Battery goes boom !

[Tarheel]

 Left my Sunstar's battery on charge over night. Walked out today and turned the switch only to get a quite loud explosion. Looking under the hood, Not only was the cap from one half of the battery missing, But the top of the battery itself was peeled like a banana. The plastic looks to have missed the electronics of the tractor, but I'll have to clean everything with something to neutralize the acid just in case.

 I knew the battery was old and on it's last leg but don't know of a way to know or guess something like this was going to happen. Any ideas ?

 

[Hick]

There is no way to predict it. When I worked at Sears Automotive around 1991, I was doing an Electrical Charging test on a car. Battery test, alternator test, starter draw, the works. Now, the customer was not supposed to be in the shop, but it was not enforced unless the customer was being a problem.  So he and I were standing next to the corner of the car that had the battery in it as the automatic tester went through its paces.  This was not a "toaster type" Load tester, but one that somehow tested resistance in the battery. He mentioned that it was a fairly new battery, maybe less than a year old. I leaned down to look at the date code when, without warning, the battery exploded about 18" from my face!!!! Sounded like I had shot my .357!!  My glasses protected my eyes, I got a little acid on my face. It exploded upward and I was at an angle to it. 
I looked at the customer to see if he was okay. He was, with a shocked look on his face. Then I noticed the splotches of battery acid all over his nice suede jacket!  I pointed it out and I expected him to say something about Sears having to buy him a jacket.  He looked down at the jacket, then up at me and said, "Well, I wasn't supposed to be out here, guess that's on me!
No idea why it blew, just one of those things! And yes, it was less than a year old. He got a new battery, at least!
 

[PhanDad]

Many years ago I was present when my uncle disconnected one of the charging leads from the battery post without turning off the charger. There was an arc and the “battery exploded”. 

The battery was in a confined area (under a seat I think) and what actually exploded was the hydrogen gas/air mixture - lead/acid batteries emit hydrogen when charging. 
 

The lessons I learned that day (turn off power before disconnecting leads and don’t charge batteries in confined space) I still follow.  

[Tarheel]

Thanks guys. Phandad, I had given thought to something along those lines but the hood was partly open and the wind was blowing 15 mph or more. But I did not turn off the charger before trying to start so you may well be right on the money.

 Years ago, I had the battery in a B10 on charge over night. An old hand held charger that was maybe 10 amps. Around 2 AM I heard a big boom and the next morning found the battery had exploded. The leads were still in place and the hood wasn't on it. I don't know if a battery can short inside and arc or not, but something set it off.

Anyway, thanks again to you both.  In the case of the Sunstar, I think I may start removing the battery to charge it.

[Hick]

11 hours ago, Tarheel said:

I don't know if a battery can short inside and arc or not, but something set it off.

That is the only explanation for my situation. And although the battery was exposed in your case and a wind blowing, the hydrogen gas in the battery was confined.