Dnk - As I recall, we had to take care that we did not overly discharge a cell by continuing to run discharge current thru it. Think of it this way, when the weakest cell becomes completely discharged, there is still discharge current flowing thru it forced by the rest of the battery, and it will reverse it's polarity. The cell is essentially shot at that point.
Again as I recall, as a discharge continues, the rate of change of cell voltage increases. Since we had 126 cells and one physical meter with a cell selector switch, towards the end, cell voltages are changing fast and it's impossible to monitor all of them, so we'd pick the 5 or so lowest reading cells and just monitor those continually. When the lowest cell's voltage diminished to a certain (low but safe) point, we'd end the discharge.
There was ample technical description about how this happens in the Naval Ships Technical Manual, but of course I don't have one of those handy anymore. I cannot remember the specifics about why this happens, but submarine main batteries are big, expensive and no fun to change, so no one wanted to mess one up.
Radar
