Proposal of Soft SOC Balancing Method to Two Battery HEECS Chopper Used for EV Power Train

Abstract

The paper proposes a solution of State of Charge (SOC) unbalancing problem, which occurs to the two battery High-Efficiency Energy Conversion System (HEECS) chopper used for a power train of Electric Vehicle. This two battery HEECS de/de chopper shows a very high efficiency over 99%, based on the principle of a partial power conversion [1]. However, if SOC's of two batteries in this chopper do not become zero at the same time, the chopper may not work well and as a result, the vehicle can no longer run. As a solution of this problem, the soft SOC balancing method for two batteries on HEECS is proposed, in which any hard ware is required, and by the control algorithm the SOC's will be balanced. Simulation results showed that SOC's unbalancing can be solved by changing de/de chopper duty ratio from PWM to fixed duty depending on SOC's of two batteries. Based on this proposal, the driving distance can be extended 7.6 % under 10–15 driving mode test cycles. The validity of the proposal soft balancing technique was verified under another three types of driving mode cycles such as JCOS, Worldwide harmonized Light vehicles Test Cycle (WLTC) mode Low speed phase, and WLTC mode Medium speed phase.