[Mike Copioli]

Originally Posted by seg9585

I'm not an electrical engineer, but I would imagine that the higher-voltage battery would supply back-current to the lower-voltage battery, thus charging the lower voltage battery until both are at equilibrium. During this process a bit of charge would be lost due to the battery's internal resistance, but this would be a converging process, not a "domino effect".

You are correct, that is what happens. Think of it as two pressure vessels placed in parallel. One that contains 2000# of pressure and one that is empty. If both cylinder have the same liquid volume they will "equalize" thier pressure until each cylinder has the exact same volume. In this case the end pressure would be 1000# You will get some unequality at first due to a temp drop in the supply cylinder and a temp increase in the cylinder being filled but in the end the temps will stabilize and every thing will be in equalibrium. This is an analog to voltage and amp hours. Where the difference in voltage would be the pressure difference and Amp hours would be the volume of the cylinders.