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

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PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.27

cycle life tests are done at 80% DOD and typical NiMH module cycle life is from 600 to

1200 cycles. HEV testing emphasizes power capability even more, and de-emphasizes

depth of discharge further still. Typical HEV mode cycle life testing is under a high current

pulse proﬁle with a 2 to 10% state of charge variation. Typical NiMH cycle life under

these conditions is over 90,000 cycles, which corresponds to nearly 160,000 kilometers in

a vehicle.

During early development, failure modes for EV and HEV batteries can include short

circuiting due to mechanical penetration through the separator. The frequency of such events

is usually small if sound cell and electrode design is employed and if manufacturing quality

control is effective. Another failure mode may be abusive overcharge where venting results

in insufﬁcient electrolyte within the separator. Abusive overcharge may result from charge

imbalances caused by thermal differences from one part of the large battery to another. The

problem may be compounded by the sophistication of the charger, where voltage and tem-

perature sensing is not necessarily done on an individual module or cell basis. Another form

of abuse is overdischarge, where a cell or module within a high energy EV battery is dis-

charged below the minimum recommended cell voltage of 0.9 V. Overdischarge is usually

caused by state-of-charge imbalance within a high voltage string brought on by thermal

gradients within a battery. Another source of abusive overcharge and overdischarge is the

‘‘weak cell or weak module’’ concept. This involves the statistical predictability within a

large number of cells as to the decay rate of capacity, power and resistance as a function of

cycle number.

A common feature of the above-cited EV/HEV failure modes is the importance of main-

taining state-of-charge balance within a battery pack that may contain several hundred in-

dividual cells. The method used to maintain equalized state-of-charge within an industrial

NiMH battery is a complete charge, in effect to routinely bring all the cells to the same

state-of-charge. This method of using overcharge to equalize state-of-charge is ineffective if

the cell temperature within a battery pack is extreme or if cell-to-cell temperature gradients

are too large. Consequently, one of the biggest factors in replicating the excellent cycle life

of small portable NiMH batteries in EV and HEV applications is proper thermal manage-

ment, which is discussed in Sec. 30.11.

If premature failure due to short circuit and abusive overcharge/overdischarge is pre-

vented, the principal failure mode in large industrial EV and HEV NiMH batteries is in-

creasing internal resistance with cycling. To the end EV user, the observation is that accel-

eration capability will diminish on long-term use, or that vehicle range will gradually

decrease. To the end HEV user, battery failure due to increasing internal resistance and

resultant power loss will be observed as inability of the battery to assist acceleration and

inability of the battery to utilize regenerative braking energy due to excessive heating caused

by the high currents used.

The NiMH battery primary failure mode of increasing internal resistance and power loss

during cycling is caused by the same failure mechanism as observed in small portable NiMH

batteries: separator dryout as a result of electrolyte redistribution due to swelling of the metal

hydride and nickel hydroxide electrodes; consumption of electrolyte due to oxidation of the

separator metal hydride active material and positive electrode materials, and loss of electro-

lyte through venting.

These mechanisms may be exaggerated for large NiMH batteries due

to their prismatic construction. Cylindrical cells have one positive electrode, one negative

electrode and one separator. NiMH prismatic EV batteries may have 20 positives, 21 nega-

tives and a corresponding number of separator sheets. The cylindrical design is more effective

for pressure containment than a rectangular container, both for gas pressure and the force

applied by the can on the electrode stack itself. Therefore, another critical factor for large

NiMH batteries is management of the compressive forces within a module. Typically, re-

straining bands are used to secure a 10 or 11 cell module which has an endplate to equalize

lateral forces on the case end. Failure to adequately equalize the compression within each

cell in a module and within the internal cell stack itself will lead to premature failure due

to unequal electrolyte distribution within a cell.

30.28 CHAPTER THIRTY

30.9.6 Shelf Life

Shelf life, also termed calendar life, for large NiMH batteries is from 5 to 10 years based

on a variety of factors including temperature, charge equalization, electrolyte compensation,

and gas permeation. Over perhaps 6 months to a year of open-circuit storage, NiMH batteries

may completely self-discharge. Further open circuit stand may cause the cell voltage to

gradually decline to 0 to 0.4 V, which can cause a breakdown of the cobalt conductive

network in the positive electrode and /or increased surface oxidation of the metal hydride

active material.

