Linden D., Reddy T.B. (eds.) Handbook of batteries

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NICKEL-HYDROGEN BATTERIES 32.13

FIGURE 32.9 TRW 81-Ah Ni-H

battery for a ﬂight program (Photograph provided courtesy

of TRW).

FIGURE 32.10 MIDEX 23-Ah CPV Ni-H

battery assembly.

32.14 CHAPTER THIRTY-TWO

are designed for LEO operation with a 6.5-year design life expectancy and are conﬁgured

as orbital replacement units (ORU), permitting replacement of worn-out batteries over the

anticipated 30-year station life.

The baseline energy storage system design contains 2 bat-

teries of 76 cells of 81-Ah capacity, packaged as 38-cell assemblies (Fig. 32.8), or approx-

imately 184.7 kWh of stored energy.

32.5.1 Design Features

Mechanical Design. Typically, each battery will have a thermal sleeve around each cell.

The cells are mechanically restrained by clamping them in a precision-machined sleeve.

These sleeves can be made of either a metal such as aluminum or a composite made in a

manner to provide electrical isolation, high thermal conductivity and strength. The sleeve is

isolated electrically from the cell by a blanket, such as CHO-THERM which allows thermal

transfer, wrapped around the cylindrical portion of the cell between the cell and sleeve. The

space between the sleeves, blanket and cell is normally ﬁlled with a material such as an

RTV 566 to provide better thermal transfer as well as to bond the interfaces mechanically.

The sleeves are then either attached mechanically to a base plate which is the interface to

the satellite structure or are attached to an interface such as extruded heat pipe assemblies

which are a part of the satellite structure. The exposed surfaces of the cells are protected by

a coating of Solithane or a combination of paint on the cell pressure vessel and Solithane.

The desired battery voltage deﬁnes the number of cells used for the assembly.

Thermal Design. Each battery is designed to operate thermally within a speciﬁed set of

limits dictated by the mission requirements and the satellite interface. These limits are nor-

mally in the range between

⫺10⬚C and ⫹15⬚C during the periods of battery operation. During

periods of inoperation such as the equinox periods of the geosynchronous missions, the

temperature range can be reduced since the thermal output of the battery is less. Heat dis-

sipated from within the cells is conducted radially to the pressure vessel wall, through the

insulating blanket /RTV 566 to the thermal sleeves, and down the thermal sleeve to the base

plate or mounting interface. Depending on the interface, the heat is transferred through the

mounting to second surface mirrors or to thermal heat pipes for dissipation. The charge

control and heater assemblies must be operated in conjunction with the passive or active

thermal dissipation to regulate operating temperature. The battery surfaces may be anodized

to optimize thermal emissivity.

Weight and Energy Density. Most battery designs are optimized through stress and thermal

analysis. Figures 32.11 and 32.12 depict expected speciﬁc energies and energy densities for

22-cell, 28-V batteries. These, of course, may vary depending on the manufacturer and the

intended use.

NICKEL-HYDROGEN BATTERIES 32.15

0 50 100 150 200 250 300 350 400 450

Capacity (Ah)

Specific Energy (Wh/kg)

3.5 inch

4.5 inch

5.5 inch

FIGURE 32.11 Speciﬁc energy for 28-V Ni-H

battery.

0 50 100 150 200 250 300 350 400 450

Capacity (Ah)

Energy Density (Wh/liter)

3.5 inch

4.5 inch

5.5 inch

FIGURE 32.12 Energy density of 28-V Ni-H

battery.

32.16 CHAPTER THIRTY-TWO

32.6 APPLICATIONS

Aerospace applications of Ni-H

batteries can be divided into two categories: LEO and GEO

applications. These two applications stress different requirements for the batteries. LEO life-

time-in-orbit requirements are typically only 3 to 6 years at approximately 6000 cycles per

year for a total cycle requirement of 18,000–36,000 cycles. The GEO applications stress

lifetime in orbit, namely 15 to 20 years. The batteries are cycled about 100 times per year

for a total of 1500 to 2000 cycles. Ni-H

batteries are also being developed and evaluated

for terrestrial applications.

20,21

32.6.1 GEO Applications

Eclipse Seasons. Communications satellites are required to operate continuously without

interruption, which includes operation during eclipse seasons. These satellites pass through

eclipse periods each day during the equinox seasons. An eclipse occurs when the satellite is

in the shadow of the earth. The eclipses start with a few minutes duration and build up to

a maximum length of 72 min, which occurs midseason (about March 21 or September 23),

and then drop off again symmetrically. The total duration of the season is 45 days. The

batteries supply power to the spacecraft during eclipse periods and are recharged during the

sunlight portion of each eclipse day. During the summer and winter solstice periods (ap-

proximately 138 days each) between eclipse seasons, the batteries are kept on trickle charge.

