each channel. Spaces in the data are 

caused by data drop-out and are not intentional 

omissions. The data clearly shows that the batteries 

are performing within their design specifications over 

the operational range. 

For the referenced data: 

• Battery voltage (76 cells) 95 to 117 Vdc 

(Figures 6 and 7) 

• Maximum charge rate 50 Amps (note that 

due to ISS EPS conventions, charging 

current is shown as negative) 

• Cell voltages ~1.25 to ~1.55 Vdc (Figures 8 

and 9) 

• Average ORU temperature range ~0.0 to 

4.4°C (Note heater cycling due to ISS 

operation at less than ORU power design 

loads) (Figures 10 and 11) 

• Average battery pressure ~563 to ~720 psi 

(note: 4B2 delta pressures between ORUs) 

(Figures 12 and 13) 

• Average SOC ~80% to ~104% (Note: 

Batteries on both channels are operating 

well below DOD design point, with 4B 

running at lighter load than 2B) (Figures 14 

and 15) 

The cycling regime has been fairly benign over the 

last 18 months, averaging closer to 20% DOD than 

the designed-for DOD of 35%. Figure 4 is a plot of 

actual and predicted DODs based on projected power 

levels. As shown in the plot, actual DODs to date 

have ranged from a low of 10% to a high of 35%. 

Power level projections (Gonzalez, 2001) have been 

used to predict the DOD during the remainder of the 

P6 batteries on-orbit operation. These predictions 

range from 16% to 38% DOD, with an average of 

Deadface load resistor 

Fuse module 

Power connector Signal connector 

BSCCM 

Upper bus bar/V sense wire 
NASA/TM2002-211721 4 

about 26% DOD. Using the Space Systems Loral 

performance-based battery design life model, figure 

5, and these predicted DODs, the P6 battery life is 

expected to exceed the 6.5 year life requirement. 

 FIGURE 4. DOD FOR P6 BATTERIES 

 FIGURE 5. BATTERY DESIGN LIFE MODEL

Note in Figure 13 that battery 4B2 has exhibited 

a pressure divergence between the two ORUs. The 

divergence has grown with cycling. Battery 4B2 

contains a mismatched set of ORUs. In the Battery 

4B2, ORU 4B2A has had more ground testing than 

ORU 4B2B. The ORUs are charged as a pair, but the 

SOC, and thus the charge rates, are based on the 

average of the SOC for the two ORUs. This led to an 

undercharge of 4B2B, and a slight overcharge of 

4B2A. 

A reinitialization procedure for the battery was 

performed in February, 2002. The battery was 

discharged during eclipse, with no charge during 

insolation periods, until the first cell reached 0.7 V. 

The individual ORU drain resistors were then 

engaged. Each ORU was discharged through the 

resistor until the first cell reached 0.1 V. At the 

completion of this procedure, the pressure difference 

had dropped from a high of about 160 psi to 42 psi. 

The capacity of 4B2B was estimated to be 56 Ah and 

the capacity of 4B2A was around 76 Ah. The total 

useable battery capacity will be limited by the 4B2B 

capacity of 56 Ah, however, at this time it is sufficient 

to meet ISS requirements. 

Additional attempts to equalize the pressures 

and capacities of these two ORUs will be performed 

later this year. We have proposed increasing the 

operating temperature by 2°C and engaging the drain 

resistor on 4B2A, as well as raising the taper charge 

current on both ORUs. A more detailed description of 

the reinitialization will be reported in a second paper 

(Hajela and Cohen, 2002). 

CONCLUSIONS 

The ISS EPS is successfully maintaining power 

for all on-board loads. This power is currently 

supplied by six NiH2 batteries (three per channel) 

during eclipse periods. The batteries are designed 

for a LEO 35% DOD cycle, however, due to the low 

power demands at this point in the ISS assembly 

phase, they have been operating between 10 and 

35% DOD. The batteries are operating nominally and 

have exceeded all ISS requirements. The power 

system will be complete following the scheduled 

launch of the second, third, and fourth PVMs in April 

and August of 2002 and January of 2004.

