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

Abstract

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.

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“…This type of circuit is the most adaptable for bidirectional balancing and it has slightly more efficiency than the CTP methods due to less energy losses during switching, although it has similar balancing speed. Some PTC methods are the voltage multiplier method, the full-bridge converter [171], the multiple transformers method [172], the switched transformer method [172], and the multisecondary windings transformer method [172,173]. -Cell-to-Pack-to-Cell Methods (CTPTC or C2P2C): These are higher in complexity as they operate in several directions, from the higher charged cell to the pack (CTP), from the pack to the lower charged cell (PTC), or from Pack to Pack (PTP) when the energy is transferred between stacks in the same pack.…”