抗反互震盪之PWM電池平衡之研究

Abstract

隨著現今環保意識抬頭，許多自動車產業都需要使用到高電壓的電池組來完成全電力裝置或是油電混和裝置，而這些高電壓電池組需要由電壓較低的電池芯大量串並聯後完成。個別電池的溫度、容量、內阻抗、老化程度、化學特性等的不同，當電池串聯充電或放電時，這些因素就會反應在電池電壓的不平衡，電池間的不平衡可能會因為過充或過放而降低電池容量或壽命。平衡電路中，切換式並聯電阻平衡因為成本與複雜度低，目前是較廣為使用的一種平衡法，此方法使用電阻對電壓較高的電池並聯以消耗多餘的電量因而降低其電壓。而此方式潛藏一個問題:當電阻並聯後流經電池的電流減少，電池內部之串聯內阻使電池電壓瞬間下降，使得電池電壓比其他電池還低，導致其他電池亦開啟平衡電路，電池間不斷開關平衡電路互相追趕彼此電壓，而產生反互震盪的現象，一旦反互震盪產生，充電時間會被延長，消耗在開關上的損耗也會增加。PWM電池平衡法在平衡電路工作週期關閉時擷取電池電壓，確保電池芯在相同充電電流的基準下比較其電池電壓差，反互震盪的現象因而消失，充電時間從原本切換式並聯電阻平衡需要6200秒降低至PWM平衡電路只需要3200秒，減少了48%的充電時間。

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.