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ELECTRIC VEHICLE BATTERY CHARGING coefficient built into the charge management system should allow the charge current or clamp voltage to be regulated with any change of battery temperature.
Multistep Algorithm for Charging a Series String of VRLA Batteries VRLA batteries, nominally rated at 12.0 V are best charged using a multistep charging algorithm that follows these steps: • Step 1 A constant current is applied to the series string of VRLA batteries. The charging current is applied until the first battery (nominally rated for 12 V) in the string reaches a voltage of 15.5 V or until the last battery in the series string reaches a voltage of 14.5 V. At this point, the current being applied to the series string is reduced to approximately 50% of the initial start value to prevent loss of water due to gassing. • Step 2 The reduced constant current is applied until the first battery again reaches a voltage of 15.5 V or until the last battery in the series string reaches a voltage of 14.5 V. At this point, the current being applied to the series string should be reduced to approximately 50% of the current applied to start Step 2. • Step 3 The current is again reduced by half as in Steps 1 and 2 until the current being applied to the first battery in the battery pack is at 1% of the battery’s three-hour rated capacity. For example, a 1% current of 90 Ahr battery is 0.90 A (approximately 1 A). • Final Step The constant current is applied until all battery voltages have risen less than 0.01 V in a 15-minute time period. This equalization time period is important as it brings all the batteries in the pack within a 5 to 10% range of the first battery achieving the charge in Step 3. Furthermore, it is important that this step must not exceed six hours to prevent gassing of the batteries, resulting in the loss of water vapor.
TEMPERATURE COMPENSATION DURING BATTERY CHARGING The on-charge voltage limits must be compensated for temperature to account for the variation of the useful battery capacity with temperature. Table 4–1 summarizes the compensated lower and the upper
