Burning, Boiling Battery - What caused it

[Jack C]

Last night a friend called me over urgently. Their boat was full of pungent fumes. The 12V sealed lead acid starting battery was smoking and the case was too hot to touch. Acid was leaking from the top of the battery.

The boat was in the slip connected to shore power. The boat has been almost 100% of the time at its slip until my friends bought it recently. They have taken the boat out and were motoring for about 3 hours last weekend.

Last year I had a battery do the same thing. It got so hot that it charred the floor of the wooden battery case. My battery was a sealed lead acid bank battery wired in parallel with a similar battery. I assumed that a cell in the hot battery short circuited and the other battery discharged through the hot battery. The charger was running at the time.

In my friend's case it was a starting battery. There is no OFF/1/2/BOTH switch so the starting battery cannot be in parallel with the bank. The battery is dated 2014.

I checked the charger and its output voltage is 14.5v on fast absorbtion with the new battery. Before installing the new battery the bank was at 13.5v. There are separate output circuits on the charger.

So what caused the problem and how can a fire be avoided in the future?

I have some ideas but would like the opinion of the experts on this forum. Preferably opinions with scientific or engineering reasoning.

Many thanks

[sailorboy1]

Got old and shorted out due to crud between the plates. Nothing you do will prevent then if the battery stays around long enough.