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

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26.16 CHAPTER TWENTY-SIX

26.6.2 Manufacturing Flexibility

The power capability of a battery will affect its potential applications. A high power battery

will be capable of delivering most of its capacity in a few minutes. To maximize battery

power, it is necessary to minimize cell resistance. To that end, high power cells are designed

with high surface areas, thin electrodes and with high metallic contents. However, the above

measures will increase battery weight, volume, and cost. Given these tradeoffs, it is desirable

to optimize the cell design for the application. Thus, it is more efﬁcient and economical to

build a cell for lower power applications utilizing thick electrodes with low metallic contents.

Practical manufacturing constraints inhibited the development of high-capacity low-rate

sintered-plate batteries. In contrast, the FNC technology covers a wide range of power ca-

pabilities. The thickness of the ﬁber electrode, and the amount of conductive metallic nickel

on it, are varied within an order of magnitude. This results in the capability to produce high-

capacity, low-weight, low-cost batteries or ultra-high power, higher weight, and higher cost

cells using the same processes and equipment. For the user, this means that the characteristics

of sinter foils and the various types of pocket or foam-plate batteries no longer have to be

considered separately. The FNC system has the same properties and basic characteristics

over the entire range of applications.

26.6.3 Sealed Versus Vented Designs

The charging of aqueous-nickel batteries always occurs in competition with water electrol-

ysis. Toward the end of the charging cycle, oxygen is typically evolved at the positive

electrode and hydrogen may be evolved at the negative electrode. The way that the cell deals

with these evolved gases will determine whether the cell can be sealed. In sealed cells, the

gases are recombined internally. In an open cell, the gases are allowed to vent, hence the

name vented cell.

The reactions that produce the gases are called the overcharge reactions. These reactions

differ depending on whether one deals with a sealed or vented cell.

Overcharge Reactions:

Vented (open) cell:

⫺⫺

4OH → 2H O ⫹ O ⫹ 4ePositive:

⫺⫺

4H O ⫹ 4e → 2H ⫹ 4OHNegative:

2H O → 2H ⫹ ONet:

222

The net result here is the electrolysis of water to hydrogen and oxygen.

Sealed Cell:

⫺⫺

4OH → 2H O ⫹ O ⫹ 4ePositive:

⫺⫺

2Cd(OH) ⫹ 4e → 2Cd ⫹ 4OHNegative:

2Cd(OH) → 2Cd ⫹ 2H O ⫹ ONet Electrochemical:

222

Chemical

Recombination on Negative: 2Cd

⫹ O ⫹ 2H O → 2Cd(OH)

22 2

INDUSTRIAL AND AEROSPACE NICKEL-CADMIUM BATTERIES 26.17

FIGURE 26.13 Electrode structure of ﬁber nickel-cadmium cell.

The result in a sealed cell is that electricity is converted into heat without any net chemical

change in the cell. The overcharge reaction is exothermic, particularly the chemical recom-

bination reaction in the sealed cell.

The overcharge reaction on the positive plate starts before the cell is fully charged, so

that some oxygen evolution on charging is unavoidable. At higher temperatures, the oxygen

evolution starts at a lower voltage. This results in lower charging efﬁciencies at higher

temperatures and, in the case of the vented cell, an increased need for the addition of water.

Additionally, this leaves the positive plate undercharged while the negative plate continues

toward a full charged state. The resulting plate imbalance reduces battery capacity. To regain

the lost capacity, vented batteries require deep discharge conditioning with each cell being

clipped out (shorted).

26.6.4 Sealed FNC Maintenance-Free Batteries

With the development of sealed FNC technology, the ﬁrst maintenance-free high rate pris-

matic nickel cadmium battery was introduced. In the sealed cell, an unﬁlled nickel-coated

ﬁber plate is placed between two cadmium ﬁlled negative plates. This effectively results in

a split negative with an unﬁlled central region. This inactivated section serves as a catalytic

site for rapid oxygen reduction. The main oxygen pathway to the recombination site is

through the plate pores, which, in the FNC plate construction, are relatively large. This

provides the oxygen with easy access to the large recombination reaction surfaces, as shown

in Fig. 26.13.