The length of time under this low voltage condition, the temperature of

low voltage storage, and cell design inﬂuence the ease and degree of recovery of the battery.

For example, a few cycles of low rate charge and discharge may be needed to recover cell

capacity and power. If the degree of low voltage degradation is severe, the battery may not

be recoverable.

Design factors which the NiMH manufacturer must consider for good shelf life are the

oxidation and corrosion resistance of the metal hydride alloy, the amount of precharge on

the metal hydride electrode, the nickel hydroxide active material formula, and the quality of

the cobalt conductive network in the positive electrode.

Since large industrial NiMH batteries represent a signiﬁcant ﬁnancial investment, most

users leave the battery on trickle charge if the battery will not be used for extended periods.

Alternately, the battery may receive a periodic top-off charge designed to compensate for

normal self-discharge capacity losses.

30.9.7 Coulombic/Energy Efﬁciency

A strength of NiMH technology is high efﬁciency due to low internal resistance. As shown

in Fig. 30.16, over 90% energy efﬁciency was observed for 60 Ah prismatic NiMH EV

batteries at 100 A; over 75% efﬁciency at 300 A.

Energy efﬁciency is largely determined by the linear resistance components in the cell,

the electronic and ionic resistances, which can be lowered by further engineering improve-

ments. Coulombic efﬁciency was determined to be 99% at 50% state of charge under an

aggressive simulated HEV driving cycle which is a typical operating point for charge-

sustaining HEVs. Under the EPA combined city-highway FTP driving schedule, energy ef-

ﬁciency is about 93 to 95%.

Current (A)

Energy Efficiency

FIGURE 30.16 Energy Efﬁciency of 60 Ah NiMH HEV battery module as a function of discharge rate.

PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.29

30.10 CHARGE METHODS

30.10.1 Normal Charge

NiMH batteries are extremely ﬂexible in accepting diverse charge methods. The principal

design factor is to prevent excessive overcharge, especially at high rates, in order to avoid

heat buildup and electrolyte venting losses. Several methods of sensing overcharge are com-

mon, including absolute temperature,

⌬T, dT/dt, ⫺⌬V, and pressure rise (see Chap. 29). In

all cases, the oxygen evolution/recombination mechanism which creates heat is the basis for

each sensing method.

30.10.2 Regenerative Braking

Regenerative braking is a vehicle characteristic which helps NiMH dominate EV and HEV

applications. Vehicle range is an area of competition for EV and HEV manufacturers, and a

method of extending vehicle range from 5 to 20% is to utilize the energy lost during con-

ventional braking to charge the battery. Regenerative braking energy is available to the

battery at extremely high power (

⬃500 W/ kg), and many rechargeable battery technologies

simply cannot efﬁciently utilize the energy supplied at such a high power. NiMH batteries

are able to accept regenerative braking energy over a wide state of charge range, and over

a wide temperature range.

Figure 30.17 shows a plot of voltage-current measurements on the Ovonic NiMH 13-

HEV-60 (13 V, 60 Ah) module during an aggressive, simulated HEV driving cycle. In con-

trast to results of a typical high power lead-acid battery also shown, the V-I slope is relatively

constant and of lower slope. The lead-acid battery showed a signiﬁcantly higher resistance

for charge. For the Ovonic 13-HEV-60 module, the effective resistance on charge and dis-

charge were the same, about 6 m

⍀. This provides a substantially higher capability for ac-

ceptance of regenerative braking energy.

FIGURE 30.17 Regenerative braking charge acceptance comparison of 13 V, 60

Ah, NiMH HEV battery module and high-power lead acid battery.

30.30 CHAPTER THIRTY

30.10.3 Charge Algorithms

Charge algorithms refer to the programming used to charge the NiMH EV and HEV battery.

The concept is that the battery can accept charge input at extremely high rates until the

battery is about 80% fully charged, at which point the charge current must be reduced. In

overcharge, the charge current should not exceed the 10-hour charge rate, and generally, the

total charge input should be below about 110 to 120%.

