Portable independent electric power source

An internal storage battery (30) and an external storage battery (40) that is freely attachable/detachable to/from a power source system (10) are connected to a solar battery (20) and a charge controller (50) via first to fourth switch circuits (SW1-SW4). A control unit provided in the charge controller (50) turns on the first switch circuit and turns off the second switch circuit so that the internal storage battery (30) is assigned priority to be charged. After the internal storage battery (30) is charged, the first switch circuit is turned off while the second switch circuit is turned on to charge the external storage battery (40). Further, the control unit turns off the third switch circuit and turns on the fourth switch circuit so that the external storage battery (40) is assigned priority to be discharged. After the external storage battery (40) is discharged, the third and fourth switch circuits are turned on and off respectively to discharge the internal storage battery (30).