Is revving the engine a good idea during a jump-start? Find out.

Dear Car Talk

Dear Car Talk | Apr 01, 2007

Dear Tom and Ray:

My boyfriend insists on revving the engine when he's performing a jump-start. He revs the good car when he starts it up, and then revs the bad car when he starts it up. Is there any reason for doing this? Is there anytime when it is good or when you need to rev your engine? Thanks. -- Amanda

RAY: Bad news, Amanda. Your boyfriend is more or less right. I know that's not the news you were hoping to hear.

TOM: Revving up the engine during a jump-start is not a bad idea, within reason. You never want to "floor" the gas pedal, under any circumstances. But when your engine turns faster, the engine's alternator also turns faster. And when an alternator turns faster, it produces more electricity.

RAY: So by revving your engine up to 2,000 rpm or 2,500 rpm and holding it there while the other car tries to start, you're increasing the output of your alternator and giving your battery a little bit of extra power, which it can donate to the other car. And depending on the respective engine sizes and battery conditions, that extra power could make a difference.

TOM: It's also smart to make sure that when you jump-start another car, all of the electrical accessories are turned off. The less electricity you're demanding for your own car, the more you can donate. So you want to turn off the headlights, the air conditioner and subwoofer, and the electric butt-scratcher.

RAY: For the donee -- the car that's receiving the jump-start -- the same rules apply. You want to turn all of the accessories off. That way, all of the alternator's power can be directed to recharging the battery. Once the car starts, you CAN rev it up to charge the battery faster, but the best way to do that is to just drive it.

TOM: Depending on how dead the battery was, you may need to drive it for 20 minutes or a couple of hours.

RAY: Right. If you just left the dome light on, 20 minutes or half an hour of driving should be enough. But if you left the headlights and the Easy Bake Oven on all night, it might take a lot longer for the battery to fully charge again. In fact, if it's really run down, you might have to take the car to a service station and have the battery put on a charger.

TOM: Of course, there's also the possibility that your battery might be truly dead -- i.e., at the end of its life. In which case, you can drive to Fairbanks, Alaska, and back but it won't recharge. You'll need a new battery. Which is why, after you jump-start your car and drive it for a while, make sure -- before you try turning it off and restarting it -- you stop it somewhere that's "in the way," like in front of some gas pumps. That guarantees that someone will have to give you another jump start!

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