Linden D., Reddy T.B. (eds.) Handbook of batteries
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PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.17
TABLE 30.2 Performance Specifications for Ovonic HEV NiMH
Battery Modules
13-HEV-60 7-HEV-28 12-HEV-20
Nominal voltage (V) 13.2 7.2 12
Nominal capacity (Ah) 60 28 20
Weight (kg) 12.2 4.3 5.2
Volume (L) 5.1 2.0 2.3
Specific energy (Wh/ kg) 68 50 48
Energy density (Wh/ L) 160 102 110
Specific power (W/kg) 600 550 550
Power density (W/ L) 1400 1200 1300
30.6.2 Charge Depletion
Charge depletion concept hybrid vehicles are designed to have a certain vehicle range during
the complete electric vehicle mode. The motivation for this requirement is that in heavily
congested urban areas where air quality is a vital concern, the vehicle be able to operate
with zero emission. Therefore, in the charge depletion mode the specific energy of the battery
remains important and a specific energy above 60 Wh /kg may be required. As an example,
a Toyota Prius HEV using 1.8 kWh NiMH D cells has a pure electric vehicle range of less
than 6 miles. However, when replaced with a 6.0 kWh NiMH battery pack, vehicle range
on electric drive only may be extended to over 25 miles.
30.6.3 Charge Sustaining
The more common HEV concept is termed charge sustaining, where the vehicle is designed
to be operating under the various modes giving the greatest overall fuel economy. In this
case, a primary function of the battery is to utilize the otherwise wasted energy of braking
and another major function is to assist the vehicle during acceleration. This concept essen-
tially utilizes the knowledge that ICE engines are very efficient on the highway at constant
speed but very inefficient under the starting and stopping conditions of the city traffic. It is
a common strategy for the engine to be shut off while the vehicle is stopped at a traffic
light, and for the battery to supply the energy needed for the initial vehicle acceleration
while the ICE is restarted.
For NiMH HEV batteries in the charge sustaining mode, the crucial performance target
is a specific power of 1000 W/ kg. For these applications, typical NiMH HEV batteries have
specific energy in the 45 Wh/kg range and have thus attained a specific power of about 600
to 850 W/kg.
30.18 CHAPTER THIRTY
30.7 FUEL CELL STARTUP AND POWER ASSIST
Fuel cells are a significant development focus for many companies worldwide, offering the
potential for equivalent fuel economy over 100 miles per gallon. The most commonly dis-
cussed fuel cell is the Proton Exchange Membrane (PEM) type (see chapters 42 and 43).
The relevance to NiMH batteries is that fuel cell vehicles will still require a substantial
(
⬃1 to 10 kWh) battery for several reasons:
•
Startup. The PEM fuel cell and reformer may operate at a temperature of about 100⬚C
and the battery must have sufficient energy for the vehicle to operate until the system
comes up to temperature.
•
Acceleration. A critical PEM fuel cell obstacle will be cost due to the use of expensive
catalysts and membranes. The fuel cell will be sized for an average vehicle power, not for
peak power conditions where the battery will dominate. Essentially, the fuel cell in a fuel
cell hybrid electric vehicle is used to charge the battery and operate the vehicle, while the
battery is used for power load-leveling.
•
Efficiency. The fuel cell can be operated under close to steady state conditions in an
optimum region of fuel utilization.
•
Regenerative braking. As with ICE powered hybrid vehicles, the fuel cell has no mecha-
nism to absorb the energy of braking without the battery.
30.8 OTHER APPLICATIONS
30.8.1 36 to 42 V SLI Batteries
While EV, HEV and Fuel Cell electric vehicles receive tremendous attention, the largest
worldwide rechargeable battery application is still the common lead acid Starting Lighting,
Ignition (SLI) battery.
As modern ICE vehicles add more electrical devices such as electric power steering,
braking, and accessories, carmakers will be using a nominal electrical operating voltage of
36 to 42 Volts compared to the normal 12 V system. Advantages including improved engine
performance and efficiency through electrically actuated valves are offset by added battery
cost and redesign of vehicle electrical systems. One option would be to use several lead-
acid batteries to meet this voltage, but size and weight considerations make this approach
impractical. Figure 30.8 illustrates a relative size comparison of lead-acid and NiMH for 36
to 42 V SLI applications. NiMH batteries are considered the leading advanced battery for
high voltage SLI applications due to the following advantages compared to lead-acid:
•
Higher gravimetric and volumetric energy
•
Longer cycle life, especially under high depth of discharge
Commercialization issues will include improving low temperature power at –30⬚C and
initial cost at low to moderate production volumes.
PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.19
FIGURE 30.8 Relative comparison of 36 V lead acid and 36 V NiMH batteries for SLI application.
30.8.2 Aircraft Applications
Prismatic nickel-metal hydride cells with 43 Ah capacity are under development for use in
military aircraft.
33
The objective of this program is to demonstrate an environmentally safe
retrofit for the U.S. Air Force F-16 Pre-Block 40 Main Aircraft battery and to pursue a
specific energy goal of 75 Wh /kg for the More Electric Aircraft Generation II Program.
Target performance parameters are listed in Table 30.3. Four-cell test sets have been evalu-
ated for capacity at several charge and discharge rates, for self-discharge and for ambient
and environmental cycling behavior. The ambient cycle life tests showed stable performance
over 200 cycles and environmental cycling behavior over 80 complete cycles of 10 minutes
at 20 amps at specified temperature. Charging efficiency was determined by charging at
⫺20,
0,
⫹23 and ⫹50⬚CatC /2, C and 2C rates followed by a five-hour rest at ⫹23⬚C and
discharge at C /2 to 0.9 V/cell. The results are shown in Fig. 30.9 for the 2C rate for four
different cell designs. Charge acceptance is good in the range of
⫺20 to ⫹23⬚C but drops
significantly at
⫹50⬚C. This effect is undoubtedly due to the fact that oxygen evolution
occurs at lower voltages at higher temperatures. Discharge characteristics were also deter-
mined vs. temperature. Cells were charged at
⫹23⬚C at the C /2 rate followed by 3 hours at
C/ 20 followed by a five-hour rest and then discharged at
⫺40, ⫺30, ⫺20, 0, ⫹23, ⫹50 and
⫹70⬚CatC/2, C and 2C rates to 0.9 V /cell. Single cell test on four different designs are
shown in Fig. 30.10 at the C/ 2 rate and in Fig. 30.11 at the 2C rate. The C/2 discharge is
well behaved between
⫺30 and ⫹70⬚C with a maximum at ⫹23⬚C but dropped below 5 Ah
at
⫺40⬚C. At the 2C rate, the discharge capacity of one configuration dropped to a low value
at
⫺20⬚C, while the other design interations provided 40 Ah at that temperature. Self-
discharge was also evaluated from
⫺10 to ⫹50⬚C. One design configuration met the require-
ment of less than 25% self-discharge per week at all temperatures up to
⫹50⬚C. Based upon
this test program, one design iteration has been selected for optimization and five baseline
batteries projected to provide 43 Ah capacity at 28 V and a specific energy of 64 Wh /kg
will be evaluated for full performance characterization.
30.20 CHAPTER THIRTY
TABLE 30.3 Envvironmental Nickel-Metal Hydride Aircraft Battery
Parameters
Nominal Voltage (V) 28
Capacity (Ah) 50
End of Life Capacity (Ah) 18 (Maintained at 14.5)
Current (A) 48 (Maximum)
Operating Temperature Range (
⬚C) ⫺40 to ⫹71
Battery Energy at C/ 2 (Wh) 1,325
Specific Energy Density (Wh/ kg) 75
Volumetric Energy Density (Wh/ L) 177
Total Battery Weight (kg {lb}) 18.18 {40}
Mean Time Between Failures (MTBF)
⬎6000 h
Maintenance Interval Maintenance Free for Three Years
Self-Discharge (
⬍25% over 7 days) All Operating Temperatures
FIGURE 30.9 Discharged capacity for 2C rate charge vs. temperature. (Courtesy of GRC and
SAFT.)
PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.21
FIGURE 30.10 Discharged capacity for C / 2 rate discharge vs. temperature. (Courtesy of GRC
and SAFT.)
FIGURE 30.11 Discharged capacity for 2C rate discharge vs. temperature. (Courtesy of GRC
and SAFT.)
30.22 CHAPTER THIRTY
30.9 DISCHARGE PERFORMANCE
30.9.1 Specific Energy
NiMH specific energy can vary anywhere from 42 to 100 Wh/ kg depending on the particular
application requirements. For laptop computers where run time is paramount, NiMH battery
makers need not have high power capability or even the need for ultra-long cycle life. On
the other hand, for extremely high power charge and discharge, extra current collection, high
N/ P (negative/ positive) ratios, and other cell design and construction decisions can cumu-
latively affect specific energy. For the most common small portable NiMH batteries, specific
energy is usually about 75 to 90 Wh /kg, for EV batteries usually about 65 to 75 Wh /kg,
and for HEV batteries and other high power applications about 45 to 60 Wh/kg.
