A power supply device integrally combined with an electrical device body including a load, the power supply device comprising: a rechargeable battery for supplying power to the load; charge terminals (4a, 4b) electrically connected to the rechargeable battery; residual capacity detecting means supplied with power from the rechargeable battery for operation to determine and store a value of a residual capacity of the rechargeable battery based on an amount of charged current to the rechargeable battery and an amount of discharged current from the rechargeable battery to the load; a switching element provided between the rechargeable battery and the residual capacity detecting means; battery voltage detecting means for detecting a value of battery voltage of the rechargeable battery; power supply stop control means for controlling to turn off the switching element to stop the power supply from the rechargeable battery to the residual capacity detecting means when the residual capacity detected by the residual capacity detecting means becomes less than a predetermined threshold value; and characterized by charge control means for stopping charging the rechargeable battery when the residual capacity detected by the residual capacity detecting means has become 100%, and when the value of the battery voltage detected by the battery voltage detecting means has become a value lower than a peak value after the value of the battery voltage passes the peak value, wherein the rechargeable battery has: a charge characteristic that if charging of the rechargeable battery is started when the rechargeable battery has a residual capacity less than the predetermined threshold value, the rechargeable battery reaches a state of full charge when the value of the battery voltage of the rechargeable battery has become a value lower than a peak value after the value of the battery voltage passes the peak value; and a charge characteristic that if the charging of the rechargeable battery is started when the rechargeable battery has a residual capacity equal to the predetermined threshold value or more, the rechargeable battery reaches the state of full charge before the value of the battery voltage of the rechargeable battery shows the peak value, wherein the power supply device is further configured such that the charge control means does not change the value of the residual capacity to 100 % until a predetermined time elapses after starting charging the rechargeable battery, even if the specific battery voltage condition is satisfied, and when the specific battery voltage condition is satisfied at the predetermined time after starting charging the rechargeable battery or thereafter, the charge control means changes the stored value of the residual capacity to 100 % regardless of the stored value of the residual capacity, and stops charging the rechargeable battery, wherein the specific battery voltage condition is that the value of the battery voltage of the rechargeable battery becomes a value lower than a peak value after the value of the battery voltage passes the peak value. a rechargeable battery for supplying power to the load; charge terminals (4a, 4b) electrically connected to the rechargeable battery; residual capacity detecting means supplied with power from the rechargeable battery for operation to determine and store a value of a residual capacity of the rechargeable battery based on an amount of charged current to the rechargeable battery and an amount of discharged current from the rechargeable battery to the load; a switching element provided between the rechargeable battery and the residual capacity detecting means; battery voltage detecting means for detecting a value of battery voltage of the rechargeable battery; power supply stop control means for controlling to turn off the switching element to stop the power supply from the rechargeable battery to the residual capacity detecting means when the residual capacity detected by the residual capacity detecting means becomes less than a predetermined threshold value; and