Design methodology and experiment on battery SOC balance circuit of HEECS for EV power train

For the extension of driving distance of Electric Vehicle (EV), a new power train with high efficiency DC-DC converter was reported by authors [1]. To reduce the conduction loss of this DC-DC converter, High Efficiency Energy Conversion System (HEECS) using two batteries was proposed in [3]. The maximum efficiency of HEECS is reported over 99.5 % in [4]. However, HEECS has one drawback that SOC(State of Charge) of two batteries may become unbalanced at the end of discharge. To avoid the SOC unbalance problem, three methods are proposed and investigated based on several driving patterns. It was observed that depending on driving patterns, the SOC unbalance level changed. Thus, SOC balancing circuit is required and the power rating of the balance circuit should be designed based on the proposed methodology. Finally a prototype of a high efficient battery SOC balance circuit was constructed and operated with efficiency 96.2% at 1 kW output.