Check the voltage output of your cars battery quickly and easily. Simply plug this handy voltmeter into the cigarette lighter socket and get an instant readout of the electrical system's voltage. This digital gauge displays the voltage of batteries in the car. With high brightness of LED there is no difficulty in seeing the display. Powered by car cigarette lighter charger. Constantly monitor battery voltage， extend and protect the battery and engine life. Installment: Clean the cigar socket. Insert the jack of the product into a cigar socket.