Stuffed battery

[SteveW]

Come to think of it .... a sky rocket.

[T1 Terry]

Check the voltage across the terminals of that battery. I think it has had an internal short, overheated and then exploded, but the case held it in as the plastic was soft from the heat. Over voltage charging causes the sides to bulge in 6 clearly marked sections along the sides where the cell has heated and gassed yet the walls between the cells have held that part of the case at the correct shape.

[SteveW]

So I was on the right track then, Terry.

[T1 Terry]

Yep! Keep charging it and it will become a balloon.

A Led Zeppelin perhaps?
I don't think it was over charging that caused that, rather an internal battery fault causing a slow heat up till the final failure created a spark and that ignited the gases.
An excellent example of why lead acid batteries should never be fitted under a bed, imagine an even longer heat up and gas build up before the ignition and the effect if you happened to be sleeping on the bed at the time I guess most have seen what happens when an LPG and air mix is ignited, the explosion from a perfect mix of 2 parts hydrogen to 1 part oxygen (the gas produced inside a battery) would make the LPG explosion look like the explosion from a penny bunger in comparison.
This one is a mix of acetylene and oxygen is hotter (3500*C) than hydrogen and oxygen (2800*C) but the hydroxy flame front is much faster so the explosion results would be far greater, battery explosion when starting a car have been known to tear the bonnet and left hand guard off, the gas build up there would have been very small.
This one shows put a lithium polymer battery in a fire and the effect of water on a very hot fire https://www.youtube.com/watch?v=8SFSSqJkGQs