### Author:  [@Oz\_Trekker](https://boards.straightdope.com/u/Oz_Trekker)
#### Post date: [May 2, 2006, 10:03am UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/21 "2006-05-02T10:03:28Z")
> [@DougC](#):
>
> * * *
>
> …As Rick said a charged battery acts as an capacitor so removing the cable as quick as possible is the wrong way to go,leaving them in place for a while allows the flat battery to collect some charge and reduces the chance of a spike… - Oz Trekker
>
> - No, this is false. _As soon as the dead car begins to run_, you should be able to disconnect the jumper cables and the car should continue to run. If it doesn’t, then either the alternator or the battery in that car is bad already–and like I said–jumping won’t fix that, and may harm the car that does the jumping. All that jumping such a car accomplishes is that the “good” car stores up a charge in the battery that the bad car will ignite off of for a mile or two, and then will stop running again.
> ~
Jumper starting always good for a debate isn’t it, I believe its car with flat battery that’s at risk from spiking . I run dual batteries in my RV and no trouble so far charging my second deep cycle battery from almost empty.
---
### Author:  [@DougC](https://boards.straightdope.com/u/DougC)
#### Post date: [May 2, 2006, 10:16am UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/22 "2006-05-02T10:16:31Z")
> [@](#):
>
> …Hmm about that huge rush of current. So here is what I did, I parked the electron donor car next to the front of the dead car. I hooked the jumper cables to three of the connections. I hooked up the Sun VAT 40 with the amp clamp set to register the load placed on the system by the dead battery. With the keys off in both cars, I hooked up the last connection. The draw was, are you ready for this? ONE Amp. One single solitary amp. About what a taillight bulb draws… - **Rick**
-
-
- Are you certain the equipment you used can measure peak currents properly? A “dead” car battery is around 10.5 volts, and a “good” one is about 12.5 volts. So there was a 2-volt difference between the two batteries when you first hooked them together. What’s the internal resistance of a car battery?
~
---
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 2, 2006, 12:58pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/23 "2006-05-02T12:58:02Z")
😃 So yesterday afternoon I was discussing this thread with a couple of technical specialists from our company and getting their opinions. Then I told them of the experiment I was going to perform. My colleague then said, after you prove him wrong he is going to ask why you did not hook up a scope, and look for a nano second long amp spike.
It appears that my colleague is can read the future. Maybe I will ask him for Saturday’s lotto numbers.
First off, the equipment used was a SUN VAT 40. While old it is still considered to be the standard of the industry for testing batteries and alternators and starters. Many schools still use a VAT 40 as to teach on since it is a strictly manual machine, the operator must know what they are doing. If you learn battery/alternator/starter testing on a VAT 40 you can do it on any of the newer computerized digital machines.
Is it accurate? Without a doubt.\*
Will it measure a peak amp load? If it lasts long enough, of course. Being an analog meter it does not read instantaneously. It takes a second or so for the meter to deflect. That is why I disabled the ignition, so that I could crank the car long enough to get a good reading.
Now I did not measure for a peak transient, but I am not concerned about it. Why? First off there was no spark when the last cable was attached. Spark = high current, no spark = low(er) current. If there was a high current transient there would have been a spark. Secondly and most importantly I was not worried about a transient I was looking for a drain to answer your assertions
> [@](#):
>
> - Just hooking the batteries up is bad as well–discharging a fully-charged battery too fast (like, connecting a good battery to a uncharged battery) can damage the good battery as well. (post #7)
>
> 1. You aren’t supposed to rapidly discharge a car battery, as it suffers cumulative damage–and may explode. This is why there are deep-cycle batteries made for running trolling motors. Yes you can do it to an automotive battery once, twice, a few times–but you are drastically shortening the battery life if you do. (post #11)
As far as your question about the internal resistance of a battery goes, off the top of my head I don’t know. I do know that the greater the state of charge the greater the resistance. Other than that who cares? It is not germane to the discussion.
