A charging control unit decides the number of battery cells on the basis of an output voltage of a battery pack, and an output voltage of a charging circuit is determined in accordance with the decided number of battery cells. Even though supply of an external power supply is detected, when a battery connection is not detected, the power supply circuit of the charging control unit is set in a stop state. In addition, when disconnection of the battery is detected, a canceling voltage for canceling a cutoff state caused by overdischarging of the battery is output. In a power supply device for a portable terminal, a capacitor electrically charged by an output voltage of main DC/DC converter is connected as an auxiliary power supply of a backup DC/DC converter to make exchange of the auxiliary battery unnecessary.