ELECTRICITY-STORAGE DEVICE THAT CAN OUTPUT ALTERNATING CURRENT

Provided is an electric energy storage apparatus capable of generating an AC output in a low-loss and low-noise manner without using a DC-DC converter or an inverter. The electric energy storage apparatus comprises: an electric energy storage module group formed by connecting in series two or more electric energy storage modules each comprising one or more electric energy storage elements; a balancing circuit electrically connected to the electric energy storage module group and configured to adjust a voltage to be applied to each of the electric energy storage modules; a first switch group comprising two or more switches each provided in a path connecting a first terminal and a terminal of one of the series-connected electric energy storage modules; and a second switch group comprising two or more switches each provided in a path connecting a second terminal and a terminals of one of the series-connected electric energy storage modules. The electric energy storage apparatus is configured to perform a switch changeover in the first and second switch groups so as to select a magnitude and polarity of an output voltage depending on a configuration of the electric energy storage elements present in a path connecting the first and second terminals.