If we connect a normal battery to a load circuit, its stored electric potential energy on its two terminals (+) & (-) will be consumed by work done on the load and eventually the battery is "drained dry" and there is zero voltage measured across its + and - minus terminals. At that stage we say that the battery is empty.
I'm asking what is exactly the physical process taking place that caused this result assuming there is no leakage current between the two terminals of the battery?
What happens to the stored charges in the battery? Do the two terminals of the battery reach electrostatic equilibrium and therefore zero voltage across them?
Does this mean that if I use an empty 9V battery's (+) terminal and an other empty say 3V battery's (-) terminal then I will have created possible a 6V potential difference and connect them to a circuit then electric current will flow again in the circuit? Even if both batteries were empty?