Why did starting my car with a trickle charger attached to the battery cause said battery to explode?

Dear Car Talk

Dear Car Talk | Feb 01, 1999

Dear Tom and Ray:

Several months ago I was having some difficulty with my car battery. It was not
holding a charge, so I had the bright idea of putting a trickle charger on it and
charging it up overnight so that I could get my '84 Prelude running and drive it
down to the shop for a battery check. The next morning, I started the car with
charger still attached and I blew up the battery. Fourteen pounds of sodium
bicarbonate later, I had the battery acid neutralized and I called a tow truck.
So that I and others can avoid future pyrotechnics, can you tell us how to avoid
blowing ourselves up along with our car batteries? -- Ray

TOM: Yes. The best way to avoid blowing yourself up is by calling AAA and letting
them risk blowing themselves up trying to start your car. That's worth 40 bucks a
year, isn't it?

RAY: In your case, Ray, I'd have to guess that your battery was out-gassing. It
probably over-charged on the trickle charger, which caused it to produce
hydrogen, and when you started it the next morning, something created a spark,
and recreated the Hindenburg disaster in your driveway.

TOM: If I had to guess, I'd say the spark was probably caused by a loose
connection at the battery terminal, which may have also caused your original
battery problem.

RAY: Just be glad you were in the car, rather than standing in front of the

TOM: In general, batteries explode because of the convergence of hydrogen and
sparks. So you want to do everything you can to avoid both of those.

RAY: Hydrogen is harder to avoid, because you can't see it, smell it or hear it.
My brother likes to detect it by lighting a large Havana cigar, but that, in
part, explains the unusual appearance of his facial features.

TOM: Batteries that have been overcharged are the most likely to emit hydrogen,
but because batteries that are faulty in other ways can also outgas, you have to
assume that there is hydrogen in the area of your battery. That's the safest
thing to do.

RAY: That's why you concentrate on preventing sparks. You do that by being
careful with the jumper cables. Don't let them dangle around and bounce into each
other or bounce off parts of the engine while you're hooking them up. Connect
them carefully, in the order prescribed by the instructions.

TOM: And the instructions on almost every set of jumper cables we've ever seen,
tell you not to make the final connection to a battery terminal. Since that's the
last connection -- completing the circuit -- you increase the chance of a spark
jumping from the jumper cable end to the terminal. So instead, make the final
connection to a grounded, solid metal piece of the engine -- away from the

RAY: And another precaution is to stay away from the battery when you're actually
starting the car. After hooking up the jumper cables, most people stand around
and watch the engine to see if it starts. This is dumb. You'll hear it start.
Trust me. You're safer inside your car.

TOM: Of course, you're safer still if you're watching AAA do the whole thing from
your living room window while politely asking them, "You guys done yet?"

?(C) 1999 by Tom and Ray Magliozzi and Doug Berman
Distributed by King Features Syndicate, Inc.

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