One method used to charge NiMH EV batteries reliably utilizes constant current steps to

a temperature compensated voltage limit. Practically, this means that charging is done at a

high current until a predetermined voltage is reached which is an indication of a certain state

of charge. At this voltage, the current is reduced or stepped down, until another predeter-

mined voltage is reached. An important aspect of the charge algorithm is that the prescribed

voltage set points are variable, based on temperature and current.

30.10.4 Fast Charge

While the infrastructure to fast charge EV batteries is not fully in place, a strength of NiMH

batteries is their ability to accept fast charge if the required power is available. As an ex-

ample, a typical EV cell capacity may be

⬃100 Ah. Therefore, if 15 minute charging is

desired, currents of

⬃400 Amps (at 360⫹ volts) must be available.

NiMH batteries can accept from 60 to 80% charge within 15 minutes, at which point

current must be reduced. Temperature rise due to internal resistance heating is small, on the

order of 15

⬚C for a 33 kWh battery, whereas heating due to oxygen recombination is very

large. Again, the crucial aspect of fast charge is proper sensing of the onset of overcharge.

30.11 THERMAL MANAGEMENT

30.11.1 Cell, Module and Pack Design

A typical 4.0 Ah C size NiMH portable cell has an AC impedance on the order of 8 to 15

⍀, and a ⌬V /⌬I resistance of 15 to 30 m⍀. In contrast, a 100 Ah NiMH EV cell has an

AC impedance on the order of 0.4 m

⍀, and a ⌬V /⌬I resistance of 0.9 m⍀. Despite the low

resistance, heating is still a concern due to the extremely high current pulses resulting from

regenerative braking and during vehicle acceleration. Even the I

R heating effects at these

high currents are small in comparison to heating due to overcharge. Consequently, an essen-

tial aspect of NiMH EV and HEV battery design involves thermal management.

Proper thermal management begins at the metal hydride negative electrode, with recog-

nition that overcharge heat is generated at the surface of the hydride electrode where oxygen

is recombined. Heat must migrate from the negative electrode to the cell case and therefore

good thermal conductivity within the electrode and electrode stack bundle is very important.

Cells are usually packaged into 12 V modules, with the important concept being the cells

are bound together in a back-to-back arrangement. An important design feature is that the

cells at each end will have a much higher amount of exposed surface available for convective

cooling, and raises the concern of thermal imbalance within a module and resultant state of

charge imbalance which will lead to premature failure. Therefore, proper module design

must include endplate and heat sink considerations within a module.

Modules are packaged in a variety of conﬁgurations. Within the various packaging ar-

rangements, important design considerations are distance between modules, air or water ﬂow

channels and, battery tray heat sink characteristics to equalize cooling from module-to-

module.

PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.31

30.11.2 Water Cooling Versus Air Cooling

A key ﬁrst step in NiMH EV and HEV thermal management is a decision on whether to

use water cooling or air cooling, with no clear cut consensus yet available on which approach

is preferable. Present electric vehicles using air cooled batteries include the General Motors’

EV1 and S-10, the Toyota RAV4 and the Ford Ranger. Present electric vehicles using water

cooled batteries include the Honda EV Plus and the Daimler-Chrysler EPIC minivan. Merits

of each approach include:

Air cooling Water cooling

Advantages • Light weight

• Simple

• More effective heat

transfer

• Average ﬂuid temperature

more consistent

• Integrated with vehicle

cooling

Disadvantages

• Complicated distribution

of air within pack

• Incoming air must be free

of dirt and water from road

• Variable ambient

temperature

• Increased weight

• Elaborate module design

• Higher average ﬂuid

temperature

The NiMH battery manufacturer prefers metal cases if air-cooling is to be used for cool-

ing, and prefers plastic cell cases if water cooling is to be used.

30.12 ELECTRICAL ISOLATION

Isolation of the EV and HEV battery from the vehicle is essential due to the high voltages

involved. Isolation begins at the cell level, where plastic cell cases are preferred from an

isolation perspective, and metal cased cells must have some kind of insulative coating which

must be stable and free of pinholes.

The battery module (usually 12 V) must also be isolated from the battery tray; this is

usually accomplished with plastic isolation mounts. It is important that the cell and module

interconnecting straps also be isolated from the battery tray.

