Battery energy storage system（BESS） has the potential capability of regulating its real/reactive power independently with fast response. It has shown great importance in load leveling， uninterruptable power supply， and power quality conditioning， etc. The NaS（sodium sulfur） battery is taken as an example. The terminal voltage of the battery changes sharply under different charging/discharging conditions. This is taken into consideration in designing a power conversion system including a DC-DC voltage regulator and a DC-AC converter. A unified mathematical model of the BESS is established and a proper control strategy is proposed according to the nonlinearity of the model， which is based on exact linearization via feedback and PI controller. Simulation results verify the stability and the anticipant dynamic response of the control strategy， which can keep the battery currents equal when the parameters are different between groups of batteries.