You can model the battery as an infinite current source with 12v potential in series with a small resistance (this is the battery's "internal resistance"). The wire or whatever you use to short the battery terminals also has some small resistance -- unless you have invented a room temperature super-conductor. If you know those two resistances you can calculate the voltage divider drop at the battery terminals.
Then you need to realize that the "infinite current source" is actually finite and can only provide a fixed number of electrons...just like a tank of water. So, as timthereaper says, the voltage at the source will start to droop and eventually become zero when there's no more water in the tank.