An energy storage system including one or more battery cells connected in series, one or more isolated DC/DC converters, each isolated DC/DC converter having one end connected to one battery cell and an other end connected to another battery cell adjacent to the one battery cell so as to equalize a cell voltage through charging and discharging of the two battery cells which are each electrically connected to one of the plurality of battery cells, and a plurality of battery modules each including a controller configured to control a charge mode and a discharge mode of the plurality of isolated DC/DC converters.