Most car batteries come with 6 cells , and each cell provides 2.1 volts , so 6 X 2.1 volts = 12.6 volts. By connecting the two ends of a Voltmeter on the positive and negative terminals of your car battery you will get the volt reading. The reading in your voltmeter for a fully charged battery is 12.6 volts   Battery Charge Level Table Voltage reading Fully Vharged battery 12.6 Volts Partially Charged battery 12.40 to 12.6 Volts Discharged battery Equal to or Below 12.39 Volts