Battery energy storage system are widely used and become the most popular form of energy storage system. This paper proposes a grid-tie Lithium-ion battery based energy storage system, which consists of LiFePO4 battery based energy storage and a high-efficiency bidirectional ac-dc converter. The battery management system (BMS) estimates the state of charge (SOC) and state of health (SOH) of each battery cell in the pack 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 1-kW prototype has been designed and implemented to further validate the proposed architecture.