A buffer battery power supply circuit is provided. In one embodiment, a battery charger circuit supplies a total output current which is delivered to both an active system and a battery. The total output current and the current delivered to a battery are sensed and compared to a preset threshold total output current signal and threshold battery current signal, respectively. The compared signals generate error signals which are provided as feedback to the battery charger circuit, so that the total output current can be controlled. In another embodiment, in addition to sensing the total output current and the battery current, the total output voltage is sensed and multiplied by the total output current, generating a total output power error signal. The error signals are provided as feedback to the battery charger circuit, so that the total output current and/or the total output voltage can be controlled.