Abstract: The lifetime of battery is an important factor affecting the economics of battery energy storage system（BESS）for smoothing wind power fluctuation.To extend the BESS lifetime,the charging or discharging model of dual-battery energy storage system（DBESS）with the most appropriate depth of discharge（DOD）is established based on the DBESS operation principle of separately charging or discharging.And the extreme operating conditions are studied due to the unbalanced energy between the process of charging and discharging.According to real-time status of DBESS and wind power fluctuations,the charging and discharging saturation index and stability index of DBESS are listed.To avoid DBESS operating in the unstable regions with less charging and discharging capacity,the fuzzy control strategy is adopted to adaptively adjust the time constant of low-pass filter so that the battery state of charge（SOC）status can be optimized.Finally,the proposed control strategy is simulated and analyzed on MATLAB/Simulink simulation platform.The results show that the proposed fuzzy control strategy can not only maintain the long-term stable operation of DBESS but also ensure the suppression effect.