Abstract
In the application of a long series battery group, an inter-pack imbalance is inevitable. No intra-pack cell equalizer can prevent pack-level over-discharge. A bimodal, multichannel battery pack equalizer based on a quasi-resonant, two-transistor forward converter is proposed to solve this problem and achieve a tradeoff between balancing efficiency and speed. This equalizer has two modes: pack-to-pack-group and pack-to-any-pack (P2PG&AP) mode and direct-pack-to-pack (DP2P) mode. In P2PG&AP mode, this equalizer can realize the full-switching-cycle (FSC) equalization through three balancing channels, and transfer energy from any pack to both the whole group and any pack inside the group. In addition, it can effectively clamp the transformer-induced voltage using a secondary side two-transistor magnetic reset structure (STMR) and reduce the total turns of transformer coil from 70 to 50 turns via a secondary side boost converter (SBC). In DP2P mode, this equalizer can realize zero voltage gap (ZVG) equalization. A prototype was tested at different switching frequencies and LC values to validate the theoretical analysis and optimize the bimodal hybrid operation. Experiment results including higher than 89.66% efficiency and minute-level balancing time under different pack voltage distributions show that the proposed topology demonstrates excellent balancing performance.