Battery Diode

The battery diode is used as a safety function in case the secondary battery cannot supply enough energy to the vehicle's consumers when the starter motor runs during an engine restart after autostop.

If the voltage difference between the battery circuit and other vehicle exceeds ~0.4 V (at + 20 °C), the battery diode opens (lead) and supplies the vehicle's other consumers during autostart. This occurs during autostart when there is no voltage supply from the secondary battery for some reason. This can, for a short time, create a large voltage drop in the electrical system and cause lamps to flash for example.