Environmental Engineering Reference
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as 40 psig. Due to their novel construction, VRLA batteries are orientation flexible
and can operate lying on their side, or under rotation when used in wind turbine
blade pitch adjustors. But these applications either have or are being replaced with
ultra-capacitors.
10.1.2 Nickel-metal hydride
It is derived from what are commonly referred to as mischmetal compositions of
either lathium-nickel (AB 5 -LaNi 5 ) or titanium-nickel (AB 2 -LaNi 2 ) alloy. If these
alloys are referred to as 'M', the NiMH cell with potassium hydroxide (KOH)
electrolyte becomes
M(H) þ 2NiO(OH) ¼ M þ 2Ni ð OH Þ 2
ð 10 : 17 Þ
where the mischmetal, or hydrogen electrode, coupled with the nickel oxyhydr-
oxide electrode reacts with base mischmetal and nickel hydroxide.
The capacity of NiMH cells is relatively high, but its cell potential is low, only
1.35 V as it was with NiCd systems, in fact as it is in all the nickel chemistries such
as NiZn. Gravimetric energy density is ~95 Wh/kg and volumetric energy is
~350 Wh/L. NiMH does not have the high discharge rate capability of NiCd, but it
shares a cell structure similar to NiCd. NiMH also suffers from relatively high self-
discharge, it is more sensitive to overcharge/overdischarge than NiCd, it requires
constant current charging and it is more problematic for hybrid propulsion systems
because of very reduced performance at cold temperatures. The problems with
overcharging and discharging mean that some form of battery management system
is necessary, as with all high performance advanced batteries. Figure 10.7 illus-
trates the discharge behaviour of NiMH cells, where nominal potential at 20 Cis
1.35 V. The NiMH cell capacity diminishes rapidly as discharge rate is increased.
1.35
1.2
1.1
1.0
0.9
4C
2C
1C
C/8
C/10
0
20
40
60
80
100
120 %DOD
Discharged capacity (%) at 20 °C (68 °F)
Figure 10.7 Discharge curve for nickel-metal hydride (NiMH) cell
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