Energy storage is needed in micro-grid to help solve the problem of intermittency introduced by renewable energy sources, enhance power quality and improve controllability of power flow. This paper presents an energy manager for energy storage system (ESS) in micro-grids. The objectives of the energy manager are focused on improving the energy efficiency and extending the life expectancy of ESS while ensuring constraints of energy storage modules are complied with. To this end a smart local prediction and local scheduling algorithm is proposed. A battery lifetime model that uses the proposed Peukert lifetime energy throughput based on the workload of the battery is developed. Verification shows that in the long run, the energy manger can improve overall energy efficiency of ESS from 74.1% to 85.5%, and improve estimated lifetime of 2 Battery Packs in ESS from 3.6 years and 2.4 years to 5 years and 5.7 years respectively.