The surface voltage of a fully charged battery when a vehicle is not running should be between 12.5 to 12.8 volts.
Most automotive batteries are lead acid 12-volt DC (Direct Current). Each of the six cells in a 12-volt DC battery produces 2 to 2.1 volts. When the engine is running, the alternator will typically put out between 13.8 to 14.8 volts.