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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.