Environmental Engineering Reference
In-Depth Information
NiMH
d
V
0
20
40
60
SOC (%)
80
100
Figure 10.10 NiMH charge characteristic
The charge characteristic for an NiMH cell is illustrated in Figure 10.10.
Because of the shallow slope in voltage for SOC values from 40% to 80%, it
becomes difficult to implement a simple charge controller.
Because the cell voltage increment is very shallow with increasing SOC, and
perhaps not even monotonic, charge control of NiMH is more difficult than for
lithium ion systems. Even more serious of an issue with NiMH is their precipitous
drop in pulse power with decreasing temperature. It is common for NiMH to
be limited to less than 40% of its 20
C capacity at
20
C. This is illustrated in
Figure 10.11, where both discharge and charge power characteristics are shown.
In Figure 10.11 a 30 cell, 16 Ah, 42 V nominal pack having pulse power
capability of 15
C
(3 s at 20
C) is shown to decrease to 30% of this when cold. At
20
C the pack is only capable of ~5 kW. A 36 cell module, on the other hand, is
capable of 6 kW at
20
C for 3 s. The module's internal resistance is on the order
NiMH pack at 50% SOC
30 cell 50 V, 36 cell 58 V
36 cell
15
10
30 cell
5
0
-5
-10
-15
-30
-20
-10
0
10
20
30
40
Temperature (°C)
Figure 10.11 NiMH power capability versus temperature (30 cell versus
36 cell string)
