# UL time: Jumpstarting a battery harmful?
**URL:** https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834
**Category:** Factual Questions
**Created:** [April 30, 2006, 3:35pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834 "2006-04-30T15:35:38Z")
**Posts on this page:** 5
**Page:** 3
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 6, 2006, 1:37pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/41 "2006-05-06T13:37:38Z")
> [@mangeorge](#):
>
> Last time I jumped started a car was for my gf’s brother, back in the 70’s. I had just started my car when he “tested fire” by touching the clamps on his battery. He had guessed wrong. My alternator idiot light immediately came on, and the next day I found that the diodes (in the rectifier) had fried.
> Cost me some bucks.
> I don’t jump cars now, but I’ll call AAA for you. I’ll even give you a little push if you’re driving a stick and don’t have AAA.
I guess that I should change my answer to jumpr starting a car correctly is not harmful. Jump starting a car is not idiot proof. In your case you had an idiot under the hood of your car. The idiot lights on your dash came on to warn you that you had an idiot under the hood. Needless to say this guys was a :wally
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### Author:  [@enipla](https://boards.straightdope.com/u/enipla)
#### Post date: [May 6, 2006, 1:57pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/42 "2006-05-06T13:57:26Z")
> [@](#):
>
> The idiot lights on your dash came on to warn you that you had an idiot under the hood.
Thanks, needed a laugh.
Sheese. I’ve jump started cars trucks and tractors; I would guess 50-75 times. Ever since I was about 10 years old. It’s not that big of a deal.
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### Author:  [@DougC](https://boards.straightdope.com/u/DougC)
#### Post date: [May 6, 2006, 5:44pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/43 "2006-05-06T17:44:30Z")
> [@chaoticbear](#):
>
> Look, I’m no mechanic, but I’ve paid enough attention to the rest of the thread to realize that you’re not talking about jumping a car. You’re talking about installing a new alternator into a car with a dead battery. This is different from jumpstarting a car because in your case, the energy is coming from the alternator. In jumpstarting a car, the energy is coming from the battery of the “good” battery. Although the alternator in the other car is having to run also, I don’t think that it’s going to be as big of a deal since its work load is buffered by the “good” battery.
-
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- Yes but that’s the part most people don’t get–it doesn’t matter if the dead battery is in the “good” car or another connected “bad” car. It’s still harmful to the good car’s alternator.
- When the engine of the good car is running, the alternator voltage is higher than the battery voltage. What happens when you connect two different-voltage power supplies in parallel? The higher-voltage supply “runs out” current until it drops down to the voltage of the lower-level supply, and then they both “share” the remaining load. So which of the two is really putting out most of the power used, the good car’s alternator or the good car’s battery?
…Also Rick–a normal voltmeter of either analog or digital type is not accurate for checking very short voltage spikes.
~
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### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 6, 2006, 6:13pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/44 "2006-05-06T18:13:29Z")
> [@DougC](#):
>
> -
> -
> - Yes but that’s the part most people don’t get–it doesn’t matter if the dead battery is in the “good” car or another connected “bad” car. It’s still harmful to the good car’s alternator.
Over and over again you have been told by people who work with this stuff every day that you are incorrect. You have been asked for cites repeatedly to back your position. The only “cite” you have come up with has to do with alternators and dead batteries. You have been told at least 4 times in this thread that this is not the same as a jump-start. Yet you persist.
> [@](#):
>
> - When the engine of the good car is running, the alternator voltage is higher than the battery voltage. What happens when you connect two different-voltage power supplies in parallel? The higher-voltage supply “runs out” current until it drops down to the voltage of the lower-level supply, and then they both “share” the remaining load. So which of the two is really putting out most of the power used, the good car’s alternator or the good car’s battery?
The alternator ramps up and covers the additional current load almost immediately. And before you hit replay and say AH HA I TOLD YOU SO this is not the same as hooking a new alternator to a dead battery.
> [@](#):
>
> …Also Rick–a normal voltmeter of either analog or digital type is not accurate for checking very short voltage spikes.
> ~
:rolleyes: OK, then you force me to dig out a [Snap-On Vantage Pro](http://buy1.snapon.com/products/diagnostics/vantage_pro.asp?tool=diagnostics&Item_id=76905&PartNo=EETM303A&Price=2999&Desc=&SUB_Cat_ID=125684&SUB_Cat_NAME=Vantage+Pro+Power+Graphing+Meter&Cat_ID=125627&Cat_NAME=Hand+Held+Diagnostics&group_id=1357&group_NAME=Vantage+PRO+Power+Graphing+Meter&store=snapon-store&dir=catalog&pix=objects%2F7800%2FEETM3.JPG&email=true) to do a retest next week after next when I am back at that training center. Are eight million samples per second fast enough for you? Will a 50-microsecond sweep speed meet your “requirements”?
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### Author:  [@AncientHumanoid](https://boards.straightdope.com/u/AncientHumanoid)
#### Post date: [May 6, 2006, 6:33pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/45 "2006-05-06T18:33:33Z")
We should probably ask the MPSIMS psychic how this will all turn out!
**Rick** , your devotion to this question is awesome. Thanks.
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