Optimal Performance of a Full Scale Li-Ion Battery and Li-Ion Capacitor Hybrid Energy Storage System for a Plug-In Hybrid Vehicle

A semi-active hybrid energy storage system, consisting of a Li-ion battery pack, dc/dc converter, and Li-ion capacitor pack is developed for a range extended plug-in vehicle application. The vehicle has a series-parallel drivetrain with two electric motors, a gas engine, gearbox, and a clutch to allow the engine to run decoupled from the gearbox in range extending mode. The peak dc electrical requirement of the electric drivetrain is about 175kW, which is similar to the peak power capability of the developed hybrid energy storage system. A model of the prototype hybrid energy storage system, which has the Li-ion capacitor pack connected directly to the motor drive's dc bus and the battery pack connected to the Li-ion capacitor pack via a dc/dc converter, is developed and used to determine the optimal power split between the battery and Li-ion capacitor packs. A dynamic programming algorithm is used to determine the optimal power split, with the optimization goals of reducing energy storage system loss, maximizing regenerative braking energy capture, and minimizing motoring power limiting. The hybridized system is shown to reduce battery pack losses and increase vehicle range compared to a system only utilizing the battery pack.