Grid-connected power storage system and method for controlling grid-connected power storage system

A grid-connected power storage system (200) for coupling a power generation system (230) to a grid (240), including: a main battery (220a) for discharging stored power to the load system (250); at least one additional battery (220b) coupled to the main battery (220a) for discharging stored power to the load system (250); a bidirectional converter (213) coupled to the main (220a) and additional (220b) batteries, and including a plurality of switches for performing a conversion between a DC link voltage, between the power generation system (230) and the grid (240), and a battery voltage (220), a first switch of the plurality of switches corresponding to the main battery (220a) and a second switch of the plurality of switches corresponding to the additional battery (220b), wherein the first and second switches are connected to each other in parallel; and an integrated controller (214) for selectively controlling operations of the first switch and the second switch based on an amount of power used by the load system (250).