While gravimetric energy usually receives the attention for advanced battery technologies,
in many cases volumetric energy density in Watt-hours per liter is actually more important.
NiMH has exceptionally good energy density, achieving up to 320 Wh/ liter.
30.9.2 Specific Power
NiMH power capability is an advantage relative to other advanced battery chemistries. NiMH
is now replacing NiCd in many applications due to higher energy and environmental con-
cerns.
Voltage profiles up to 10-C discharge rate for high power cylindrical NiMH are presented
in Fig. 30.12, attaining a specific power of 835 W/kg (Fig. 30.13). NiMH HEV module
power is shown in Fig. 30.14 in excess of 600 W/kg for both charge and discharge.
FIGURE 30.12 Voltage-Capacity profiles for high power NiMH cy-
lindrical 3.5 Ah C-size batteries at different rates on continuous dis-
charge.
PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.23
FIGURE 30.13 Specific power for ultra-high power NiMH cylindrical 3.5 Ah C-size batteries.
FIGURE 30.14 Specific power of 12 V, 20 Ah NiMH HEV battery module as a function of
depth of discharge.
30.24 CHAPTER THIRTY
FIGURE 30.15 Temperature performance of 100 Ah NiMH EV prismatic batteries using commercial
nickel hydroxide.
30.9.3 Effect of Temperature
NiMH, due to its KOH/H
2
O electrolyte, provides performance as function of temperature
similar to nickel-cadmium with some exceptions. Capacity dependence as a function of
temperature is strongly influenced by the active material selection of the metal hydride and
nickel hydroxide, and typical commercial NiMH performance is presented in Fig. 30.15. At
low temperature, NiMH has reduced discharge capacity and power compared to NiCd be-
cause the metal hydride electrode is polarized due to the generation of water on discharge.
On the other hand, whereas NiCd has some difficulty with charging at cold temperatures,
and particularly fast charge at cold temperature, NiMH is much less sensitive to this effect.
High temperature charge efficiency is a matter of significant importance for propulsion
applications. Electric and hybrid electric vehicles will be used in warm weather climates,
where vehicle range in the summer is of critical importance to the end user. First generation
NiMH batteries for EV application would lose almost 50% of room temperature capacity
when charged at 60
⬚C. The problem with high temperature charge acceptance is the oxygen
evolution characteristics of the nickel hydroxide positive electrode. Normally, at room tem-
perature, the nickel hydroxide electrode has almost complete charge acceptance until about
80% charge input when the competing reaction of oxygen evolution begins. At full charge,
continued charging causes 100% oxygen evolution at the nickel hydroxide electrode, and
the oxygen migration to the metal hydride or cadmium electrode provides the well known
oxygen recombination overcharge mechanism.
At elevated temperatures, the issue is therefore that oxygen evolution occurs at much
lower states of charge, and total charge acceptance is reduced. To combat this premature
oxygen evolution mechanism, manufacturers have introduced oxygen evolution suppressants
PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.25
such as Ca(OH)
2
, CaF
2
,orY
2
O
3
. Another method is to modify the formula of the nickel
hydroxide itself. The most common NiMH positive active material is NiCoZn. To improve
high temperature performance, multielement formulas such as NiCoZnCaMg materials have
been developed. As shown in Table 30.4, introduction of these additives can reduce capacity
loss during 65
⬚C charging from about 50% to below 10%. NiMH manufacturers must care-
fully select the oxygen suppressant type, amount and location to avoid deleterious effects
such as power loss and cycle life reduction due to the nonconductive nature of many of
these materials.
Another aspect of performance is the effect of storage at temperature on life (discussed
later). Extended storage over temperatures of about 45
⬚C can reduce life due to degradation
of the separator, oxidation and corrosion of the metal hydride alloy, and disruption of the
cobalt conductive network in the positive electrode. Each of these mechanisms is highly
dependent on the manufacturer’s choices of the materials.
TABLE 30.4 High Temperature Charging Efficiency
Performance in NiMH C-Cell Battery
Charging Efficiency for C-cell batteries at 65⬚C
Commercial Nickel Hydroxide (Ni
94
Co
3
Zn
3
) 52%
Commercial Controls
⫹ Ca(OH)
2
External Additive 83%
Advanced NiCoZnCaMg Formula 92%
30.9.4 Charge Retention
Charge retention is also highly affected by temperature depending on the manufacturers’
choice of the specific metal hydride alloy formula, separator material, and the quality of the
nickel hydroxide active material. Because these same selection criteria also apply to small
portable NiMH batteries, large industrial NiMH batteries have very similar charge retention
performance. There are two main self discharge mechanisms; the oxygen instability of the
nickel hydroxide electrode and shuttle mechanisms where chemical species may oxidize at
the positive, diffuse to the negative electrode and be reduced, with the redox mechanism
repeated.
