Batteries seem to be team players

[allatsea99]

Battery banks seem to be like sports teams. One battery fails and the whole team comes crashing down?
My boat is kept on a mooring, plenty of sun, regulated solar panels provide ample charge. So it was a surprise one morning when I discovered the whole bank had an output of <11v from the previous night's 12.6v. Eh? Two 80AH Marine Deep Cycles and one 70AH high CCA starting battery... all were apparently drained. A 'short' is the obvious culprit, but where?
Have learnt 2 lessons here: a) use all your senses to find the problem, b) low maintenance batteries have drawbacks.
There was a 'hot smell', one battery was pretty warm. There was a crackling sound from the end of the same battery. The visual window indicator on the battery showed "white" i.e. replace. There was my short caused by a dried out wet cell lead acid battery.
I have since found all the 'Low Maintenance' batteries I have (tractor etc.) are likely to fail from dehydration, not cholesterol like sulphation. So I neatly drilled holes into the tops of the cells, topped up with d.water, stage charged all & fitted sealable caps. Advice suggests adding magnesium sulphate to older batteries. Am I doing the right thing? What do you think?

[john61ct]

No!

Replace with new matching, all at once.

[allatsea99]

Thanks john61ct, yes I have replaced the whole bank with 'new' for the boat, but for applications less critical its a waste not to restore the older ones which I have to say are performing really well.
Naturally the battery with the short went to the cemetery ... its body parts may well pop up in my future replacements?!
Your comment is worthy of note.