What causes a battery to explode?

Dear Car Talk

Dear Car Talk | Dec 01, 1996

Dear Tom and Ray:

My daughter's 1985 Dodge Caravan has a 2.6-liter Mitsubishi engine. Until
recently, the 3-year-old maintenance-free battery functioned flawlessly.
Recently, after a 15-mile drive, after the engine was off for five minutes
the battery exploded upon restart! It blew off its top, cracked the case on
each side, dented the hood, and severely annoyed my daughter!

After the explosion, the cables were still attached to the battery posts.
The battery components were found to be in order, as was the electrical
charging system. What is your best guess as to the cause of the explosion?
-- Ed

RAY: Oh, she must have ordered the Caravan HE -- the Hindenburg Edition!

TOM: The explosion was undoubtedly caused by a battery that was outgassing
hydrogen. For some reason -- either a defect in the battery or a faulty
voltage regulator that was causing the battery to be overcharged -- that
battery had apparently filled the engine compartment with hydrogen gas.

RAY: And then a spark from the ignition system, or perhaps something else,
caused the hydrogen to explode.

TOM: And I don't believe for a moment that they were able to determine,
after the fact, that the battery components were "in order." They weren't
in order. They were all over the driveway! And if, as you say, the charging
system checked out fine (i.e., it was not overcharging), then I'd conclude
that the battery was definitely not "in order."

RAY: The only question is where the spark came from. On this car, the
ignition system is all the way on the other side of the engine. So either
there was a lot of hydrogen, or the spark was created by a loose connection
in the starter circuit or at the battery terminal itself.

TOM: It's unlikely this is going to happen again, Ed, so you can tell your
daughter that she needn't live in fear of another explosion. But if I were
her, I would do two things:

RAY: I'd get a second opinion on the charging system to make absolutely
sure the battery isn't getting overcharged.

TOM: And I'd always be sure to drive with the hood closed!

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