Charge Control. In the middle of the eclipse season, the battery is typically discharged up

to 70% of its beginning-of-life rated capacity. At the end of life, after 15 years of operation,

the battery still must meet the same initial load requirements. The preferred charge method

for GEO satellites is to recharge the battery at a ﬁxed c/d ratio, returning 105 to 115% of

the capacity removed on discharge at a high charge rate, and then switch to a low trickle

charge rate for the remainder of the 24-h eclipse day to maintain the batteries at the full

(100%) state of charge. For the 135 days between eclipse seasons, the batteries are main-

tained on trickle charge in the fully charged condition.

Reconditioning. Batteries are typically reconditioned prior to each eclipse season.

32.6.2 LEO Applications

Battery Requirements. A 96-min orbit is typically used to characterize LEO satellite ap-

plications. The time it takes a satellite to orbit the earth at 555 km is 96 min. The satellite

orbits the earth 15 times in one day. The orbital duration remains ﬁxed, but the sunlight and

eclipse periods vary with each orbit. For example, a LEO satellite orbiting 555 km above

the earth, at an inclination of 28.3

⬚, has an orbital period that is constant at 96 min; but

during a given month, such as December 1991, the eclipse durations vary from a maximum

of 35.58 min on December 1 to a minimum of 26.97 min on December 30.

Charge Control. The battery is charged during the sunlight period and discharged during

the eclipse period. With this high duty cycle, it is essential to minimize overcharge (heat

dissipation) and maximize overall Watthour efﬁciency for the battery. A charge method is

needed to compensate for the variation in the depth of discharge (variation in eclipse dura-

tion) and to minimize overcharge. If the battery can be maintained at a low temperature on

charge between 0 and 10

⬚C, the Ampere-hour charge efﬁciency approaches 100%, and the

Watthour efﬁciency approaches 85%.

NICKEL-HYDROGEN BATTERIES 32.17

NASA’s Marshall Space Flight Center (MSFC) and Lockheed Missile and Space Company

(LMSC) selected a temperature-compensated voltage-limit charging method for charging the

88-Ah Ni-H

batteries that replaced the Ni-Cd batteries on board the Hubble Space Telescope

(HST) satellite.

22,23

Figure 32.13 shows the battery voltage limits used for the HST program

as a function of temperature. At the beginning of life, the batteries were charged to the K1-

L3 and K2-L3 voltage limits at the high rate, then switched to trickle charge, which is

approximately a C/100 rate. The K1-L3 setting has a cell voltage limit of 1.513 V at 0

⬚C,

or a battery voltage limit of 33.28 V for the 22-cell battery. The battery is not fully charged

at this voltage limit but rather charged to about 73 Ah (83% of its rated capacity). The

battery is thermally stable with this charging method. The overall battery Watthour efﬁciency

is 80 to 85% during cycling with these level 3 control limits. Charging the battery to a higher

voltage limit would decrease the coulombic efﬁciency, reduce the overall energy efﬁciency,

increase the heat dissipated internally within the battery, and possibly exceed the constraints

for thermal control of the battery. The cell pressure serves as an indication of the state of

charge of the battery and is very useful in this type of application where the battery is not

fully charged.

FIGURE 32.13 Charge control V-T limits for HST 22-cell Ni-H

battery, y ⫽ 34.7412 ⫺ 0.0329x,

R ⫽ 1.00; y ⫽ 34.316 ⫺ 0.0318x, R ⫽ 1.00.

32.6.3 Terrestrial Applications

The advantages offered by Ni-H

batteries, including long life, low maintenance, and high

reliability, make them very attractive for terrestrial applications such as stand alone photo-

voltaic systems or stand-by power for emergency or remote site use. The major drawback

to the wider use of the Ni-H

battery is its high initial cost. The following are two examples

of terrestrial applications addressing Ni-H

batteries.

Starting in 1983, Sandia National Laboratories sponsored a cost-sharing program with

COMSAT Laboratories and Johnson Controls, Inc., for the design and development of a

sealed Ni-H

battery for deep-discharge terrestrial applications that would be cost-competitive

with lead-acid batteries in a system designed for a 20-year life. The main thrust of this

program was to reduce the cost of the aerospace technology without comprising the desirable

features of the Ni-H

system. Figure 32.14 shows a 5-cell, 6-V 100-Ah Ni-H

battery assem-

32.18 CHAPTER THIRTY-TWO

FIGURE 32.14 6-V 100-Ah terrestrial Ni-H

battery. (Courtesy of COMSAT Laboratories.)

bled in a pressure vessel. Assemblies have been tested by Sandia National Laboratories in

combination with photovoltaic arrays. Conclusions were expressed that the cells ‘‘continue

to perform well reinforcing the projection of a 20-year life, matching that of photovoltaic

panels.’’

