Taken from Service Android (SA-80 series) user manual:

Should your SA-80 unit be disconnected from its recharging station, the unit will be powered by its internal TR928 battery. In the event the battery is depleted, the unit’s reserve power will take over, allowing the unit to either properly shut down or, if its recharging station is in close proximity, will automatically return to the station for recharge via power cord.

Danger Zone One. Story by Midnight. Art by Ryo Kawakami.