True or false: It's possible to blow up your battery by starting your car with the AC on.

Dear Car Talk

Dear Car Talk | Apr 01, 1998

Dear Tom and Ray:

I think Disney may make a movie about me: "Honey, I Blew Up the Car." I ran a
12-mile errand with the company car, a '93 Escort wagon with automatic
transmission. It was a hot day and I had the air conditioner on. The next
morning, I had to run another errand with the car, and when I turned the key,
there was an explosion under the hood. I had blown up the battery. The battery
was replaced, but the mechanic told my boss that the reason the battery blew up
was because I tried to start the car with the air conditioner on. Is this an
accurate diagnosis? I'd sure appreciate your opinion on this. -- Randy

TOM: Wow! And I thought we were the masters of bamboozlement!

RAY: Yeah. This mechanic has his headlight firmly implanted in his taillight
socket, if you know what I mean, Randy. There's no way that trying to start the
car with the AC on could cause the battery to blow up.

TOM: Even if you leave the AC switched "on" inside the car, it is automatically
cut off (along with most other electrical accessories) when you turn the key to
the "start" position. The car is set up that way so that all of the battery's
power can be focused on getting the car started.

RAY: What happened is that you had a bad battery that started out-gassing
hydrogen. And a spark created by a loose connection when you turned the key
probably ignited the hydrogen and caused the explosion.

TOM: The battery could simply have been out-gassing because it was old and
rotten, or it could have been ruined by an overactive charging system.

RAY: I hope this mechanic had the sense to check out the charging system when he
replaced the battery. If not, the same thing could happen to the new battery,
too. And when the new battery blows up, he's going to have to come up with a
whole new ridiculous excuse.

* * *

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