Look we are in general questions Factual answers to questions. I have supported my position with facts and cites. You have not. If jump starting a car were in fact dangerous, there should be cites all over the net from reputable sources to this fact. The fact that there are not warnings all over not to jump start a car from the people that would have the most to lose if it were dangerous (car companies, alternator makers / rebuilders / battery companies) should tell you that this is not something that will
A) Destroy your battery
B) Kill your alternator
C) Get your girlfriend pregnant.
So I say again
**CITE?** to your claims
\*Frankly I was expecting a larger than 1 amp draw when I hooked up the cables. I was on a 100 amp scale with 2 amp divisions. The meter deflected about halfway to the first division. Hell it might have been 1.2 amps in reality, but who cares. It was not destroying my battery as **DougC** maintains.
---
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 2, 2006, 1:27pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/24 "2006-05-02T13:27:05Z")
Some other answers to questions brought up by other people
> [@enipla](#):
>
> In the case of an absolutely dead battery, I have had to leave them hooked up for a few minutes before trying to start the dead vehicle. And of course the car giving the jump will be running.
>
> I’ve got good quality jumper cables, and make sure to make a good connection on both vehicles. I usually ground the negative to the ground cable that is often hooked up to the alternator mount. It sometimes seems that an absolutely dead battery in the hook up is draining to much power getting charged and not enough amps can be diverted to properly turn the starter. After a few minutes, it’s usually no problem. \But, I prefer to never leave two running unattended cars in this position. Vibration can make one of the clamps fall off.
This is probably a limitation of the current carrying capacity of the cables you are using. If your cables can only transmit 80 amps, and the car takes 100 to crank, you will have to allow 20 amps to flow into the battery before you can jump start it.
I agree with your statement about a clamp vibrating loose. I don’t like leaving cables on too long myself from a safety point of view, not from an danger to an electrical system point of view. I am always worried that they might fall into the fan. One more thing only 2 batteries in 13 years? [Mr Burns]excellent![/MB]
> [@Cheesesteak](#):
>
> I wouldn’t want to leave your stone dead battery connected to my good battery any longer than necessary either. I don’t want my battery getting drained trying to charge up yours.
Assuming that you have your donor car running, your battery won’t get drained, the car does not run off the battery, it runs off of the alternator. Your alternator is just working a little harder that is all.
> [@David Simmons](#):
>
> Unless you buy the cables with really good clamps I not sure I would even put their current capacity that high.
I buy good cables.
> [@Buck Rodgers](#):
>
> I’ve seen people blow the rectifier out in the Alternator in either vehicle. If a problem is going to take place it’s going to be at the connection if a surge takes place or if the car with the dead battery puts too much load on the alternator. Sparks have blown up leaky batteries that are producing a gas.
Two conditions could cause the diodes to blow in the alternator First if the battery is stone dead and the cables are hooked up backwards, the diodes will blow (along with some other stuff, this can be ugly) or the second possibility is that the alternator was on its last legs, and the extra load caused it to fail a couple of days early. As far as battery explosions go, they are dangerous and scary as hell. That is why your last connection should always be done to an engine or body ground point away from the battery.
> [@OZ Trekker](#):
>
> I believe its car with flat battery that’s at risk from spiking .
You are correct.
---
### Author:  [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)
#### Post date: [May 2, 2006, 3:44pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/25 "2006-05-02T15:44:55Z")
Well, I appreciate all of your testing, and when I get home later today, I’ll be sure to show your results to Anagramless Guy in hopes that he will believe it. He’ll probably just defend what he was always told as a child, anyway, though.
---
### Author:  [@DougC](https://boards.straightdope.com/u/DougC)
#### Post date: [May 2, 2006, 5:30pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/26 "2006-05-02T17:30:04Z")
-
-
- The numbers point to damage:
1. several places online gave a figure of .003 ohms as the internal resistance of a car battery and the difference between a charged battery and a non-starting battery is at least 2 volts, and may be higher if the dead battery is drained even more. Two volts into a .003 ohm load is around 666 amps. A regular meter (analog or digital) is not going to catch this.
