I have witnessed several lead acid battery explosions. It is quite a violent event. As Jim said, the top of the battery literally blows off. I found several pieces of battery plastic at least a hundred feet away. Funny thing is a battery with the top blown off will still start and run an engine. Little risk of explosion at that point, but plenty of chances of sulfuric acid getting where it shouldn't be.
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A while back I tried to start my '65 Ford and all I got was about 1 second of cranking and then nothing. When I investigated closer I noticed that when slightly loaded the battery would gurgle and the voltage on the posts would drop to nothing. I assumed something broke loose inside and promptly got rid of it.
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Another time I recall trying to start an engine and ended up with a carburetor fire (yes, I am old) due to a backfire. No fire extinguisher, nowhere close to water. The engine had caught a couple of times so I just kept on cranking it hoping to get it to run. Yes, this will extinguish carb fires. Long story short is that it worked. But that battery was never the same. Something must have gotten hot enough inside to come apart from extended heavy loading because when I returned it they said it got hot when charging. It had good electrolyte levels and the unloaded voltage was way up there. But it could not run any significant load. In my opinion, no decent car battery should be able to destroy itself from cranking, but that's just my opinion.
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At any rate, be careful with batteries. I would ditch the battery in question if I were you foolios.