According to the features of lithium ion battery packs, a distributed battery management system (BMS) for battery electric vehicle (BEV) is designed in this article. The BMS consists of a master module with several sampling modules. The kernel of master module is TMS320C2812 digital signal processor, and the kernel of sampling module is P87C591 singlechip. The main functions of master module include estimation of state of charge (SOC) and security management of lithium ion battery packs, and the main functions of sampling module include battery information collection and CAN bus based communication. SOC estimation method based on Extended Kalman filtering (EKF) theory is adopted in this article to precisely estimate the SOC of lithium ion battery packs.