Boiling Batteries -- Shorted Cell ?

[Dockhead]

I'm about to leave the boat for a few weeks and I am somewhat nervous about a rerun of the boiling batteries incident I just had.

I am not really sure that I understand what happened and how to prevent it.

My guess was that one cell shorted in one battery, raising the voltage seen by the remaining 11 cells in the virtual 24v battery (two 12's wired in series), which was enough voltage to boil those two batteries.

Is that what happened, or is there some other thing I should be worried about?

What happens if that process goes on unchecked? Does the battery catch on fire and burn the boat down? Or does it melt down and stop conducting? If this were common surely we would hear about more boats burning down that way.

And why? Just because the battery was heavily sulfated? The residue built up on the bottom until it touched the bottoms of the plates?

My plan is to leave on-line only the one pair of batteries (out of four pairs) which don't have any completely dead cells, so that I have power for bilge pumps if necesary while I'm gone.

Any comments? We have such great electrical expertise on here -- I was hoping to learn something.

Cheers.