A UPS comprising an input, an output, a battery circuit, a PFC stage, a switch configured to selectively couple an interface of the PFC stage to the input in an online mode and to the battery circuit in a backup mode, a positive DC bus, a negative DC bus, and a controller configured to operate, in the online mode, the PFC stage to provide DC power, derived from the input AC power, to the DC busses, to operate, in the backup mode, the PFC stage to provide DC power, derived from the backup DC power, to the DC busses, to operate, in a first stage of the backup mode, the battery circuit to couple a positive terminal of a DC source to the interface, and to operate, in a second stage of the backup mode, the battery circuit to couple a negative terminal of the DC source to the interface.