In air cooled battery packs, pack-to-vehicle resistance is strongly inﬂuenced by road

contaminants such as dirt and salt, and pack case design must take this into consideration.

For water cooled designs, the issue is the resistance of the plastic casing materials to the

coolant (e.g. ethylene glycol). There is a strong dependence of the plastic resistance to

temperature where a pack isolation resistance of 500 M

⍀ at 20⬚C may be reduced to 5 M⍀

at 65⬚C. A ﬁgure of merit for HEV and EV battery pack isolation is a resistance of 1 to 10

⍀.

30.32 CHAPTER THIRTY

30.13 DEVELOPMENT TARGETS

Despite the huge commercial production of portable NiMH batteries, the technology is still

in its infancy.

37,38

Since the initial introduction of portable NiMH in 1987, great strides have

been made in terms of speciﬁc energy, speciﬁc power, cycle life, charge retention, and cell

sizes. Beginning with only small low-power cylindrical cells, NiMH is now available in

high-power cylindrical cells and large high-power prismatic designs. Initially only metal case

cells were available but NiMH is now produced in both metal and plastic prismatic cells

and even in plastic monoblock conﬁguration.

Intensive development of NiMH battery technology is underway in many places world-

wide. Widespread activity has been reported on magnesium-based metal hydride alloys for

capacity and cost advantages, evaluation of bipolar NiMH designs, satellite NiMH battery

development, and a myriad of technical goals too lengthy to list here. However, the most

intensive NiMH development at this time involves efforts to raise speciﬁc power to new

levels and to reduce cost.

30.13.1 Cost Reduction

Cost reduction is at the forefront of NiMH development. High volume portable battery

production has seen NiMH cost equal to or less than the $/Wh cost of NiCd, previously

thought to be unobtainable. Key achievements in NiMH which have allowed this signiﬁcant

cost reduction include:

•

Nickel hydroxide cost reduction from $30 per kg to under $10 per kg

•

Nickel foam substrate cost reduction of ⬃50%

•

Increased cell capacity by ⬃50% with virtually no additional cost

•

Replacement of pure nickel wire mesh metal hydride electrode substrates with copper and

nickel-plated steel on expanded metal and perforated sheet

•

Elimination of metal hydride electrode sintering

•

Improved NiMH battery activation and formation

Despite these dramatic achievements, EV and HEV applications demand even further cost

reduction. Initial NiMH EV battery cost was about $1000 per kWh at a prototype basis, and

has steadily progressed to the $250 to 400 per kWh range (at projected production volumes

of about 7,000 to 20,000 vehicles per year). At the $250 per kWh level, and with a 30 kWh

battery as an example, the vehicle battery cost is $7500. Most vehicle manufacturers agree

that the cost of an internal combustion engine vehicle is about the same as a comparable

electric version, not including the battery cost. While the operating costs of an electric car

are less than half that of an ICE vehicle (comparing the cost of electricity vs. gasoline), the

upfront costs are prohibitive. This paradigm has resulted in the emphasis on hybrid electric

vehicles where the battery may be only 10 to 25% of the EV battery size, and the tremendous

development push for reducing NiMH cost to $150 per kWh, a target set by USABC.

NiMH cost reduction begins with the recognition that the technology cost is primarily

dictated by the materials cost. Important development activities to meet this goal include:

•

Raising NiMH EV speciﬁc energy range from 63 to 75 Wh/kg to 95 Wh/ kg

•

Replacement of expensive inactive components such as nickel foam substrate and cobalt

additives

•

Use of inexpensive plastic cell cases, and

•

Monoblock construction to reduce the number of parts

PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.33

Effort to raise speciﬁc energy involves development of metal hydride alloys with higher

hydrogen storage capacity (from 320 to 385 mAh/ g active material to 450 mAh/ g)

and higher utilization nickel hydroxide (from 240 mAh/g active material to 280 to 300

mAh/ g).

39,40

Each of these higher utilization active materials involves innovative materials

research involving modiﬁed alloy formulas and advanced processing techniques.

