A high efficiency isolated bidirectional equalizer for Lithium-ion battery string

Equalization for battery pack to enhance its usage life and performance is important for xEVs. In this paper, an isolated bidirectional equalizer based on full-bridge for Lithium-ion battery string is proposed. If a battery charge is excessive, the equalizer transfer charge from this cell to battery string; if a battery charge is insufficient, the equalizer will charge this cell from battery string. Closed-loop control keeps the equalization current constant, allowing precise control of the transferred charge. On the basis of considering OCV hysteresis, we proposed an equalization strategy based on SOC estimation for the equalizer. The equalizer features small size, high efficiency, and high accuracy. Experimental results confirm the applicability of the equalization scheme.