2. New alternators come with instructions warning not to connect them to a dead battery; they say to trickle-charge the battery before starting the car. If connecting a running alternator to a dead battery is harmless, then why do they state this? And if the two batteries are simply attached in parallel, how do the electrons from your car know to only go into the “good” battery and not the “bad” one?
~
---
### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [May 2, 2006, 6:12pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/27 "2006-05-02T18:12:36Z")
> [@DougC](#):
>
> -
> -
> - The numbers point to damage:
>
> 1. several places online gave a figure of .003 ohms as the internal resistance of a car battery and the difference between a charged battery and a non-starting battery is at least 2 volts, and may be higher if the dead battery is drained even more. Two volts into a .003 ohm load is around 666 amps. A regular meter (analog or digital) is not going to catch this.
Where do you come up with these untrue, unsubstantiated pronouncements? There’s no way that anywhere near that kind of amperage is going to be present in any of the situations under discussion here. And if it were, I guarantee you that the test equipment used by **Rick** and me would react to it.
> [@](#):
>
> 1. New alternators come with instructions warning not to connect them to a dead battery; they say to trickle-charge the battery before starting the car. If connecting a running alternator to a dead battery is harmless, then why do they state this? And if the two batteries are simply attached in parallel, how do the electrons from your car know to only go into the “good” battery and not the “bad” one? ~
The reason alternators have that instruction isn’t because it’s harmful to connect a running alternator to a dead battery. It’s because it’s harmful to make an alternator try to do the job of a battery charger, bringing a dead battery up to full charge. But that’s not what’s involved in a jump start.
Look, **Doug** , I know you mean well, but you’re out of your league here. **Rick** is an expert in this area. He teaches it. He knows what he’s talking about. I’ve been an auto repair professional for over 30 years, and I know what I’m talking about. There are 8 or 9 factual misstatements in your posts on this thread. It’s clear you don’t know what you’re talking about. Give it a rest.
---
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 2, 2006, 6:20pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/28 "2006-05-02T18:20:07Z")
> [@DougC](#):
>
> -
> -
> - The numbers point to damage:
>
> 1. several places online gave a figure of .003 ohms as the internal resistance of a car battery and the difference between a charged battery and a non-starting battery is at least 2 volts, and may be higher if the dead battery is drained even more. Two volts into a .003 ohm load is around 666 amps. A regular meter (analog or digital) is not going to catch this.
> 2. New alternators come with instructions warning not to connect them to a dead battery; they say to trickle-charge the battery before starting the car. If connecting a running alternator to a dead battery is harmless, then why do they state this? And if the two batteries are simply attached in parallel, how do the electrons from your car know to only go into the “good” battery and not the “bad” one?
> ~
No the difference between a fully charged and flat battery may be as little as .75 of a volt. [Cite](http://autorepair.about.com/library/weekly/aa101604b.htm) There are many more authoritative sources on the net that give these same (or very similar) numbers. Where did you get the “fact” that a dead battery is 10.5 Volts?
Are you seriously saying to me that I, as a trained automotive technician, with over 30 years of experience would fail to notice the huge arc as 666 amps arc-welded my jumper cables to the engine block of the car with the dead battery? Do you really think that I am that brain dead? Stop and think for just one second 600+ amps is a huge load and would melt most if not all jumper cables. The only place where the internal resistance of the battery comes into play is if you lay a metal bar across the battery terminals. The internal resistance is governs the amount of current that can flow across the metal bar. And why do you say my meters would not pick up such an incredible load?
Tell you what, I have just turned the head lights that car with the bad battery. This afternoon after class I will repeat the experiment using the latest digital equipment set to capture max amp reading. IIRC correctly this equipment will capture a 1 ms glitch. I will report back with my results. So if your 666 amp load exists for more that 1/1000 of a second my equipment will capture it.
About the alternator hooked to a dead battery thing in my very first response to you I said
> [@](#):
>
> –You don’t hook up an alternator to a fully dead battery, because alternators are not designed to charge a dead battery, they are designed to keep a fully charged battery fully charged, or to top up an almost fully charged battery.
