An electricity storing device includes a high-voltage terminal, a low-voltage terminal, a plurality of rechargeable battery modules, a plurality of first switches each coupled between one rechargeable battery module and the high-voltage terminal, a plurality of second switches each coupled between one rechargeable battery module and the low-voltage terminal, a plurality of third switches each coupled between two of the rechargeable battery modules, and a control module for outputting a control command to control couplings of the plurality of first switches, the plurality of second switches and the plurality of third switches.