Study on anti-race PWM-based battery pack equalization
|Keywords:||平衡電路;電池管理系統;PWM控制系統;反互震盪;Equalization circuit;Battery management system;PWM-based control;Racing phenomenon|
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 lead to the imbalance of the battery voltages. Furthermore, the imbalance turns out over-charged or over-discharged and also 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 approach shunts the high-voltage battery with a resistor to dissipate the excess energy directly. The battery behavior can be modeled as a circuit of an electromotive force (EMF), internal impedance and capacitors in series or parallel connections. When the equalization is turned on, the resistor is shunted to the battery to bypass part of charging current. However, the decline of the charging current results in the voltage drop across the internal impedance. That is, the battery voltage drops while equalizing. When the equalization is on, the battery voltage drops; when the equalization is off, the battery voltage is back to the level based on the charging current. Yet here is a hidden problem: 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.
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