Power supply control system

A power supply control system that is used when performing program rewriting of an electronic control device by using an external power supply, the power supply control system comprising:

• a first relay configured to connect a first in-vehicle battery to the electronic control device to allow electric power to be supplied when the program rewriting is not performed;
• a second relay configured to connect the external power supply to the electronic control device to allow electric power to be supplied when the program rewriting is performed;
• and
• a controller configured to control a connection state of the first relay and the second relay,
• wherein the controller is configured to, after the external power supply is connected to the second relay controlled to be in a cutoff state and before the program rewriting is started,
• control the first relay to be in a unidirectional conduction state in which a current direction is restricted to a direction from the first in-vehicle battery to the electronic control device,
• control the second relay to be in a unidirectional conduction state in which a current direction is restricted to a direction from the external power supply to the electronic control device,
• control the first relay to be in a cutoff state, and
• control the second relay to be in a bidirectional conduction state in which a current direction is not restricted.