Onboard energy storage system (ESS) plays a major role for vehicle electrification. The performance of hybrid electric vehicle (HEV), plug-in HEV (PHEV), extended range electric vehicle (EREV), fuel cell vehicle (FCV), and electric vehicles (EV) heavily depends upon their ESS technology. The ESS must be able to store sufficient energy for adequate pure electric range, provide adequate peak power for needed vehicle performance under various driving cycles, absorb energy efficiently during regenerative breaking, and have long operation life and low costs. At present, pure battery based ESS often cannot effectively meet all these requirements due to many trade-offs. In order to improve the overall performance of ESS, integration of two (or more) energy sources have been studied to best utilize the unique characteristics of each, leading to a hybrid energy storage system (HESS). Hybridization of high-energy batteries and ultracapacitors with complementary characteristics present a common choice today. In this paper, the necessity and superiority of a HESS are illustrated considering system performance, efficiency, costs, functional life, and temperature requirements. Three major types of battery-ultracapacitor HESS, passive, semi-active and fully active, are presented. Various HESS control strategies proposed in the past are then reviewed, including rules or reference curves and tables based control, fuzzy logic control, and closed-loop control. Building upon these review and analyses, a novel control strategy based on signal separation using sparse coding is proposed at the end.

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