Silly question: using only the positive terminal on a battery
If you leave either terminal of the battery unconnected, no current will flow from it.  For current to flow there must be a complete loop, or circuit, and the resistance along the path of the loop and the voltage(pressure) applied will determine how much current will flow.

Note that in electrical and electronics it is not uncommon for things to be referred to as ground that are not actually grounded, and are simply a dedicated return path for current.  For this reason, the new convention in electrical is to refer to things that are directly connected to ground up to the point the connection diverges(IE a ground rod pounded directly into conductive earth, the wire attached to it up to the first enclosure in which it splits are ground.  Anything past that point in green or bare copper is "bonding")

Your question seems to stem partly from the idea that the "ground" on your car is related in some way to the "ground" that is the earth.

The electrons that flow out of the battery must be replaced by the electrons flowing into it.  A battery does not supply you with electrons, it provides you with the emf(voltage/pressure) to make them flow.  The electrons are already present in the materials the electrons will flow through(conductors).  If you think of a copper wire as a pipe for electrons, you must think of it as a pipe that is already completely full of them.