Can a hybrid jump-start another car?

Dear Car Talk

Dear Car Talk | Sep 01, 2008

Dear Tom and Ray:

Can you use a hybrid vehicle to jump-start another car? Recently when my girlfriend was visiting me in Cleveland from D.C., her car wouldn't start. The first thing I tried was jump-starting it from my hybrid. Before I knew it, there was smoke coming out of both cars! Naturally, I panicked and quickly disconnected the cables. I soon realized that the insulation on the cable's pincers was melting and generating the smoke. I'm sure the cables were connected to both 12-volt batteries correctly. Any idea what might cause this? Did I do something wrong? Nobody I talked to knew, and I hoped the Car Talk guys would know the answer. Happy ending: Eventually, we got my girlfriend a new battery, and she was safely on her way home to D.C. -- Tim

TOM: Tim, you might be sure that you hooked up the cables correctly, but we're not.

RAY: Actually, we DO believe you, Tim. If you mixed up the positive and negative terminals, there would not only be smoke, but probably fire, too. And you didn't report any fire.

TOM: Plus, your girlfriend drove away under her own power, and she's still talking to you. Neither of those would have been the case had you crossed the cables. You would have fried a bunch of important electronics in her car -- and yours too. So, I think we can eliminate human error as the cause of the smoke.

RAY: Which leads us to believe that you used a cheap set of jumper cables. If you use really thin cables and send lots of current through them, they're going to get hot. There are ways in which you can exacerbate the situation and make them get VERY hot.

TOM: If your girlfriend's car is much bigger than yours, her battery will try to draw a lot of current from yours. That'll generate extra heat. So will a failing starter motor; if her starter is giving out, it will keep trying to draw current until it either starts, melts or fries the wiring, whichever comes first.

RAY: And all that heat can set some frayed insulation a-smokin'.

TOM: It's possible that if you had left things alone, her car would have started before the fire did. But if you're not a mechanic, and you see smoke coming from the site of a repair you're attempting, abandoning ship is exactly the right thing to do.

RAY: Just to be sure, we checked with Toyota, which sells more hybrid vehicles than any other manufacturer, and it said that there are no restrictions against using the Prius, for example, to jump-start another car.

TOM: The Prius, like most other hybrids, has a 12-volt battery, in addition to its larger, hybrid battery pack. It's a small 12-volt battery, and it's located in the trunk. But there's a pair of jumper terminals, marked with plus and minus signs, under the hood, to allow the car to jump-start or be jump-started.

RAY: As long as that 12-volt battery is in good working order, and is properly charged, it should have done a perfectly adequate job of jump-starting your girlfriend's car.

TOM: But despite that, you did exactly the right thing, Tim. You saw smoke, gave up on doing it yourself and threw money at the problem. Women like that. It shows that your judgment is stronger than your ego. I'm sure it added to your girlfriend's confidence in you.

RAY: And that will pay dividends long after this small, smoky stain on your self-esteem has faded away, Tim.

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