The voltage in any 12 volt system, which includes cars, is 12.66-12.72 volts.  A common misconception is that if your battery has 12 volts in it, the battery is charged.  12 volts in a battery means that the battery is basically dead.  When the vehicle is running the voltage at the battery should read 13.9 volts to 15.0 volts.  With computer systems becoming much more advanced, your vehicle's charging system may charge at a different voltage based upon the temperature of the battery.  This is something to keep in mind if you have access to a simple DC voltage tester and want to check your own alternator or charging system.  Thank You