A high performance bi-directional AC-DC converter for charge equalization

To achieve high voltage and power in propulsion systems, cascade battery stacks are essential. Because of small discrepancies in charge or discharge characteristics, even between identical batteries, voltage imbalance would happen in cascaded battery stacks which get worse over the time with battery cell degradation. In this paper, a multilevel bi-directional battery charger with two operation modes for cascaded battery stack is developed. Grid to Vehicle (G2V) mode is an Electric Vehicle (EV) battery charger with reactive power control, controllable input current and charge equalization over cascade battery cells. Vehicle to Grid (V2G) mode is considered as an auxiliary energy storage device with active and reactive power control. The proposed charge equalization control uses each voltage cell as a state of charge indicator to prevent over charge or discharge of the cascaded cells which is very harmful for battery cell lifetime.