Bidirectional Constant Current String-to-Cell Battery Equalizer Based on L2C3 Resonant Topology

In battery equalization systems, equalization speed, control complexity, and length of energy flow path are the important figures of merits. This article presents a novel bidirectional L2C3 resonant converter in a unique equalizer architecture with constant balancing current and fixed frequency control. The constant balancing current can be customized to achieve a stable balancing speed. The common equalizer unit transfers power bidirectionally between the entire string and any single cell. Design considerations of L2C3-based bidirectional equalizer are analyzed in detail, which ensures zero-voltage switching among all mosfets during the equalization process. The circuit is designed with synchronous rectification using the first harmonics approximation. The key contributions of this article include: a new bidirectional resonant topology for battery equalization; bidirectional constant current balancing with open-loop control; simplified synchronous rectification. An equalizer prototype with four lithium-ion battery cells at a balancing current of 500 mA is built and tested. The L2C3 circuit operates at 200 kHz with a peak efficiency of 89.4% and 90.1% under two balancing modes. Experimental results show the proposed scheme exhibits outstanding balancing performance.