Abstract: A battery-powered rotation detector detects passing of a rotating magnet attached to a vehicle wheel to generate and transmit a rotation signal. A receiver receives the rotation signal. A processor computes the vehicle running condition (e.g., speed, traveled distance) from the received rotation signal and displays the results on a display device. The processor monitors intervals between the rotation signals to detect when the vehicle is stopped. When the vehicle is stopped, the processor disconnects a battery power supply from components such as the display device for power savings. In addition, the processor checks the strength of the received rotation signal to determine a capacity of the battery in the rotation detector, and operatively indicates a power failure of the rotation detector.