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Lithium-ion-based battery energy storage system has started to become the most popular form of energy storage system for its high charge and discharge efficiency and high energy density. This paper proposes a high-efficiency grid-tie lithium-ion-battery-based energy storage system, which consists of a LiFePO4-battery-based energy storage and a high-efficiency bidirectional ac-dc converter. The battery management system estimates the state of charge and state of health of each battery cell and applies active charge equalization to balance the charge of all the cells in the pack. The bidirectional ac-dc converter works as the interface between the battery pack and the ac grid. A highly efficient opposed-current half-bridge-type inverter along with an admittance-compensated quasi-proportional resonant controller is adopted to ensure high power quality and precision power flow control. A 1-kW prototype has been designed and implemented to validate the proposed architecture and system performance.