Bidirectional Constant Current S2C Battery Equalizer Based on Fixed-Frequency L2C3 Resonant Converter

In battery equalization systems, equalization speed, control complexity and length of energy flow path are the important figures of merits (FoMs). In this paper, a bi-directional equalizer is proposed to achieve a good compromise among those three FoMs. The proposed equalizer is based on a shared L2C3 resonant converter. The common equalizer unit is in charge of bidirectional charge transfer between the entire string and any single cell. A constant balancing current is achieved with simple open-loop control at fixed frequency. The constant balancing current can be customized to achieve a fast balancing speed. Design considerations of L2C3 based bidirectional equalizer are analyzed in detail, which ensures zero-voltage switching (ZVS) among all MOSFETs during the equalization process. A prototype with eight lithium-ion battery cells is implemented. Experimental results show the proposed scheme exhibits outstanding balancing performance.