Tom admits to overcharging

Dear Car Talk

Dear Car Talk | Feb 01, 1991

Dear Tom and Ray:

I had a nearly new battery. It went dead all of a sudden. The mechanic said it was overcharged and sold me a new alternator. It's been OK since then. I've never heard of overcharging before. A 195 dollar rip-off?

TOM: Well, Sam, if you're unfamiliar with overcharging, come down to our shop. We overcharge everybody!

RAY: Actually, Sam, a battery can be overcharged. The part that went bad is probably the voltage regulator, which, as the name implies, is supposed to control the amount of voltage going to the battery. When it fails, the electrolyte in the battery can get over-stimulated. That may sound like fun to you, but it's death for your battery. You're actually lucky you didn't need another new battery as well as a voltage regulator.

TOM: And the reason your mechanic sold you an alternator is that in most modern cars, the voltage regulator is built-in to the alternator. If the voltage regulator went bad, your mechanic probably figured that the alternator wouldn't be far behind. The smart thing to do was to replace the whole thing. So your battery did get overcharged, Sam, but you probably didn't.

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