Hybrid energy storage systems and battery management for electric vehicles

Electric vehicles (EV) are considered as a strong alternative of internal combustion engine vehicles expecting lower carbon emission. However, their actual benefits are not yet clearly verified while the energy efficiency can be improved in many ways. The carbon emission benefits from EV is largely diminished if we charge EV with electricity from petroleum power plants due to power loss during generation, transmission, conversion and charging. On the other hand, regenerative braking is direct power conversion from the wheel to battery and one of the most important processes that can enhance energy efficiency of EV. Power loss during regenerative braking can be reduced by hybrid energy storage system (HESS) such that super-capacitors accept high power as batteries have small rate capability. Conventional charge management does not systematically exchange charge between the supercapacitor and battery. However, asymmetry in acceleration and deceleration as well as battery charging and discharging capability make the supercapacitor state of charge (SoC) management override the efficiency optimization. Unlike previous works, we show how charge migration during idle and cruise/stopping time can be beneficial in terms of energy efficiency and cruise range. Systematic charge migration decouples SoC management and charging efficiency optimization giving a higher degree of freedom to charging efficiency optimization. We demonstrate the proposed charge migration between the supercapacitor and battery improves energy efficiency by 19.4%.