VEHICLE POWER SUPPLY SYSTEM

In a vehicle power supply system, a backup power supply device includes a backup 12 V LI battery and a control ECU. Here, a load unit, a load unit, and a load unit are load units having a specific function related to a vehicle. In this case, the load unit is a load unit having another function in addition to the specific function, and has a discharging function of discharging power when estimating the battery state of the backup 12 V LI battery as the another function. The control ECU is switchable among a normal mode, an abnormal mode, and a battery state estimation mode.