As an energy source of mobile system including robot, Li battery is extensively applied. To increase durability and efficient usage of battery, it is to require the rigorous management to battery controller, BMS,. Of them, the exact estimation of SOC is very important for correct usage. In this paper, introducing the design of BMS simply and, as a estimation method of SOC, dual extended Kalman filter is proposed and proven through simulation based on the real data of current and voltage of battery.