An Isolated Resonant Voltage Balancing Charger of Series-Connected Lithium-Ion Batteries Based on Multi-Port Transformer

The inconsistency of individual lithium-ion batteries causes the voltage imbalance of the batteries. An effective voltage-balancing circuit is essential to improve the inconsistency of series-connected batteries. This paper presents an isolated resonant voltage-balancing circuit for series-connected lithium-ion batteries based on a multi-port transformer. This circuit utilizes the multi-port transformer to enable free energy flow among the batteries. Without any direct transmission path between the capacitor and the battery string, it achieves full isolation between them. The resonant circuit is adopted to realize the soft-switching operation. Compared with other active balancing circuits, the proposed circuit requires only half a winding and one transistor per individual battery. Consequently, the proposed circuit enhances power density and further improves efficiency and reliability. Additionally, a fixed-group-number control strategy is introduced to enhance the circuit’s equalization voltage capabilities. Finally, a prototype of the voltage-balancing circuit for 24 series-connected lithium-ion batteries is established to verify the effectiveness and feasibility of the proposed circuit.