Since it is not possible to power the system load from the battery through the MAX1870A itself, a pMOSFET can be used in the configuration shown. When the wall adapter is powering the system load, the gate is powered to shut off the pMOSFET to cut off the battery from the system load circuit. when the wall adapter is removed, the gate is powered off and the pMOSFET turns on to supply to system load from the battery.