Reduction of expensive inactive cell components is focused on the positive electrode,

including the nickel foam substrate and cobalt metal and cobalt monoxide used to form the

conductive network. Several approaches are being studied: replacement of the cobalt com-

pounds by metallic nickel ﬁbers; use of reduced quantity and less expensive cobalt com-

pounds. An innovative approach being studied is the use of inherently more conductive nickel

hydroxide accomplished by multielement modiﬁcation, and by heterogeneous nickel hy-

droxide powder particles where ﬁlamentary metallic ﬁbers have been embedded into the base

nickel hydroxide to make contact with the overall conductive network. To reduce the cost

of the foam substrate, development activities include less expensive nickel coating processes

and elimination of the foam entirely by enhancing the conductivity of the nickel hydroxide

and the conductive network.

Cell construction cost reduction involves development of novel plastic monoblock hous-

ings; reduction of piece parts through monoblock construction, low cost plastic materials

where possible, shared terminals and vents and standardized sizes.

30.13.2 Ultra-high Power Designs

For many years, it was commonly believed that NiMH would never be able to replace NiCd

in portable battery applications where extremely high rates of discharge were required. In

particular, power tools such as cordless drills require almost continuous discharge capability

at the 10 C rate. Today’s NiMH cylindrical cells have even exceeded the power of NiCd.

Achievement of such high rate discharge capability has been accomplished through appli-

cation of low resistance current collectors as in NiCd technology and improved surface

catalytic activity of metal hydride materials. High power 3.5 to 7.0 Ah NiMH cylindrical

cells for power tools and HEV application have an AC impedance of about 1.7 m

⍀ and a

⌬V/ ⌬I internal resistance of about 4 m⍀. NiMH products in a variety of sizes (most com-

monly Cs, C, and D) are being employed in portable power tools, scooters and electric

bicycle applications.

While NiMH for HEV application already demonstrates speciﬁc power in excess of 500

W/ kg, there is a worldwide development effort to achieve 1000 W /kg. NiMH cylindrical

and prismatic prototype batteries have been announced with room temperature peak power

in excess of 700 W/ kg and power over 800 W /kg at 35

⬚C. In some cases, power has been

increased by the sacriﬁce of energy to about 44 Wh /kg while in other designs inherently

higher power materials and construction have allowed extremely high power without energy

tradeoffs. The concept is to reduce battery cost further by recognizing that for power assist

HEVs, the energy of the battery is not crucial but rather the size of the battery is set to

provide a predetermined power. Battery cost is primarily determined by the amount of the

battery materials, and higher power utilization materials reduces the amount of material used.

A crucial factor in high power NiMH batteries has been the development of highly cat-

alytic metal hydride active materials. In particular, the interface between the metal hydride

itself and the electrolyte has been identiﬁed as essential for low voltage loss under pulse

discharge. The surface oxide thickness and microporosity inﬂuence the reaction of H

⫹

and

⫺

, and a critical observation has been made that within the high power surface oxide

metallic catalysts of enriched nickel having sizes less than 70 A

are especially important for

reducing activation polarization.

41,42

30.34 CHAPTER THIRTY

FIGURE 30.18 Stackable Wafer Cell Concept. (Courtesy of Electro-Energy Inc.)

30.13.3 Other Design Options

In another approach to larger NiMH batteries, a quasi-bipolar design is being developed for

a variety of applications.

In a true bipolar design, each electrode is made with a positive

and negative surface mounted back-to-back on a conductive substrate which serves as an

intercell conductor. A separator is placed between each electrode and the bipolar plates are

sealed with an insulating material to form the cell stack. The advantage of the bipolar design

is a shorter, more uniform current path which reduces the intercell resistance and improves

the power capability of the battery. In the quasi-bipolar battery design under development,

individual cells are fabricated using one positive and one negative electrode, both on a

metallic conductive substrate with a separator between them. An insulating perimeter seal is

employed around the edge of each cell to contain the electrolyte and gasses which form

during operation. Each cell is fabricated as a discrete element designed for the particular

application. The cells are stacked with positive to negative faces in contact to construct multi-

cell batteries of the required voltage. The battery stack is held in compression by end plates

and the battery housing. The end plates also contain positive and negative terminals for the

battery. Conductive heat transfer plates can also be placed within the cell stack to improve

thermal management. Electrodes are formed by rolling plastic-bonded mixes of active ma-

terials on the conductive substrates. No screens or grids are employed in the electrodes.

