In the lithium-ion batteries for an electric-drive vehicle, the cell balancing is required to enhance life time of batteries and to guarantee safety. In the vehicle-to-grid (V2G) system, batteries of electric-drive vehicle are more frequently charged or discharged, which increases probability of cell imbalance. Therefore, battery management system (BMS) with efficient cell balancing operation is necessary. To achieve efficient management of lithium-ion batteries with high capacity for the V2G system, this paper proposes a modularized BMS structure with an active cell balancing circuit. The proposed BMS shares a multifunctional switching block for cell voltage monitoring and cell balancing. By sharing monitoring and balancing operation, a cost-effective BMS with small size can be achieved. In this paper, the structure and operational principles of the proposed BMS are presented. To confirm the validity of the proposed scheme, a prototype of 20 lithium-ion batteries with 25Ah is designed and implemented. Cell balancing performance is also verified by an experiment.