Is Art dreaming, or were copper pennies once used to save batteries?

Dear Car Talk

Dear Car Talk | Sep 01, 2010

Dear Tom and Ray:

I may be dreaming, hallucinating or even suffering from dementia. I seem to remember something from my childhood that other people my age do not. When I was but a tadpole, I seem to remember my dad checking the water level in the car battery. The top of the battery was covered with a thick, tarlike substance, into which he would push copper pennies next to each battery terminal. The reasoning behind this was to draw the potential corrosion away from the terminal and toward the penny. Did I dream this, or did people used to do this? Please tell me if I'm demented or not. -- Art

RAY: We don't have enough information to answer your last question, Art. And the fact that you're writing to us for advice definitely is a strike against you. But you're not dreaming about the pennies and the batteries.

TOM: It's based on the theory of sacrificial anodes, in which you "sacrifice" a more reactive metal -- copper, in the case of pre-1983 pennies -- to protect a second, less reactive metal -- the lead battery terminal and connector.

RAY: And in the old days, when batteries were covered with tar on top, you could warm up a penny with a match or a cigarette lighter, and then slide it into the tar half an inch away from the battery terminal. The penny always would corrode first.

TOM: Nowadays, most batteries are sealed in plastic, so the acid -- which is what causes the corrosion -- rarely escapes the inside of the battery. That makes a sacrificial anode far less necessary. And besides, these days, we all have to save our pennies.

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