Storage of electrical energy is the most important now a days. Why because renewable energy plants (i.e. solar power plants and wind power plants) are growing like anything, these are purely dependent on wind-availability and sunshine- availability. So, BESS-battery energy storage systems are mandatory to store this renewable energy in the high-capacity batteries. In this paper, it describes MW-Range BESS system overview including main components, charging / discharging and maintenance tips.