Electric energy storage system and method of maintaining the same

A method of maintaining an electric energy storage system in which:

• a battery unit and a converter to control charge and discharge the battery modules are provided;
• in the battery unit, a plurality of battery modules, and a battery management unit to manage charge and discharge of the battery modules are provided;
• and in each of the battery modules, a secondary cell line composed by connecting a plurality of battery cells, and a cell monitoring unit to monitor states of the respective battery cells and to control charge and discharge thereof are provided;
• and
• a maintenance connector for connecting the replacement battery module and a charge and discharge control unit for a replacement module are provided;
• the method of maintaining an electric energy storage system, comprising:
• providing a charge and discharge control unit for a replacement module in the converter, connecting the replacement battery module to the maintenance connector, and obtaining information relating to charge and discharge states of the respective battery modules by the charge and discharge control unit for a replacement module;
• and
• controlling the converter by the charge and discharge control unit for a replacement module, based on the information relating to the charge and discharge states obtained from the respective battery modules, to thereby charge and discharge the replacement battery module, so as to balance the charge and discharge state of the replacement battery module with the charge and discharge states of the respective battery modules in the battery unit.