Lightweight Electric/Hybrid Vehicle Design

32 Lightweight Electric/Hybrid Vehicle Design
Ni–Cad cells
BatteryType RR C Sub D D
Cell capacity (Ah) 1.0 1.7 2.8 4.0
Int. resistance* (milliohms) 4.5 3.5 3.0 2.8
Cell dimensions, DxL (mm) 22x34 22x56 32x42 32x56
Weight (gm) 42 65 130 200
Int. Resistance of 4 Ah milliohms 1.1255 1.6 2.5 2.8
*Internal Resistance is at 20° C and 50% state of charge.
Fig. 2.2 Typical Ni–Cad packages and capacities.
2.2.2 INTERNAL RESISTANCE PERFORMANCE OF FAST-CHARGE NI–CAD CELLS
The results discussed here are based on Sanyo products but the same trends are seen in Varta,
Panasonic and Saft cells. See Fig. 2.2.
Why does internal resistance matter so much? This is because at 25° C discharge rate, the
voltage drop on a 1.2 V cell is as tabulated below:
Voltage drop on a 1.2 V cell at 25° C discharge rate
RR C Sub D D
112.5 mV 160 mV 250 mV 280 mV
The 1.2 V cell is thus no longer a 1.2 V cell but closer to 1 V at 20° C. Why does the 1 Ah cell
win? It is because it is short and fat – the others are long and thin. Cell geometry is the decisive
factor for low internal resistance.