An auxiliary power supply unit which is mountable to a portable electronic device having a battery, includes at least one capacitor, and an I/F connector which is connectable to an I/F connector of the portable electronic device, the I/F connector of the portable electronic device including power terminals connected to the battery. The I/F connector of the auxiliary power supply unit includes power terminals connected to the at least one capacitor, wherein the at least one capacitor is connected in parallel to the battery when the auxiliary power supply unit is mounted to the portable electronic device via the power terminals of the I/F connectors.