The hybrid energy storage system (HESS) uses the retired power battery can prolong the battery life cycle and lower the system's cost. A series-parallel resonance switched-capacitor equalizer for the HESS is proposed in this paper, which uses series-parallel resonance switched-capacitor to realize the balance. In addition, the use of series-parallel resonance circuits can realize the conversion of different voltage levels, and energy can flow through the HESS via equalizer. Therefore, the entire HESS only needs one equalization circuit to achieve system equalization. Moreover, a prototype consisting of a ternary-lithium battery pack, a polymer-lithium battery pack and a supercapacitor pack is built

—Hybrid Energy Storage System;switch capacitor;voltage balance