Power storage system

A power storage system that performs charging and discharging with an external system, the power storage system comprising:

• a first battery unit and a second battery unit connected in parallel to each other, and each including
• a power converter and
• a plurality of battery packs connected in parallel with each other to the power converter;
• and
• a controller that controls operation of the power converter of each of the first battery unit and the second battery unit, wherein
• the first battery unit and the second battery unit each include at least one first battery pack and at least one second battery pack as the plurality of battery packs, and the second battery pack is different in type from the first battery pack and higher in internal resistance than the first battery pack, and
• the controller
• selects two second battery packs from a plurality of the second battery packs included in the power storage system, and
• performs first control and second control at respective timings different from each other, in a standby state in which charging and discharging are not performed between the power storage system and the external system, wherein the first control is performed to discharge one second battery pack of the selected two second battery packs and thereby charge the other second battery pack, and the second control is performed to discharge the other second battery pack and thereby charge the one second battery pack.