Your car’s 12-volt battery is split into six cells, each holding 2.1 volts when fully charged. Therefore, your car battery is fully charged at 12.6 volts or higher.
If your voltmeter shows a number lower than 12.6, this means your battery has fewer volts—or energy—available to power your car. Your car’s battery requires a certain amount of voltage to start and function properly. Less than 11.9 volts often isn’t enough power to start your car.
Without getting too scientific, the voltage of a battery depends on the levels of sulfuric acid. Sulfuric acid and water react within the battery to create the battery’s energy. As the battery’s charge goes down, the levels of sulfuric acid decrease as well, reducing your battery’s voltage.