In essence the alternator is a trickle charger. You probably won’t drive the car long enough to fully charge the battery. Picture you are an alternator rebuilder. You know that the alternator is a trickle charger. You also know your customers are not professionals and do not know this. Further you know that if your idiot err not professional customer hooks up a new alternator to a dead battery then they drive the car for 15 minutes parks it. If it does not restart (it probably won’t), they are going to return your alternator and tell you it is defective. Given that, don’t you think it would be wise to include a reminder that you must charge the battery before hooking the alternator up? I do.
As far as where the electron knowing where to go, electricity always takes the path of least resistance. As I mentioned before the internal resistance of the battery goes up as it approaches a full charge. Conversely the resistance goes down as the battery discharges (I am not going to discuss sulfation, that is a subject for a different thread) Therefore the electron will go to the discharged battery since it has a lower resistance. Simple.
---
### Author:  [@Skywatcher](https://boards.straightdope.com/u/Skywatcher)
#### Post date: [May 2, 2006, 9:23pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/29 "2006-05-02T21:23:59Z")
> [@Rick](#):
>
> 666 amps
The Current of the Beast?
---
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 2, 2006, 10:17pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/30 "2006-05-02T22:17:52Z")
> [@Lute Skywatcher](#):
>
> The Current of the Beast?
That’s what I am guessing. 😃
---
### Author:  [@Rick](https://boards.straightdope.com/u/Rick)
#### Post date: [May 3, 2006, 4:05am UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/31 "2006-05-03T04:05:53Z")
OK, as promised I left the key on the car with the bad battey until the voltage was so low the relay controlling the headlights would no longer stay engaged. This is pretty seriously dead. The dash lights were very dim. Voltage measured at the battery was 10.75 volts. Since 11.89V or so is considered dead, this is way dead.
Using a [Fluke](http://us.fluke.com/usen/products/Fluke+78.htm?catalog_name=FlukeUnitedStates) meter I went and set up the jumper cables as I described in the post from yesterday.
With the key off on both cars and I hooked up the the last connection the draw was
Drum roll please
.66 Amp\* which over the next 30 seconds or so increased to 1.45 amps Pretty much agreeing with what I observed yesterday.
Starting the good car gave me a draw of 9 amps (8 yesterday)
Turning the key on the dead car gave me reading of 20 amps\*\* (18 yesterday)
Cranking the car the draw increased to 95 amps.\*\* this makes perfect sense because almost all the power this time had to come from the good car. I really put that battery down for this test.
- This meter only reads to .01 amp if it read to .001 amp I am sure it would have read .666 amp which would have been the current of the milli beast. 😃
\*since the Fluke only has the ability to read 10-15 amps without a seperate amp clamp, these last two reading were done with the VAT 40 I used yesterday.
---
### Author:  [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)
#### Post date: [May 3, 2006, 4:09pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/32 "2006-05-03T16:09:13Z")
Beautiful, brings a tear to the eye. 😉
---
### Author:  [@NurseCarmen](https://boards.straightdope.com/u/NurseCarmen)
#### Post date: [May 4, 2006, 5:49pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/33 "2006-05-04T17:49:20Z")
I just gotta say, Rick and GaryT, you guys rock.
---
### Author:  [@robcaro](https://boards.straightdope.com/u/robcaro)
#### Post date: [May 4, 2006, 9:52pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/34 "2006-05-04T21:52:02Z")
Here is a link that I found about this subject:
[http://www.nsc.org/library/facts/batjump.htm](http://www.nsc.org/library/facts/batjump.htm)
---
### Author:  [@DougC](https://boards.straightdope.com/u/DougC)
#### Post date: [May 5, 2006, 6:22pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/35 "2006-05-05T18:22:05Z")
-
-
- If jump-starting another car is not harmful, then why do all alternators come with instructions telling you NOT to jump-start new alternators on a dead battery? And if you are jump-starting someone else’s car, how do the electrons from your car know that the dead battery is in someone else’s car and not yours?
[http://www.boschusa.com/AutoParts/FAQs/AlternatorsStarters/](http://www.boschusa.com/AutoParts/FAQs/AlternatorsStarters/)
> [@](#):
>
> Q. Can I use my alternator to recharge a discharged battery?
