May be used a parallel connection between internal power supply and battery (eventualy small resistor for current limiting). In this way power supply keep charged battery, and feed power to circuit. If battery is externaly charged or have a separate charging circuit, may use a diode (2 or 3 in series) to isolate battery from internal power supply (if needed / required). When power supply drop below 11.3V (10.6 or 9.9V), automaticaly battery will became source for circuit.