A Fast Equalizer with Mechanisms of Bidirectional Energy Transfer and Balancing Current Control

In this study, an active equalizer for the rapid balancing of series-connected lithium-ion battery strings is proposed. A buck-boost converter with bidirectional energy conversion serves as the power stage to make the energy shuttle back and forth from cell to sub-pack or sub-pack to sub-pack realizable. In order to deal with the problem of slow balancing rate, which inherently arises from the reduction in balance current as the SOC/voltage difference between the cells/sub-packs decrease in the later period of equalization especially, an adaptive varied-duty-cycle (VDC) strategy is proposed to shorten the balance time. The devised method has taken the battery nonlinear behavior and circuit parameter nonideality into consideration and can adaptively modulate the duty cycle with the change in SOC/voltage differences to maintain balancing current nearly constant through the equalization process. Simulated and experimental results are provided to validate the correctness and effectiveness of the equalizer prototype constructed. Comparing with the conventional fixed duty cycle (FDC) methods, the improvement of 56% in balance time has been achieved.