Islanded microgrid systems are highly susceptible to disturbances. Fluctuating loads, low system inertia, the intermittent output of renewable energy sources (RES) alSOContribute to system disturbances. The use of storage devices, especially the battery energy storage (BES) system, provides the answer that the system can maintain its stability. The BES can participate actively in regulating the system frequency. However, the battery's active participation causes fluctuation in its state-of-charge (SOC), causing unstable battery performance and shortening the battery lifetime. By increasing the battery capacity, such a problem is solved. Nevertheless, the enlarged battery capacity may make its economic value unattractive. Hence, SOC control is needed on the BES to maintain battery performance but does not override system performance. In this paper, a battery SOC control installed in BES load-frequency control (EEC) is proposed in this study. The proposed method is evaluated based on battery performance for one year. It provides technical performance, which stays within the grid code and provides low economic value compared to other control.