A Novel Balancing Algorithm for Lithium Battery Strings Using Bidirectional Charge Circulation

In energy storage systems (ESS), numerous battery cells are connected in series to elevate the bus voltage. However, the manufacturing tolerance between the cells often introduces an imbalance in the battery string, which can lead to safety issues. To address this issue, a battery equalizer is employed to harmonize the energy levels among cells. The conventional balancing method utilizes unidirectional converters to redistribute charge among cells with high efficiency and minimal energy loss. However, this approach lacks flexibility in energy transfer direction, which can impede the balancing speed. To overcome this challenge, this paper introduces a bidirectional converter-based cell-to-string equalizer featuring a high-speed balancing algorithm. The performance of the balancing process is confirmed through experiments involving 20 series-connected cells. The results show that the open-circuit voltages of the cells are balanced to within the voltage deviation of 21mV. In addition, the proposed method reduces the balancing time by 50% in comparison to conventional techniques.