Because rapid oxygen recombination eliminates the pressure buildup normally associated

with sealed nickel cadmium cells, high charging rates can be sustained even in the overcharge

mode. Also, it is possible to use conventional nylon cell construction to produce prismatic

sealed cells rather than the cylindrical design required for high pressure cells. The sealed

26.18 CHAPTER TWENTY-SIX

FNC prismatic cell case is made of either polyamide (nylon) or stainless steel. The negative

pressure within the cell (approximately 0.1 bar absolute) allows for pressure change due to

oxygen generation during overcharge without causing the cell walls to expand. In addition,

the external air pressure presses the vessel walls and plate assembly together, enhancing the

hydraulic contact between the electrodes and providing mechanical rigidity. The plastic cell

cases are covered with an aluminum foil wrap which serves as a gas barrier to block the

migration of gases and water through the plastic. Many years of operation are possible

without gasses penetrating the cell, assuring the electrode materials remain active.

All nickel cadmium batteries must be overcharged to achieve a 100% state of charge.

During the overcharge stage, excessively charged portions evolve oxygen and hydrogen. In

vented batteries these gasses, along with water vapor, are vented outside the cell. The lost

liquid must be replaced. The sealed FNC battery completely eliminates the loss of any gases

from within the cell. Oxygen generated is rapidly recombined on the negative electrode.

Hydrogen evolution is avoided by excess discharged cadmium on the negative electrode.

This recombination process also keeps the plates in balance, eliminating the capacity loss

that would otherwise result. Electrolyte spillage and corrosion is completely eliminated. In

the absence of free KOH, even aluminum battery boxes may be used to reduce weight.

In the event of cell reversal or a failure to control charging voltage, hydrogen gas will

be produced. A recombination plate of Pt /Pd-catalyzed plaque located within the cell pro-

vides for hydrogen recombination. The oxygen source, for hydrogen recombination, is pro-

vided by the self-discharge reaction on the positive electrode or the overcharge reaction on

the following charge.

A safety valve at the top of the cell is provided to allow excessive pressure to escape

should the battery be abused to the point that the electrolyte boils. This condition might be

caused by a severe overcharge where adequate heat dissipation is not provided. Under high

temperature abuse (

⫹100⬚C and above), the safety valve will open at approximately 45 psia

over pressure allowing water vapor to escape. Electrolyte will not be expelled, even with the

cell in an inverted position. When the cell is allowed to cool, the valve will reseal and the

negative pressure cell will return to a normal operating condition. A reduction in cell capacity

may be anticipated due to the loss of water from within the cell.

A high negative to positive capacity ratio (in excess of 2:1) is employed in the cell design.

This high excess of negative capacity allows for fast charge without hydrogen evolution over

a wide range of temperatures. The excess negative capacity also provides for high power at

low temperature. Forming the electrodes prior to stack assembly in order to minimize the

carbonate level within the sealed FNC cell further increases low temperature performance.

Positive and negative plates are connected to their respective terminal posts by nickel

tabs. The tabs are attached directly to the ﬁber plates by a patented welding process and

then fastened directly to nickel-plated, solid-copper terminal posts. The electrical path of

each cell type is designed for maximum electrical performance.

Plate stacking is the same as previously discussed. Single positive plates are separated

from the negative cadmium electrode by an electrolyte wet separator. The cadmium electrode

is in three parts: two ﬁber frameworks carrying the negative active material, and an unﬁlled

ﬁber recombination electrode placed between them. The large recombination surface area is

sufﬁcient to handle a 2 C rate charge on a fully charged battery. With the unﬁlled recom-

bination plate being the primary gas path for oxygen recombination, small pore size separator

can be used. The separator is designed to be completely ﬁlled with electrolyte, thus contrib-

uting to improved high rate performance. Additionally, the recombination plate acts as a

reservoir for electrolyte, allowing for volumes in excess of 4 ml /Ah. This prevents stack

dry-out as a possibility for premature cell failure.

Sealed FNC batteries are fail-safe. Even if subjected to extreme overcharge to the point

at which the electrolyte boils, the battery will not go into thermal runaway. Instead, the hot

cells dry out, with the loss of water vapor causing the battery impedance to increase. As the

impedance increases, the current will decrease. After a time, the battery will no longer accept

the charge current and it will cool down.