REFERENCES

Cohen, F., and Dalton, P.J., “Space Station 

Nickel-Hydrogen Battery Orbital Replacement Unit 

Test,” Proceedings of the 29th Intersociety Energy 

Conversion Engineering Conference, Monterey, CA., 

August 1994. 

Cohen, F., and Dalton, P.J., “International Space 

Station Nickel-Hydrogen Battery Start-up and Initial 

Performance,” Proceedings of the 36th Intersociety 

Energy Conversion Engineering Conference, 

Savanah, GA., August 2001. 

Gonzalez, F., “ISS Integrated Energy Balance 

Analysis - Preliminary DAC9, Revision F,” Houston, 

TX, January 2001. 

Hajela, G, and Cohen, F, “Battery ReInitialization on the Photovoltaic Module of the 

International Space Station,” Proceedings of the 36th

Intersociety Energy Conversion Engineering 

Conference, Savanah, GA., August 2001. 

Lowery, J.E., Lanier, J.R., Hall, C.I., and Whitt, 

T.H., “Ongoing Nickel-Hydrogen Energy Storage 

Device Testing at George C. Marshall Space Flight 

Center,” Proceedings of the 25th Intersociety Energy 

Conversion Engineering Conference, Reno, NV, 

August 1990. 

1

10

100

20 25 30 35 40 45 50 55 60

Battery EOD_DOD, %

Battery Design Life, years

10 Deg. C

20 Deg. C

ISS On Orbit, Battery DOD for P6 IEA

0

5

10

15

20

25

30

35

40

10/1/2000 2/13/2002 6/28/2003 11/9/2004 3/24/2006 8/6/2007

Date

% DOD

Average DOD

Launch

November 20, 2000

Design Point
NASA/TM2002-211721 5 

BATTERY 2B2 VOLTAGE & CURRENT

April 27, 2002 

-60

-40

-20

0

20

40

60

80

100

120

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36

GMT TIME

VOLTS/AMPS

Batt Volt Batt Curr BattA Volt BattB Volt

CHARGE CURRENT (-)

DISCHARGE CURRENT (+)

ORU VOLTAGE

BATTERY VOLTAGE

BATTERY 2B2 CELL VOLTAGES

(Min-Max and ORU Cell Average)

April 27, 2002

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36

GMT TIME

CELL MIN-MAX AND AVERAGE VOLTS 

BattA Min Volt BattA Max Volt BattB Min Volt BattB Max Volt BattA Avg Volt BattB Avg Volt

BATTERY 4B2 VOLTAGE & CURRENT

May 4, 2002

-60

-40

-20

0

20

40

60

80

100

120

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36 03:50:24

GMT TIME

VOLTS/AMPS

Batt Volt Batt Curr BattA Volt BattB Volt

CHARGE CURRENT (-)

DISCHARGE CURRENT (+)

ORU VOLTAGE

BATTERY VOLTAGE

FIGURE 6. BATTERY 2B2 VOLTAGE AND CURRENT 

FIGURE 7. BATTERY 4B2 VOLTAGE AND CURRENT 

BATTERY 4B2 CELL VOLTAGES

(Min-Max and ORU Cell Averages)

May 4, 2002

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36 03:50:24

GMT TIME

CELL MIN-MAX AND AVERAGE VOLTS

BattA Min Volt BattA Max Volt BattB Min Volt BattB Max Volt BattA Avg Volt BattB Avg Volt

FIGURE 8. BATTERY 2B2 CELL VOLTAGES FIGURE 9. BATTERY 4B2 CELL VOLTAGES 
NASA/TM2002-211721 6 

BATTERY 2B2 TEMPERATURES

April 27, 2002

30

32

34

36

38

40

42

44

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36

GMT TIME

DEGREES F.

