Battery Problem - overheating

Cherokee2015

Popped into the van yesterday to put some clothes and things in ready for the weekend and noticed a funny 'eggy' smell inside.   Checked the battery box and the battery was red hot - couldn't even touch it.   Turned off the electricity supply and the van
and left it to cool down for a couple of ours.   Seems like the battery has cooked - but wonder what caused it!

 

We had noticed that it wasn't holding it's charge as much just lately when it wasn't on electric and when using the mover, it had struggled.   Do people think that using the mover has caused the problem as we have to use it for a fair distance from garden
to road.

 

Any thoughts, thanks in advance.