A novel dual-inductor based charge equalizer for traction battery cells of electric vehicles

The battery packs in EV (electric vehicle) applications are generally composed of lots of battery cells connected in a series or parallel way. Differences in battery cells may shorten the lifetime of the battery pack, and even worse, lead to fire or explosion. Cell balancing or charge equalization is an effective way to relief the problems caused by cell imbalance. A novel dual-inductor based charge equalizer is proposed with a detailed investigation by theoretical analysis, simulations and experiments. A balancing strategy based on periodical cell OCV (open circuit voltage) measurements is also developed to facilitate the implementation of the proposed equalizer. The charge equalizer is capable of transferring excess energy from an arbitrary battery cell to the battery module through a discharge bus realized with a switch array. With the assistance of a flying capacitor, the requirement on control synchronization of frequency switches can be lowered. The proposed charge equalizer is easy to be implemented in a BMS (battery management system), and the experimental results show an acceptable efficiency of 86% in the balancing of a battery module composed of 6 series-connected LiMnO2 based cells.