PURPOSE: To offer a power source unit, designed for electric vehicles, of simple construction and capable of extending the life of a low-voltage battery.

CONSTITUTION: A power source unit, designed for electric vehicles, is provided with a high-voltage battery 53, DC-DC converter 57, and low-voltage battery 58. The unit has a charging means to charge the low-voltage battery 58 by operating the DC-DC converter 57 through the detection of the high-voltage battery being charged. The charging means is provided with a charging switch 3, current sensor 1, DC-DC converter 57, and relay 2. The current sensor 1, connected to a charger 51, generates a detection signal by detecting that the charger 51 is turned on. The charging switch 3 is interlocked with the ON-OFF of the charging switch 3 to turn on/off the DC/DC converter 57.