An energy storage system including a battery cell, a tray for receiving the battery cell, and a rack for receiving the tray. The rack includes a connector unit. The connector unit is configured to connect to the tray inserted into the rack and to vibrate together with the tray when the tray vibrates due to, for example, an earth quake or external impact applied to the rack, thereby preventing an electric connection between the tray and the connector unit from being broken.