Abstract：Lithium ion battery is the most common electrical energy storage devices in EVS. The battery management system (BMS) has been the topic of recent research in EV system, and one of the important parameters is the estimating the state of charge (SOC). Accurate estimation of SOC is difficult, because the operation condition of EV is in a wide rage. The model of battery will change as the operation and aging of the EV. This research designing a self-adaptive system to measurement the coefficients of battery follow the operation and aging. The amendment of open circuit voltage (OCV) and internal resistance will be achieved by the measurement. The amendment model and current integration technique are used to estimate the SOC.