A vehicle power supply system, comprising: a main power supply device that is mounted on a vehicle and supplies power to a first load unit and a second load unit; and a backup power supply device that is mounted on the vehicle and supplies power to the second load unit when the main power supply device is abnormal, wherein the backup power supply device includes a first switch circuit that switches a connection with the main power supply device, a backup battery that stores power supplied from the main power supply device and supplies power to the second load unit, a second switch circuit that switches a connection between the backup battery and the second load unit, and a controller that controls the first switch circuit and the second switch circuit, when the main power supply device is normal, the controller turns on the first switch circuit and turns off the second switch circuit to supply power from the main power supply device to the first load unit and the second load unit and enable to travel the vehicle in a normal mode which is a normal driving state, when the main power supply device is abnormal, the controller turns on the second switch circuit and turns off the first switch circuit to supply power from the backup battery to the second load unit, not to supply power from the backup battery to the first load unit and enable to travel the vehicle in a limit mode in which a function is more limited than in the normal mode, characterised in that the backup power supply device includes a discharge circuit that discharges the power of the backup battery, and a third switch circuit that switches a connection between the backup battery and the discharge circuit,and the controller turns on the third switch circuit and turns off the second switch circuit to cause the discharge circuit to discharge the power of the backup battery and estimate a deterioration state of the backup battery. a main power supply device that is mounted on a vehicle and supplies power to a first load unit and a second load unit; and a backup power supply device that is mounted on the vehicle and supplies power to the second load unit when the main power supply device is abnormal, wherein the backup power supply device includes a first switch circuit that switches a connection with the main power supply device, a backup battery that stores power supplied from the main power supply device and supplies power to the second load unit, a second switch circuit that switches a connection between the backup battery and the second load unit, and a controller that controls the first switch circuit and the second switch circuit, when the main power supply device is normal, the controller turns on the first switch circuit and turns off the second switch circuit to supply power from the main power supply device to the first load unit and the second load unit and enable to travel the vehicle in a normal mode which is a normal driving state, when the main power supply device is abnormal, the controller turns on the second switch circuit and turns off the first switch circuit to supply power from the backup battery to the second load unit, not to supply power from the backup battery to the first load unit and enable to travel the vehicle in a limit mode in which a function is more limited than in the normal mode, characterised in that the backup power supply device includes a discharge circuit that discharges the power of the backup battery, and a third switch circuit that switches a connection between the backup battery and the discharge circuit,and the controller turns on the third switch circuit and turns off the second switch circuit to cause the discharge circuit to discharge the power of the backup battery and estimate a deterioration state of the backup battery.