This paper presents the frequency enhancement of an isolated island microgrid by a battery energy storage system (BESS) with a frequency sensor controller (FSC). We selected the Chimei Island microgrid for our study. The total installation capacity of solar photovoltaic (SPV) plants is 410 kWp with over 50% instantaneous penetration level. A BESS with the proposed FSC was installed to alleviate the impact of SPV power output variation on the microgrid. Load flow analysis and transient stability analysis were then executed with and without employing the BESS. The events of the largest SPV power plant tripping and all SPV output powers having a ramp rate (RR) of 70 %/s were selected for investigation via a simulation of transient stability. It was found that the frequency dropped below 57.3 Hz during these events without compensation by the BESS, which may result in the tripping of the load. However, the system frequency can be well maintained if the proposed FSC of the BESS is applied to support the operation of the microgrid.