Energy Management Strategy and Size Optimization of a LFP/LTO Hybrid Battery System for Electric Vehicle

This paper proposes a semi-active hybrid battery system (HBS), composed by lithium iron phosphate battery (LFP) and lithium titanate battery (LTO) for electric vehicle (EV) to reduce the life cycle cost of energy storage system. Firstly, the topology of this HBS is introduced. The high energy-density battery, LFP is adopted as the primary energy source, while the high power-density one, LTO is connected in parallel with a bidirectional DC-DC converter and used as secondary energy source to extend the lifetime of HBS by reducing the current stress of LFP. The dynamic model of this HBS is built, in which, the LFP and LTO are both modeled as second-order RC model. In addition, dynamic semi-empirical degradation model of the LFP battery is chosen to estimate the lifetime of HBS. Secondly, a fuzzy logic controller with 3 inputs and 1 output is proposed to decide the power split between the primary and secondary power sources. LFP and LTO both could provide power to drive the vehicle, while the electricity generated by regenerative braking is only stored in LTO. The parameters of the controller and the size of HBS are optimized simultaneously to minimize the life cycle cost. Simulation results show HBS can mitigate the capacity loss of LFP, increase the battery life by 46.04% and reduce the average annual cost by 18.06% compared to single LFP battery system. Meanwhile, the optimization of the energy management strategy indicates the system prefers charge-depletion-charge-sustaining mode for reducing the degradation rate of LFP.