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Hybrid Lithium-ion/Ultracap energy storage systems for plug-in hybrid electric vehicles

Battery technologies to maximize power density and energy density simultaneously are not commercially feasible. The use of bi-directional DC-DC converter allows use of multiple energy storage systems, and the flexible DC-link voltages can enhance the system efficiency and reduce component sizing. In this paper we have conducted vehicle level study and modeling to quantify the benefit of bi-directional DC-DC converter in hybrid energy storage systems for vehicles. The goal of this study is to reduce the overall cost of plug-in hybrid electric vehicle (PHEV) and demonstrate high power density and efficiency by hybrid energy storage system, including a Lithium-ion battery, an Ultracap, and two DC-DC converters. The simulation results show the PHEV with hybrid energy storage system has better performance over the conventional PHEV. The hybrid energy storage system allows the best utilization of Ultracap and battery technologies for both high power density and high energy density.