View Here : Car Battery Voltage
For a standard "12 volt" battery, the voltage should be 12.6 to 12.8 while car is off, 13-14 while running. Some newer cars (e.g. recent model Hondas) have higher voltage batteries.
An automotive battery is a rechargeable battery that supplies electrical current to a motor vehicle. Its main purpose is to feed the starter , which starts the engine . Once the engine is running, power for the car's electrical systems is supplied by the alternator .