About a year back I was charging the battery on my cub which was parked outside.
While it was charging I set to cutting a link out of a chain with an air powered cut off wheel.
Like I said I was outside in the open air and was also a few feet away from the cub.
As the sparks flew one reached close enough to the battery and BOOM!
I have a 12 gauge shotgun that's not near as loud as the that battery was when it blew. It made my ears ring for awhile.
The end result was a busted battery case that didn't look much worse than the one you have.
Makes me wonder if a similar incident didn't occur with the PO of your tractor.