[prairieschooner]

I had a Starting Battery Exploded a couple of years ago and what a mess it makes. Here is what you need to do in my opinion;
Remove whatever is left of the old Battery.
Get some Baking Soda to Neutralize the Battery Acid, I rinsed the area down then applied the baking soda and allowed it to sit for a while until it stopped foaming, then I rinsed the area down again. Once the area is clean you can Paint it again.
The problem had to be a spark as was stated above. It was most likely from a Bad Connection and or Old Batteries. Check to see if the other Battery shows a funky look around the Battery Cables. Anyway you should replace BOTH Engine Batteries. Now you will need to clean the Battery Cables, there is a round inside/outside wire brush made for this purpose. I use a grease or one of those packets made by Loctite that you get at the counter at O'Reilly's that inhibits that from occurring.
The good news is that after I did this I have seen no negative indications.