To simplify carrying-in and carrying-out work of a temporary battery, and to make installation or connecting work unnecessary by putting a charger for charging a DC power source in which a plurality of batteries are connected, and a switch which performs voltage monitoring, power supply and its cut off, on the load-carrying platform of a car.

A DC power source 2 is composed by connecting twenty four, for example, parallel connected two lead battery units in series, and a charger 3 always keeps the power source 2 in a fully charged state. The voltage of the DC power source, the voltage of a pilot battery, their voltage difference, and room temperature are confirmed by a monitor 4, and if they are in specified conditions a switch 5 is made. After that, connection between the cells of an existing battery is removed, and the battery is carried out. A newly fitted battery is connected to the input terminals of an exchanger, and batteries are exchanged. Consequently, work mandays are extremely reduced, and faults are wholly prevented too.