This voltmeter is plugged into the socket of the cigarette lighter. The voltage displayed provides information about the battery status and whether it is supplied from the alternator with sufficient charging current. Depending on the vehicle and battery type, the values are slightly different. Order of magnitude, the battery should unloaded bring about 12 volts, the voltage during start-up should not fall below 8.5V. If there is sufficient charging current to a voltage of ca.12V ca.15V appears. But as I said, that can only be approximate, which scatter and to a large extent depend on the activated consumers. To assess the electrical system while driving and at different temperatures, this little device cost can be very helpful. Unfortunately, the first delivery of this equipment was lost during shipping. When I was complaining spare me immediately assured (or money back). The replacement I received very short notice. Good goods, optimal service, cost-effective. meaningful diagnostic tool. Strong buy recommendation.