SOC Estimation-Based Quasi-Sliding Mode Control for Cell Balancing in Lithium-Ion Battery Packs

The successful and safe operation of a serially connected battery pack necessitates dynamic energy equalizing to adjust each cell's state of charge (SOC) to the same level, since there exists energy imbalance among its cells. Bidirectional Modified Ćuk converters are utilized as the cell equalizers due to their advantages of integrated infrastructure and modular design. Distinguished from the literature, the maximum allowed cell equalizing current is designed to vary with the change of the battery pack's external current rather than a constant to avoid the cell's current exceeding its limitation. With adaptive quasi-sliding mode observers designed for the cells' SOC estimation, a discrete-time quasi-sliding-mode-based strategy with saturated equalizing current constraints is proposed to have the converters work together efficiently to achieve the cells' SOC equalization. As analyzed in mathematical proofs and shown in experimental results, the cells' actual SOC differences can converge to a tolerant range around the origin in a relatively short time under the designed SOC estimation-based cell balancing method.