>
> ```
> A. No! When installing an alternator, **the battery should be fully recharged before vehicle use**. In many cases, depending upon how thoroughly discharged the battery is, it can take 4-8 hours. Most technicians and DIYers don’t want to wait that long so they just get the vehicle running, and figure that by driving the car around the alternator will charge the battery. In some cases with weather extremes the battery never gets fully charged, and in all cases the alternator is being overly stressed asking it to do a job it was not designed to do. This leads to premature failure....
>
> ```
- I said before: "yes, it will work once, twice, a few times, " but it does do cumulative damage. At least, _somebody at Bosch thinks it does_…
~
---
### Author:  [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)
#### Post date: [May 5, 2006, 7:16pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/36 "2006-05-05T19:16:23Z")
> [@DougC](#):
>
> -
> -
> - If jump-starting another car is not harmful, then why do all alternators come with instructions telling you NOT to jump-start new alternators on a dead battery? And if you are jump-starting someone else’s car, how do the electrons from your car know that the dead battery is in someone else’s car and not yours?
>
> [http://www.boschusa.com/AutoParts/FAQs/AlternatorsStarters/](http://www.boschusa.com/AutoParts/FAQs/AlternatorsStarters/)
>
> - I said before: "yes, it will work once, twice, a few times, " but it does do cumulative damage. At least, _somebody at Bosch thinks it does_…
> ~
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.
---
### Author:  [@Gary\_T](https://boards.straightdope.com/u/Gary_T)
#### Post date: [May 5, 2006, 7:34pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/37 "2006-05-05T19:34:03Z")
> [@DougC](#):
>
> -
> -
> - If jump-starting another car is not harmful, then why do all alternators come with instructions telling you NOT to jump-start new alternators on a dead battery? And if you are jump-starting someone else’s car, how do the electrons from your car know that the dead battery is in someone else’s car and not yours?
You have raised both of these questions earlier in this thread. I know this because I read your questions. Both questions have been answered. I know this because I wrote one answer (post #27), and read the other (post #28). I suggest you read the answers, and if you don’t understand ask for clarification.
I will offer this clarification on the first question. The harm to the alternator is not in the jump starting. The harm is in the alternator working for hours trying to fully charge a significantly low battery, just like your link said. The alternator in the donor car is not at risk for this. So the warning you keep asking about is to prevent someone from putting a new alternator in his car which has a dead battery, getting a jump start, and then overworking his new alternator trying to fully charge that battery. The warning is not to prevent people from jump starting at all, and the warning is not based on concerns of the jump starting process harming either alternator. That’s something that you have (erroneously) read into it. Notice that even the question you cited doesn’t ask about any harm in jump starting, rather about harm in letting the alternator attempt to do the job of a battery charger. And the answer doesn’t say there is any harm in jump starting - it says the harm is in using the alternator to fully charge a dead battery.
So why do alternators come with instructions telling you NOT to jump-start new alternators on a dead battery? For the reasons outlined by **Rick** , Bosch, and me, and \*\*
```auto
NOT
```
BECAUSE JUMP STARTING ANOTHER CAR IS HARMFUL.\*\*
---
### Author:  [@mangeorge](https://boards.straightdope.com/u/mangeorge)
#### Post date: [May 5, 2006, 9:49pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/38 "2006-05-05T21:49:40Z")
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.
---
### Author:  [@Skywatcher](https://boards.straightdope.com/u/Skywatcher)
#### Post date: [May 5, 2006, 10:15pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/39 "2006-05-05T22:15:10Z")
Our van’s battery died yesterday and we had to get our roadside service to give us a jump.
---
### Author:  [@mangeorge](https://boards.straightdope.com/u/mangeorge)
#### Post date: [May 5, 2006, 10:18pm UTC](https://boards.straightdope.com/t/ul-time-jumpstarting-a-battery-harmful/354834/40 "2006-05-05T22:18:55Z")
> [@Lute Skywatcher](#):
>
> Our van’s battery died yesterday and we had to get our roadside service to give us a jump.
If you can afford it, that $50/year is money well spent.
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