An Anti-Racing PWM-based Battery Pack Equalization

Abstract

Following the trend of eco-conscious, the automakers tend to use high-voltage packs in the electric vehicle or hybrid electric vehicle. The packs are formed by battery cells in series and parallel connections. Because of the variation of battery production, their capacity, impedance, lifetime and chemistry characteristic vary with ambient temperature. When the batteries are charged or discharged as a pack, the differences result in the imbalance of the battery voltage. The imbalance turns out over-charged and over-discharged and shortens the capacity and lifetime of batteries. Among the equalization methods, resistively switching approach is widely used for its low complexity and low cost. The controller gauges the battery voltage no matter the equalization is on or off. The comparison of two battery voltages must be based on the charging current. If not, the racing phenomenon happens and the equalization circuit would be turned on and off frequently. PWM-based equalization can solve this problem. Detecting the voltages of the batteries when the duty is off ensures the battery voltages are both based on the charging current. The PWM-based equalization successfully eliminates the racing phenomenon and shortens charging time by 48%; from 6200 seconds of switching shunt resistor equalization to 3200 seconds.