Maintaining frequency levels within narrow tolerances around the nominal value is one of the key conditions for stable power grid operation. This document describes the problem of ensuring a high level of frequency stability in power systems with high penetration of power electronic interfaced power sources. Additional measures will be needed to ensure stable power grid operation in the future, one of which is the use of battery energy storage systems. Dynamic simulations are used to present the influence of battery energy storage system (Li-ion technology) fast responses to the frequency response of two islanded systems in the Slovenian power grid. Droop-based control was used for regulating the battery active power output. Simulation results show a positive influence on frequency stability even at relatively high droop settings. This is especially true for smaller networks where frequency is generally less stable. Since generator oscillations were triggered in some cases, the influence of battery energy storage system response on oscillatory stability was analysed by means of modal analysis. The latter didn't show a significant negative effect, either in the case of droop-based or synthetic-inetia-based active power control. Since certain assumptions are used in the simulations, further analyses and practical tests are needed to confirm them.