BattA Cell02 Temp BattA Cell08 Temp BattA Cell20 Temp BattA Cell28 Temp BattB Cell02 Temp

BattB Cell08 Temp BattB Cell20 Temp BattB Cell28 Temp AvgTemp

BATTERY AVG. TEMP 

(heavy line 2 ORUS)

Battery nominal operating range = 41 +/- 9 degrees F

BATTERY 2B2 PRESSURES

April 27, 2002

400

450

500

550

600

650

700

750

800

850

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36

GMT TIME

PSI

BattA Cell02 Pres BattA Cell08 Pres BattA Cell20 Pres BattA Cell28 Pres BattB Cell02 Pres

BattB Cell08 Pres BattB Cell20 Pres BattB Cell28 Pres AvgPress

AVERAGE PRESSURE (2 

ORUS)

BATTERY 2B2 STATE OF CHARGE

April 27, 2002

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36

GMT TIME

SOC (%/100)

SOC BattA SOC BattB SOC

DOD OPERATION OF 2B2 AT THIS TIME IS ~ 23%

BATTERY 4B2 STATE OF CHARGE

May 4, 2002

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36 03:50:24

GMT TIME

SOC (%/100)

SOC BattA SOC BattB SOC

DOD OPERATION OF 4B2 AT THIS TIME IS ~ 18%

Note: SOC imbalance between ORUs 

within the battery set

BATTERY 4B2 TEMPERATURES

May 4, 2002

30

32

34

36

38

40

42

44

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36 03:50:24

GMT TIME

DEGREES F.

BattA Cell02 Temp BattA Cell08 Temp BattA Cell20 Temp BattA Cell28 Temp BattB Cell02 Temp

BattB Cell08 Temp BattB Cell20 Temp BattB Cell28 Temp AvgTemp

BATTERY AVG. TEMP 

(heavy line 2 ORUS)

Indicates heater Operation

BATTERY 4B2 PRESSURES

May 4, 2002

400

450

500

550

600

650

700

750

800

850

23:31:12 00:00:00 00:28:48 00:57:36 01:26:24 01:55:12 02:24:00 02:52:48 03:21:36 03:50:24

GMT Time

PSI

BattA Cell02 Pres BattA Cell08 Pres BattA Cell20 Pres BattA Cell28 Pres BattB Cell02 Pres

BattB Cell08 Pres BattB Cell20 Pres BattB Cell28 Pres AvgPress

AVERAGE PRESSURE (2 

ORU-B

ORU-A

FIGURE 10. BATTERY 2B2 TEMPERATURES FIGURE 11. BATTERY 2B2 TEMPERATURES 

FIGURE 12. BATTERY 2B2 PRESSURES FIGURE 13. BATTERY 2B2 PRESSURES 

FIGURE 14. BATTERY 2B2 SOC FIGURE 15. BATTERY 2B2 SOC 
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12

International Space Station Nickel-Hydrogen Battery On-Orbit Performance

Penni Dalton and Fred Cohen

Battery; Nickel-hydrogen; ISS power

Unclassified -Unlimited

Subject Category: 20 Distribution: Nonstandard

Prepared for the 37th Intersociety Energy Conversion Engineering Conference sponsored by the Institute of Electrical and

Electronics Engineers, Electron Devices Society, Washington, DC, July 28–August 2, 2002. Penni Dalton, NASA Glenn

Research Center, and Fred Cohen, The Boeing Company, Rocketdyne Propulsion and Power, 6633 Canoga Avenue,

P.O. Box 7922, Canoga Park, California 91309–7922. Responsible person, Penni Dalton, organization code 5910,

216–433–5223.

International Space Station (ISS) Electric Power System (EPS) utilizes Nickel-Hydrogen (Ni-H2) batteries as part of its

power system to store electrical energy. The batteries are charged during insolation and discharged during eclipse. The

batteries are designed to operate at a 35 percent depth of discharge (DOD) maximum during normal operation. Thirtyeight individual pressure vessel (IPV) Ni-H2 battery cells are series-connected and packaged in an Orbital Replacement

Unit (ORU). Two ORUs are series-connected utilizing a total of 76 cells to form one battery. The ISS is the first application for low earth orbit (LEO) cycling of this quantity of series-connected cells. The P6 (Port) Integrated Equipment

Assembly (IEA) containing the initial ISS high-power components was successfully launched on November 30, 2000.

The IEA contains 12 Battery Subassembly ORUs (6 batteries) that provide station power during eclipse periods. This

paper will discuss the battery performance data after eighteen months of cycling.