Definition of VRLA Battery Capacity

Figure 6–1 Dynamic driving battery discharge test profile.

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Battery Voltage (V) 13

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Discharge Time (secs)

tunately, it is difficult to keep the battery pack temperature, housing hundreds of cells at a constant temperature. This is owing to the fact that the batteries that are on the outermost edges have a greater surface for heat exchange while the batteries in the middle of the pack have the least available surface for exchange of heat. The stacking arrangement of the batteries in the pack leads to nonuniform temperatures, which in turn lead to nonuniform discharge and charge characteristics of the battery pack.

DEFINITION OF VRLA BATTERY CAPACITY

As part of a battery pack configuration, a major problem experienced with the EVs is the premature decline of battery capacity, which ultimately leads to battery failure. The primary cause of the battery pack failure is owing to repeated nonuniform discharging and charging of the cells. Both the battery charging and discharging are highly dependent upon temperature. Owing to the large temperature difference between the coolest battery or batteries on the outer edge of the battery pack, and the hottest battery or batteries on the inner side of the battery pack, there is a corresponding variation in the available battery discharge capacity.

Figure 6–2 Variation of battery capacity with respect to discharge temperature.

Capacity (AHr) 180 160 140 120 100 80 60 40 20 0 400 350 300 250 200 150 100 50 40 30 20 10 Discharge Current (A)

T = 140F T = 90F T = 40F T = 130F T = 80F T = 30F T = 120F T = 70F T = 20F T = 110F T = 60F T = 10F T = 100F T = 50F T = 0F

The battery pack cut-off point is normally determined by a predefined total pack voltage. This pack voltage is with respect to a particular current. As the battery pack undergoes discharge, the coolest cells with less available capacity are discharged further to a lower state of charge (SOC) than their hotter counterparts. During the discharge, the capacity of the coolest batteries may be reduced enough to force them into reversal of polarity (i.e., the batteries are reverse charged while the other batteries in the pack are discharged normally). Gradually these repeated over-discharges reduce the battery life. Equalization charges applied to the battery pack balance the batteries. However, the variations in temperature bring back the same discharge and charge limitations as before the equalization.

The temperature gradient affects larger battery packs more significantly, resulting in capacity imbalance and charge acceptance problems that lead to early battery failures. The capacity (CT) of the battery pack is determined on a per cell basis using a linear two-hour discharge curve based on the equation below. This equation may overestimate the capacity at extreme temperatures.

CT = C30 ¥ [1 + 0.008 ¥ (T - 30)] where T is the temperature in °F.