Zeltauto LCD Cigarette Lighter Digital Car Voltmeter 12V/24V Vehicle Voltage Gauge Monitor for Auto Car Truck With red screen digital color, there is no difficulty in seeing the display; Apply for DC 12V or 24V car; Measuring range: DC 8V-30V+/-1.2% (Please allow 0.1-0.3 volts display error) This digital gauge displays the voltage of batteries in the car; Powered by car cigarette lighter charger Simple enough to use, just plug it into cigar lighter socket, watch alternator's output from idle to accelerated speed When engine running (do not drive), the LED display: between 13.5 to 14.5 (12V car), between 26.0 to 28.0 (24V car) When engine off, and nothing turned on, LED display: Greater than 12.0 (12V car) or 24.0 (24V car)