fuzzybabybunny:
Ok, so I guess this is where using the water analogy breaks down, right?
Not completely. Imagine a battery as a tank with a movable wall, dividing it into two chambers. Is pushed into one direction by a spring. Current needs to flow back to fill up the other chamber, else the spring will be stopped by the vacuum suction on its side. It is a little more involved, since you need to get potential energy and spring energy translated to get it into the rest of the analogy, but that is basically it.
It is not just a full and an empty chamber. Why would water flow happen in the first place in that situation?
Maybe it is better to have the chambers vertically arranged, with a thick (proportional to the voltage) but weightless wall in between. With that, you only deal with potential energy, but you cannot fill another battery, since the upper chamber needs to be empty to make room to fill the lower one.