A power supply control apparatus in a power supply system comprising a power supply unit, a main power supply, and a backup power supply having a higher voltage at a time of full charge than the main power supply, the power supply control apparatus connecting the power supply unit, the main power supply, and the backup power supply and configured to control charging and discharging of the backup power supply, the power supply control apparatus comprising: a power line to which the power supply unit, the main power supply, the backup power supply, and a backup load are connected; a first switch provided on the power line and configured to turn on/off connection between the power supply unit as well as the main power supply and the backup power supply; a second switch provided on the power line and configured to turn on/off connection between the backup power supply and the backup load; a charging DC/DC converter connected to the power line in parallel with the second switch and configured to convert a voltage supplied from a power supply unit side and supply a converted voltage to a backup power supply side; and a control unit configured to turn on the first switch and turn off the second switch when the backup power supply is to be charged, and turn off the first switch and turn on the second switch when electric power is to be supplied from the backup power supply to the backup load, characterized in that the control unit is configured to set an upper limit value of a state of charge of the backup power supply based on an output voltage (Vout) to the backup load obtained by subtracting, from an open circuit voltage of the backup power supply, a voltage drop (ΔV) from an internal portion of the backup power supply to an output terminal of the power supply control apparatus to a backup load side via the second switch when the second switch is turned on, wherein the voltage drop (delta V) is based on a minimum current (Ioutmin) of the backup load. a power line to which the power supply unit, the main power supply, the backup power supply, and a backup load are connected; a first switch provided on the power line and configured to turn on/off connection between the power supply unit as well as the main power supply and the backup power supply; a second switch provided on the power line and configured to turn on/off connection between the backup power supply and the backup load; a charging DC/DC converter connected to the power line in parallel with the second switch and configured to convert a voltage supplied from a power supply unit side and supply a converted voltage to a backup power supply side; and a control unit configured to turn on the first switch and turn off the second switch when the backup power supply is to be charged, and turn off the first switch and turn on the second switch when electric power is to be supplied from the backup power supply to the backup load, characterized in that the control unit is configured to set an upper limit value of a state of charge of the backup power supply based on an output voltage (Vout) to the backup load obtained by subtracting, from an open circuit voltage of the backup power supply, a voltage drop (ΔV) from an internal portion of the backup power supply to an output terminal of the power supply control apparatus to a backup load side via the second switch when the second switch is turned on, wherein the voltage drop (delta V) is based on a minimum current (Ioutmin) of the backup load.