The best lessons learned are those that just cost a little more time or money. However, some can be far more dangerous. The photo below shows a battery with the case blown out from a hydrogen gas explosion. This can occur when the battery is subjected to a massive dead short that quickly overheats the battery. This causes the hydrochloric acid to heat up and form explosive hydrogen gas. All it takes is a very small spark to ignite that gas and cause the explosion.
This particular battery was merely sitting on the battery charger at roughly 10 amps when it exploded. That’s reason enough to never leave a battery on a charger unattended.
In our case, we had moved the battery to the trunk for weight transfer reasons on our Chevelle. In our haste to perform the task, we drilled a hole in the trunk floor and intended to come back at a later date to fit it with a rubber grommet to protect the positive battery cable.
Good intentions aside, that grommet was never installed, and later during drag testing, the car inexplicably shut off while in the pits. I opened the trunk and reached for the positive battery terminal which was very hot to the touch. At that moment, the battery exploded, sending battery plastic and sulfuric acid flying. Luckily, I was not seriously injured, but the acid decorated my face with red acid marks for a few days and destroyed my shirt. It could have been much worse.
Twelve-volt batteries supply power for starting but they must be cared for properly. In this case, at minimum, the grommet should have been in place, or better yet, a pass-through firewall bulkhead fitting should have been employed.
Battery explosions can also occur if the battery is over-charged, which creates heat. This can then cause the battery to out-gas. Another common situation is placing a long wrench across a top terminal battery that can short out between the positive and negative terminals. In most cases, the wrench will weld itself to the terminals and quickly overheat to the point where it cannot be removed. This is almost immediately followed by a battery explosion. The message here is to not use the battery as a tray for tools or parts that can create this situation.
Ultimately if you treat all car batteries with the respect they deserve with proper maintenance, and not place them in situations that can cause damage, you will never personally experience what happens when a battery explodes. It’s never pretty.