Eagle-Picher Technologies, LLC has another design for use as stand-by power for a

remote site in a terrestrial application. Again the primary effort in the design was to create

a more cost-effective, reliable battery for terrestrial use. This design utilizes a combination

of the dependent pressure vessel (DPV) and two-cell common pressure vessel (CPV) tech-

nologies. Five 2-cell DCPV units are assembled creating a 12-V battery. The nominal ca-

pacity of the battery is 44 Ah at 10

⬚C. This battery is shown in Fig. 32.15.

NICKEL-HYDROGEN BATTERIES 32.19

FIGURE 32.15 DPV battery assembly.

32.7 PERFORMANCE CHARACTERISTICS

32.7.1 Voltage Performance

Electrochemically impregnated nickel oxide electrodes are used in Ni-H

cells because of

their excellent cyclic performance capabilities.

9,24

The capacity of these electrodes increases

as the temperature decreases. The measured capacity of an electrochemically impregnated

electrode is about 20% greater at 10

⬚C than at 20⬚C. The capacity of the NTS-2 35-Ah cell

at different temperatures is presented in Fig. 32.16 to show the variation in capacity with

temperature. These NTS-2 cells were discharged at the C/ 1.67 rate. Note that the mid-

discharge voltage is between 1.2 and 1.25 V.

FIGURE 32.16 Capacity of NTS-2 35-Ah cell at different temperatures. Discharge

rate, C / 1.67.

32.20 CHAPTER THIRTY-TWO

FIGURE 32.17 Discharge of INTELSAT V 30-Ah cell at 200-A rate. 6.7 C-rate

discharge.

FIGURE 32.18 Speciﬁc energy vs. speciﬁc power of INTELSAT V Ni-H

cell.

The high-rate discharge of an INTELSAT V cell at 200 A (12-min rate) is shown in Fig.

32.17. The discharge proﬁle is almost ﬂat at 0.6 V; the potential drop of approximately 600

mV is due to the 3-m

⍀ terminal impedance of the cell (3 m⍀ ⫻ 200 A ⫽ 600 mV). As

previously seen in Fig. 32.16, the mid-discharge voltage is between 1.2 and 1.25 V for a

cell discharge at the C /1.67 rate. The aerospace IPV Ni-H

cells are not optimized for high-

rate discharge but are optimized for maximum speciﬁc energy at discharge rates of between

C/ 2 and C /1.5. At higher rates, the usable energy drops off because of the I

R losses in the

terminals. For example, the INTELSAT V cell is capable of delivering about 50 Wh /kg up

to the C rate (1-h rate) of discharge at 0

⬚C. Above the C rate (30-A rate), however, the

speciﬁc energy starts to drop off, as shown in Fig. 32.18.

A salient feature of the Ni-H

cell is that the pressure is a direct indication of the state

of charge of a cell (Fig. 32.19). On charge, the hydrogen pressure increases linearly until

the nickel oxide electrode approaches the fully charged condition. During overcharge, oxygen

evolved at the positive electrode recombines at the negative electrode, and the pressure

stabilizes. On discharge, the hydrogen pressure decreases linearly until the nickel oxide

electrode is fully discharged. If the cell is reversed by overdischarging, hydrogen generated

at the positive electrode is consumed at the negative electrode, and again the pressure is

constant.

NICKEL-HYDROGEN BATTERIES 32.21

FIGURE 32.19 Pressure and voltage characteristics of NTS-2 Ni-H

cell at 23⬚C.

The effects on cell voltage and capacity of discharge at different rates for a 90 Ah Hubble

Space Telescope cell can be seen in Fig. 32.20. The data was gathered from cells stabilized

at 10

⬚C. As the current increases, the capacity to 1.00 V decreases. Changes in the cell

voltage and capacity can be attributed to the impedance of the cell (0.9 m

⍀).

0.7

0.8

0.9

1.1

1.2

1.3

1.4

1.5

0 0.5 1 1.5 2 2.5

Discharge Time (hours)

Terminal Voltage (volts)

40 amps

55 amps

64 amps

80 amps

96 amps

FIGURE 32.20 Discharge of Hubble Space Telescope cell at different rates.

32.22 CHAPTER THIRTY-TWO

32.7.2 Self-Discharge Characteristics of Ni-H

Cells

The self-discharge rate as a function of temperature was determined experimentally for Air

Force 50-Ah cells used in the INTELSAT VI program.

Figure 32.21 gives the data for the

self-discharge of these 50-Ah cells at 10, 20, and 30

⬚C. Figure 32.22 shows the Arrhenius

plot for these three temperatures. The slope of the straight-line regression ﬁt to these three

data points indicates an activation energy of 13.6 kcal/mol.

FIGURE 32.21 Self-discharge rates vs. temperature for 50-Ah

Ni-H

cell. (Courtesy of COMSAT Laboatories.)

FIGURE 32.22 Arrhenius plot, self-discharge rates vs. tem-

perature (Courtesy of COMSAT Laboratories.)