PURPOSE: To decide and display whether a battery and its charging system are accepted or not by fitting a current, a voltage, and a rotation sensor to specific positions of a motorcar, and operating an engine and an electric load according to a specific procedure.

CONSTITUTION: An operator sets the respective current, voltage, and rotation sensors 1, 3, and 5 at the specific positions of the motorcar. When a start switch is turned on, a control circuit 7 measures external temperature by a temperature measuring circuit 19, sets a current measuring circuit 2 to zero, and measures the no-load voltage of a battery by a voltage measuring circuit 4; when everything is normal, all loads are connected as a next step and an indication of cranking operation is sent. The operator performs operation according to the indication and the control circuit 7 measures and record the operation results. Further, operations for increasing the rotating speed of the engine up to a specific value by stepping on an accelerator and decreasing it to an idling speed are indicated and the battery and its charging system are diagnosed from data on the results.