Energy storage system

An energy storage system comprising:

• a converter connected to a renewable energy source, the converter converting a direct current (DC) voltage supplied from the renewable energy source to another DC voltage;
• a plurality of unit battery packs;
• a plurality of unit bidirectional converters electrically coupled to the converter and the plurality of the unit battery packs, each of the unit battery packs connected to one of the unit bidirectional converters;
• an inverter electrically connected to a grid, the inverter inverting an alternating current (AC) voltage supplied from the grid to a DC voltage, the inverter electrically coupled to the converter and the plurality of the unit bidirectional converters, the inverter inverting a DC voltage supplied from the converter or from the plurality of the unit bidirectional converters to an AC voltage;
• and
• a controller electrically connected to the inverter, to the converter, and to the plurality of unit bidirectional converters, the controller applying a drive signal during a drive period to at least one of the unit bidirectional converters, a voltage from one of the plurality of the unit battery packs, the converter and the inverter being applied to the at least one of the unit bidirectional converters whenever the drive signal is applied to the at least one of the unit bidirectional converters.