BATTERY FAILURE DETERMINATION DEVICE

A battery failure determination device that determines a failure of a second battery based on presence or absence of an electric discharge from the second battery when an output voltage of a DCDC converter is temporarily lowered in a power source circuit in which electric power is supplied to a load by the second battery and in which electric power is supplied to the load by a voltage stepped down or boosted by the DCDC converter from a first battery, and a cycle of temporarily lowering the output voltage of the DCDC converter is irregular.