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Car Battery Explosion

Battery exploded when the car was started. It is a 2005 Toyota Camry, 4-cylinder. What is the cause of this explosion?

The cause could depend on whether the explosion occurred on a cold start or whether it occurred on a restart soon after shutting the engine off.

It could have been a leaking battery or a problem with the plates inside the battery; probably the latter.
With the former, a microscopic spark from the alternator, ignition system, etc. can ignite the hydrogen gas fumes that are emitted and trapped under the hood when the battery is being charged by the alternator.
With the latter, sometimes a lead plate may distort inside the battery. When a couple of plates touch each other the resulting spark will ignite the hydrogen gas inside and cause the battery to explode.

Warped plates can occur if the battery electrolyte level becomes too low so a routine check of the level should be done ever so often if the battery is the type with removeable caps. Hope that helps and now you know why the Hindenburg went down.

Thanks for replying. It was a cold start. Car is only 3-1/2 years old with the original battery. Is this a factory defect? Do you think it’s still under warranty? The car’s basic warranty ended (3 years/36000 miles whichever comes first).

Batteries don’t explode very often but they can do it whenever they want to with no good reason that can be detected.

Toyota buys batteries from battery manufacturers, and installs them in new cars. The battery manufacturer warrants the battery. Check the battery warranty that came with the car. It will tell you if there is any coverage for this.

Sorry, but it’s unlikely a battery would be covered by warranty. That’s a separate deal from the factory warranty, just like tires. At 3 and 1/2 years most batteries are approaching the end of their lives or at least becoming iffy.

Since it blew up on a cold start it was probably caused by a distorted plate in the battery. The plate can distort if the electrolyte level drops too much. The alternator is continuing to charge the battery while it’s low and excess heat will warp the lead plates.
I have no idea if the battery was a maintenance free battery (in this case, one of those freak accidents) or the battery has removeable caps (they’re removed now for sure) but if it’s the latter the battery electrolyte level should be inspected about every 6 months or so. This falls into the same category as checking the engine oil on a regular basis to determine if it also low.

Sometimes, it is also due to hydrogen gas getting concentrated, and finding a spark. Hydrogen gas is given off when the battery is charging. Either way, it’s a dangerous situation, with a lot of sulfuric acid being blown onto a lot of sensitive stuff. I hope no one got hurt.