the battery lol.
The battery will output 12V till its near dead, its the amount of amps that decrease as it loses its charge AFAIK.

When you make a circuit by turning the key or by opening the door, electrons are free to go running around the circuit, and the battery happily flows, when there is no circuit, electrons go nowhere and the battery cant do too much!
Someone feel free to add something - my grasp of electricity is a little weak (and i do physics haha

lol)