Optimal sizing of hybrid high-energy/high-power battery energy storage systems to improve battery cycle life and charging power in electric vehicle applications

Design of the Electric Vehicle (EV) battery pack involves different requirements related to the driving range, acceleration, fast-charging, lifetime, weight, volume, etc. Therefore, sizing of the EV battery pack necessitates a multi-objective optimization study to achieve the right trade-off considering the aforementioned factors. This “trade-off” can vary depending on the type and size of the EV, as well as use cases. In this regard, a nice solution is to use a hybridized battery pack consisting of both High-Energy (HE) and High-Power (HP) battery cells, which will help to meet a wider range of customer requirements. Hybridization decouples energy and power and thus increases design flexibility to achieve a better trade-off for a wider range of EV applications. This paper proposes an effective framework for optimal sizing of such hybridized battery packs for a typical EV, namely the Mitsubishi MiEV. Lithium-ion cells with Nickel Manganese Cobalt Oxide (NMC) and Lithium Titanate Oxide (LTO) chemistry are chosen as HE and HP cells, respectively. A detailed analysis is fulfilled to determine the best hybridization topology, e.g. voltage profile, interface with DC-link, etc. The genetic algorithm is used to solve the multi-objective optimization problem considering two different driving cycles, namely the New European Driving Cycle (NEDC) and Worldwide Harmonised Light Vehicle Test Procedure (WLTP). The results favor the usefulness of the hybrid battery pack to simultaneously achieve lifetime and charge power requirements compared to mono battery systems. The hybrid pack offers >+40,000 km improvement in the achievable driving when an end-of-life criterion of 70 % for the cell capacity is considered. Remarkably, the optimized hybrid pack can also be fast charged up to 70 % within 6 min.