The diploma thesis is a general overview of battery systems. It starts with introduction, which presents how the power system can be improved by storing energy. My focus is on batteries and to better understand batteries and their functions in power system it is necessary to understand the characteristics and the technology of batteries. I mentioned various uses of BESS, but I focused on three most common problems, which are frequency regulation, voltage regulation and peak shaving. Then I introduced largest BESS manufacturers and their biggest projects.