INDUSTRIAL AND AEROSPACE NICKEL-CADMIUM BATTERIES 26.19

FIGURE 26.14 Short circuit current. Model KCF XX47 cell (47 Ah rated).

26.6.5 Performance

The high current performance capability of the sealed FNC battery design is exempliﬁed by

a short circuit test performed on a model KCF XX47 battery which produced currents ap-

proaching 4000 amps (see Fig. 26.14). The KCF XX47 unit was designed to meet the

requirements for large auxillary power unit (APU) and direct engine starting. The constant

voltage discharge of 12 V demonstrates the extraordinary high power capability of the battery

(see Fig. 26.15). The KCF XX47 battery’s high power performance is also demonstrated

with start curves for a large, wide-body aircraft APU. The ﬁrst two discharge sequences

represent unsuccessful start attempts with the third showing a successful start. The minimum

voltage required for this particular speciﬁcation is 13 V, while the FNC battery provides over

16 V (see Fig. 26.16).

Cold temperature performance available with the sealed FNC cells is also impressive.

Fig. 26.17 shows the capacity of a 28 V 47 Ah battery for four different discharge rates with

the battery soaked at a temperature of

⫺18⬚C.

Sealed FNC batteries have shown outstanding cycle life at both low and high rates of

discharge. Figure 26.18 provides cycle life data for Low Earth Orbit (LEO) cycle testing.

The data demonstrates a cycle life in excess of 10,000 cycles for 35% DOD, 10

⬚C, C/2

cycling. By maintaining the stable end of discharge voltage, the low recharge coefﬁcient

(approximately 3%) demonstrates the superior charge efﬁciency of the sealed FNC cell.

Deep discharge cycling is not necessary, eliminating the necessity for battery removal

from the aircraft during maintenance. Capacity checks, if desired, can be accomplished with

the battery installed by using a portable discharge/charger unit because the sealed FNC

design does not exhibit the memory effect typically found in other NiCd’s. This is illustrated

in Fig. 26.19 which consists of three capacity checks taken after 7845 60% DOD LEO

cycles. The ﬁrst capacity is directly out of orbital cycling at approximately the C rate. The

second capacity value is after a full recharge (C/10 for 16 hours) and C/ 2 discharge. The

third capacity value is after a resistive let down and subsequent full recharge. Of particular

importance is the shallow voltage depression that occurs around 48 Amp-hours into the

26.20 CHAPTER TWENTY-SIX

FIGURE 26.15 Constant voltage (12.0 Volts) discharge. Room temperature. Model XX47 battery (47 Ah

rated).

FIGURE 26.16 APU starts: 2 unsuccessful, 1 successful. Model XX47 battery (47 Ah rated).

INDUSTRIAL AND AEROSPACE NICKEL-CADMIUM BATTERIES 26.21

FIGURE 26.17 Constant current discharge. Battery charged at 25⬚C. Battery discharged at ⫺18⬚C. Model

XX47 battery (47 Ah rated).

FIGURE 26.18 LEO cycles, 35% DOD, 92 minute orbit. Model KCF X81 (81 Ah rated) battery. 10⬚C

ambient temperature. End of discharge voltage and coulombic charge factor.

26.22 CHAPTER TWENTY-SIX

FIGURE 26.19 Capacity checks after 7845 LEO cycles, 60% DOD. Out of orbital capacity, before condi-

tioning, after conditioning. Model KCF X81 (81 Ah rated). 10⬚C ambient temperature.

discharge, the point where the prior cycles discharge had ended. After 7845 cycles, the cell

continued to discharge pasts this point to an out of orbital capacity of 68 Amp hours.

Charging characteristics of the sealed FNC cells are simple, yet different from vented

NiCds. Because of the recombination that takes place during overcharge, the normal dV /dt

behavior is not always observed. In addition, heat is generated during the overcharge from

the recombination reaction, providing a reliable parameter for charge control. Changing from

main mode to topping mode and charge termination is determined by battery temperature

rise (

⌬ T).