Figure 30.18 shows this stacked cell concept.

Batteries are being developed for a variety of applications including Low Earth Orbit

(LEO) Satellite Batteries, Military Communications Batteries, Aircraft and Heart-Assist

Pump Batteries. Table 30.5 shows the requirement for the LEO Satellite battery shown in

Fig. 30.19. Charge-discharge cycles for a single-cell battery at different points in the cycle

life are shown in Fig. 30.20 at 40% depth of discharge. Five thousand cycles have been

demonstrated in test cells at 40% DOD while over ten thousand cycles have been obtained

at 25% DOD. Batteries are also being developed for the other applications described above.

PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.35

TABLE 30.5 Satellite Battery Requirements

Voltage 28 V

Current 22.7 A Charge, 35.7 A Discharge

Power 1 kiloWatt

Battery Capacity 52 Ahr (20.8 Ahr at 40% DoD)

Energy Density Goal 100 Wh /kg

Cycle Life Goal 30,000

Operating Temperature

⫺10⬚Cto⫹25⬚C

Overcharge 5%

Electrical Insulation Resistance

⬎1 megOhm @ 100 VDC

Cooling Conductive baseplate

Maximum Temperature Change 25

⬚C above cold plate

FIGURE 30.19 28 V, 52 Ah NiMH LEO Satellite Battery. (Courtesy Electro-Energy Inc.)

30.36 CHAPTER THIRTY

FIGURE 30.20 Cycle life of a single sealed 1.6 Ah single-cell battery operated at 40% DOD, 55 Minutes

Charge at 0.72 Amp. and 35 minute discharge at 1.1 Amp. (Courtesy Electro-Energy Inc.)

REFERENCES

1. S. R. Ovshinsky, S. K. Dhar, M. A. Fetcenko, K. Young, B. Reichman, C. Fierro, J. Koch, F. Martin,

W. Mays, B. Sommers, T. Ouchi, A. Zallen and R. Young, 17th International Seminar & Exhibit

on Primary and Secondary Batteries, Ft. Lauderdale, Florida, March 6–9, 2000.

2. R. C. Stempel, S. R. Ovshinsky, P. R. Gifford, D. A. Corrigan, IEEE Spectrum, Volume 35, Number

11, November, 1998.

3. G. Halpert, Proceedings of the Symposium on Nickel Hydroxide Electrodes, Electrochemical Society,

Hollywood, FL, October 1989 (Electrochemical Society, Pennington, NJ, 1990), pp. 3–17.

4. B. Reichman, S. Venkatesan, M. A. Fetcenko, S. R. Ovshinsky, United States Patent 5,851,698

(1998).

5. F. J. Kruger, 15th International Seminar on Primary and Secondary Batteries, Ft. Lauderdale, Flor-

ida, (March 1998).

6. D. A. Corrigan, S. Venkatesan, P. Gifford, A. Holland, M. A. Fetcenko, S. K. Dhar, and S. R.

Ovshinsky, Proceedings of the 14th International Electric Vehicle Symposium, Orlando, Florida,

1997.

7. V. Puglisi, 17th International Seminar & Exhibit on Primary and Secondary Batteries, Ft. Lauder-

dale, Florida, March 6–9, 2000.

8. V. Ettel, J. Ambrose, K. Cushnie, J. A. E. Bell, V. Paserin, P. J. Kalal, United States Patent 5,700,363

(1997).

9. S. R. Ovshinsky, Materials Research Society, MRS Fall Meeting, Boston, MA (November 1998).

10. K. Sapru, B. Reichman, A. Reger, S. R. Ovshinsky, United States Patent 4,623,597 (1986).

11. S. R. Ovshinsky, M. Fetcenko, and J. Ross, Science, 260, 176 (1993).

12. S. R. Ovshinsky in ‘‘Disordered Materials: Science and Technology,’’ D. Adler, B. Schwartz, and

M. Silver, Eds., Institute for Amorphous Studies Series, Plenum Publishing Corporation, New York,

1991.