34
Charge retention is an area where NiMH manufacturers compete and the end user must
compare all performance properties for a given design. For example, advanced separator
materials have the ability to reduce self-discharge from about 30% loss per month at room
temperature to about 15%. However, these same materials may reduce cycle life from 15 to
50%. The chemical mechanisms by which the separator reduces self discharge are compli-
cated, involving chemical grafting agents that can bind and thereby inactivate chemical spe-
cies which may promote self discharge. However, these separator treatments may also have
deleterious effects on the amount of electrolyte absorption and the ability of the separator
to retain the electrolyte during cycling.
The metal hydride alloy has similar design tradeoffs. Higher capacity AB
2
alloys also
have traditionally had higher rates of self-discharge compared to the lower capacity AB
5
alloys discussed previously. The mechanisms for the MH alloy effect on self-discharge are
twofold. First, corrosion products from the hydride alloy may migrate to the nickel hydroxide
positive electrode and promote oxygen evolution during storage. Second, other corrosion
products such as vanadium with multivalent oxides may form redox shuttle mechanisms
similar to that for nitrate ions. The quality of the nickel hydroxide affects self-discharge due
to amounts of residual impurities such as nitrates and carbonates, which can influence self-
discharge mechanisms. Achievement of ultra-low levels of impurities may incur increased
processing costs.
30.26 CHAPTER THIRTY
30.9.5 Cycle Life
Cycle life for industrial NiMH batteries has both similarities and differences to those of
small portable NiMH batteries. Cycle life for small portable NiMH batteries can vary from
manufacturer to manufacturer, but usually falls in the range of 500 to 1000 cycles (100%
DOD under 2 hour charge/ discharge). Design and chemistry factors affecting cycle life that
are common to both large and small NiMH batteries include:
1. Metal hydride electrode
•
Alloy formula (oxidation /corrosion properties)
•
Alloy processing effect on microstructure (particle disintegration)
•
Electrode construction (swelling in x-y-z direction, stability of conduction pathways)
2. Nickel hydroxide electrode
•
Active material formula (swelling and poisoning resistance)
•
Conductive network stability (amount and type of cobalt oxides)
•
Substrate (pore size, strength and resistance to fracture)
3. Cell design
•
N/ P ratio (amount of excess negative electrode capacity to influence cell pressure, MH
corrosion, disintegration)
•
MH discharge reserve (overdischarge protection)
•
Separator (stability to corrosion, electrolyte absorption and retention, thickness and re-
sistance to short circuit)
•
Electrolyte (composition, amount, and fill fraction)
•
Vent pressure (weight loss, charge imbalance)
•
Electrode stack design (compression, electrode thickness, aspect ratio of height to
width)
Factors affecting cycle life which are different from small portable applications for large
industrial NiMH applications include:
•
Significantly higher typical battery voltages (42 to 320 V vs. 12 V) increases risk of abusive
overcharge and overdischarge due to capacity or state-of-charge mismatch.
•
Overall higher energy (0.1 kWh vs. 33 kWh) increases heat generation and criticality of
thermal management, which can be further influenced by battery pack enclosure heat trans-
fer limitations.
•
Typically higher operating temperature. Air cooled and water cooled vehicle batteries are
usually operating at a temperature of 35
⬚C or higher whereas operating temperatures for
small portable batteries may experience transient high temperatures, but on average are at
or near room temperature.
•
Series/ parallel strings. In small portable batteries, there are a large number of cell sizes
available, ranging from 100 mAh button cells to 7 to 12 Ah D and F cells. There are many
fewer cell sizes for HEV and EV NiMH batteries. Consequently, vehicle applications may
be required to use series /parallel combinations of NiMH cells to meet the required pack
voltage and energy demands, and thereby increase the risk of pack imbalances.
•
End of life definition for small portable batteries is usually based on capacity loss. In
contrast, NiMH for EV and HEV applications find end of life is due to power limitations.
•
Qualification and operation testing. The emphasis on power greatly influences testing and
methodology. In small portable battery cycle life testing, it is most common to use 1 or 2
hour constant current charge and discharge, usually to 100% DOD each cycle. For EV
cycle life testing, discharge is usually a variable current /time profile to simulate driving
conditions—the so-called DST driving profile. The significance of pulsed discharge cycle
life testing is that the high current pulse dominates the test. On the other hand, most EV