Abstract: A power source switching system for electric motor vehicles comprises a plurality of secondary cell units as a power source, one or more discriminating sensors for detecting vehicle information, a control device for accepting a signal from the sensors and a switching circuit for switching the connection state of the plurality of secondary cell units, responsive to a control signal issued by the control device. In one embodiment of the invention, the discriminating sensor detects the occurrence of an impact shock, with the switching circuit changing the connection state of the plurality of secondary cell units into a parallel connection to prevent short circuiting.