The preferred charge is at constant current with a voltage clip (maximum voltage) of 1.55

V per cell. This provides an excellent charge rate while keeping the voltage below the level

where both hydrogen and oxygen would be generated. This voltage is also sufﬁcient to charge

the battery at temperatures as low as

⫺40⬚C. For many applications, this means that a heater

blanket is not required.

A completely charged FNC battery has sufﬁcient recombination to continue to accom-

modate a 2 C overcharge rate. Even higher rates are possible with the battery in a lower

state of charge. A 200 Amp, 47 Ah cell charge proﬁle is shown in Fig. 26.20. Voltage,

current and temperature curves are shown for the charge sequence. The discharge voltage

curve indicates the cell capacity following the 200 Amp charge.

The superior recombination rate of the oxygen produced in overcharge is demonstrated

in Fig. 26.21. Internal cell pressure was taken at three different temperatures for a C rate

charge. Note that at the end of charge and into overcharge, the internal pressure is well

below atmospheric pressure.

INDUSTRIAL AND AEROSPACE NICKEL-CADMIUM BATTERIES 26.23

FIGURE 26.20 4C (200 amp) Charge. 1 C discharge. Model KCF XX47 Battery (81 Ah rated).

FIGURE 26.21 Cell internal pressure and current for 35% DOD accelerated LEO cycle. Model KCF X81

battery (81 Ah rated).

26.24 CHAPTER TWENTY-SIX

26.7 MANUFACTURERS AND MARKET SEGMENTS

Table 26.3 contains data regarding prominent manufacturers of industrial nickel-cadmium

batteries, Table 26.4 lists the market segments and applications for these batteries.

TABLE 26.3 Major Manufacturers of Industrial and Aerospace Nickel-Cadmium Batteries (Does Not

Include Sintured-Plate Designs—See Chap. 27)

Manufacturer/ country

Product range

Trademark Pocket-plate Fiber plate Plastic-bonded plate

Acme Electric, U.S.A Acme X

Alcad Ltd., U.K. Alcad X

HBL-NIFE Power Systems, India HBL X

Hoppecke Batterien, Germany Hoppecke X

Japan Storage Battery, U.S.A. GS X X

Marathon Battery, U.S.A. Marathon X X

SAFT, S.A., France SAFT X X

Tudor S.A., Spain Tudor X X

Varta, Germany Varta X

Yuasa, Japan Yuasa X

Table 26.5 lists parameters of one line of standard sealed FNC cells currently manufac-

tured. As detailed in the prior text, the manufacturing processes and cell design have the

ﬂexibility to develop capacities and packaging to ﬁt a variety of application.

Figure 26.22 is a picture of a 28 V, 47 Ah airborne battery system with battery and

dedicated charger which uses the model XX47 unit described in Table 26.5.

Table 26.6 lists the characteristics of a second line of sealed FNC batteries which are

available in X, H, M and L designs. These operate with lower pressure vents than the batteries

listed in Table 26.5.

26.25

TABLE 26.4 Market Segments and Applications for Vented Industrial Nickel-Cadmium Batteries

Cell range*

Pocket plate

HM L

Fiber plate

XX H or X M L

Plastic-bonded

plate

Capacity, Ah 10–1000 10–1250 10–1450 23–47 10–220 20–450 20–490 11–190 20–200

Applications UPS,

starting,

switchgear

UPS,

switchgear,

auxiliary

power,

emergency

power

Lighting, alarms,

signalling,

communications,

standby power

Aircraft UPS, satellites,

starting,

switchgear,

traction, power

stations and

substations

UPS,

switchgear,

auxiliary

power,

emergency

power

Lighting, UPS,

alarms, signalling,

telecommunications,

standby power

UPS,

starting,

switchgear,

traction,

aircraft

Lighting,

auxiliary

power,

traction

Railroad X X X X X X X X

Mass transit X X X X X X X X

Industry X X X X X X X

Buildings X X X X X X X

Hospitals X X X X X X X

Oil and gas X X X X X X X

Airports X X X X X X X

Marine X X X X X X X X

Military X X X X X X X X

Telecommunications X X X X X X X

Photovoltaics X X

AGV / hybrid vehicles XX

X-H—high rate; M—medium rate; L—low rate.

XX—ultra high-rate.