Modeling of Low-Temperature Operation of a Hybrid Energy Storage System with a Butler-Volmer Equation Based Battery Model

Lithium-ion battery performance is significantly reduced at low temperatures, where substantially increased resistance reduces power capability and lithium plating causes charging limitations. To reduce the low-temperature limitations of an electric vehicle battery pack, a hybrid energy storage system consisting of a battery pack, an ultracapacitor pack, and a dc/dc converter is investigated. A low-temperature battery model that includes a nonlinear resistance based on the Butler-Volmer equation and an ultracapacitor model are developed, and the model parameters are experimentally measured for temperatures from °20°C to 25°C. The models are then appropriately scaled for a full-size electric vehicle and paired with a dc/dc converter loss model. The optimal power split is determined for various drive cycles using a dynamic programming optimization algorithm. It is shown, using both analytical and experimental results, that the hybrid energy storage system is an excellent approach for substantially reducing the total energy storage system losses at low temperatures, as well as increasing regenerative braking energy capture, reducing output power limiting, and increasing vehicle range.