I heard a muffled pop 2 days ago and looked in the direction of the sound. Nothing but thick black smoke was filling the sky. A friend about 200 yards away had any car nuts worst nightmare. His battery in the 57 Chevy had gotten down so he put a charger on it...15 amp setting. He got distracted and went to his other building. He finally saw smoke comming from the small wodden structure with his 57 chevy, Willys pickup and conversion van in it. He saw it was out of control and got a wiff of the black smoke and was headed away when he passed out from the bad smoke. The battery in the trunk was the hot spot according to the fireman. When the gas tank blew accelerated by the melting NOS bottle it was all over with at that point. Fire trucks were there in 5 minutes.
He knows the battery charger killed his rides and almost got him. DON'T go off a leave a battery charger unattended on anything but a trickle charge. It ain't worth it.
Also the fire marshall told him that his trophys also added to the intensity. He told him one good sized trophy was like a cup of gasoline when they get hot. They are all petroleum products. His back wall was full of them.
What brought tears to his eyes weren't the rides burning but the friends that showed up before the fire department did to help out.

Most battery explosions are caused by an internal spark setting off the hydrogen gas. That internal spark is usually a cracked post attachment inside the case that finally separates or a cell connector strap that breaks. The gases in a battery are quite normal but heavy charging, large loads or just plain high temperatures can make the whole thing worse. Quite often a battery explosion occurrs during a jump start attempt when one of the internal pieces crack & break allowing a spark to jump the gap. Smart chargers and battery tenders are the best thing to happen to lead acid batteries in ages.

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It can even happen while using a trickle charger. A shop down the street (an ignition shop none the less) was charging the batteries on a medium sized loader after working on the charging system. The tech figured it would be O.K. to leave it on over night as they were a large bank of batteries. NOT, the tech woke up early and went to work @ 6:00 'cause he had a feeling, a good one too. He got the loader pulled out of the building before it burnt the place down as your buddies place did.
sniff, those were some nice automobiles he lost but the higher cost wound have been his life. He's lucky and not so lucky. Damn shame.