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

If you clean your battery cables, contrary to belief, you should have lead to lead or lead to cable end direct connection.  This means that you should not disrupt current flow with dielectric grease or some other substance.  I'm not saying not to use grease.  Clean the connections and when the cables are connected, cover them with dielectric grease.  This will prevent any corrosion from building up on your cable ends.  If done properly, your connections will never need to be cleaned again.