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ELECTRIC VEHICLE BATTERY CAPACITY
Load Voltage (V)
Figure 3–3 Battery pack depth of discharge (%DOD) at room temperature.
10
5
0 0
20
40
60
80
100
Battery Capacity (Ahr) C/3 Discharge @ 31.7 A
C/2 Discharge @ 41.5 A
C/1 Discharge @ 78 A
However, if this discharge is not regulated, it leads to premature battery failures due to deep discharge, leading to loss of electrolyte and gassing of the VRLA battery. As the battery pack discharge rates vary, so does the battery pack performance. An identical depth of discharge (%DOD) may be achieved, at room temperature, using different rates of discharge at varying current levels. A fully balanced battery pack consisting of 80 Ahr VRLA batteries, delivers identical performance at C/3, C/2, and C/1 discharge rates.
STATE OF CHARGE OF A VRLA BATTERY The state of charge (SOC) of a sealed VRLA battery is defined as the percentage of full charge capacity remaining in the battery. This information is identical to the combustion engine fuel gauge. In case of the EV, the SOC provides an indication of the amount of electrical energy remaining in the battery pack. The SOC is accurately determined by the measurement of the stabilized open circuit voltage (OCV). Unfortunately, the VRLA batteries require at least five hours to stabilize after the recharge is applied. Typically, the 100% SOC OCV for an 80 Ahr battery ranges between 12.9 V to 13.0 V (under room temperature conditions). The 0% SOC OCV
