Actually a 12 volt auto battery already has 6 batteries (or more correctly, 6 cells) connected in series and you are just adding 6 more cells.
If you have a 12 volt or a 24 volt battery without a load connected, there is no circuit. Look at it as a light in your house. If the switch is open, or off, there is no circuit, no current flow, and your light is off. If you flip the switch on, you complete the circuit, current can flow, and the light goes on.
Oops, error in number of cells.