# Jump starting cars and reversing the poles **URL:** \ **Category:** Factual Questions\ **Created:** [January 13, 2010, 7:18pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820 "2010-01-13T19:18:18Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 14, 2010, 3:05am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/21 "2010-01-14T03:05:48Z")
> [@electronbee](#): > > I doubt the current rating of a cigarette lighter can handle a starter for a car. Just look at the thickness of the wires going to your starter and that “cigarette jumper cable” and then a proper pair of jumper cables. There is a reason why they are so thick. If your car is weak that might work. But, if it is totally dead, I would not do that. I can easily see a blown fuse if not some toasted wires within the dashboard. It won’t handle starter current _(hundreds of amps)_, but you can trickle-charge through it for an extended period _(half an hour? depends how dead the battery really is)_, and then disconnect the charging battery and hit the starter. ---
**Author:** ![FoieGrasIsEvil](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/foiegrasisevil/32/16638_2.png) [@FoieGrasIsEvil](https://boards.straightdope.com/u/FoieGrasIsEvil)\ **Post date:** [January 14, 2010, 3:30am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/22 "2010-01-14T03:30:10Z")
As I understand it, the proper way to jump a car (and I’ve done this dozens of times without issue) is: connect positive to good battery, then positive to dead battery connect negative to good battery, ground negative to metal surface in engine compartment of dead car. don’t ever let leads touch each other, and when finished, disconnect in exactly reverse order. works like a champ. ---
**Author:** ![Critical1](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@Critical1](https://boards.straightdope.com/u/Critical1)\ **Post date:** [January 14, 2010, 5:36am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/23 "2010-01-14T05:36:09Z")
> [@electronbee](#): > > I doubt the current rating of a cigarette lighter can handle a starter for a car. Just look at the thickness of the wires going to your starter and that “cigarette jumper cable” and then a proper pair of jumper cables. There is a reason why they are so thick. If your car is weak that might work. But, if it is totally dead, I would not do that. I can easily see a blown fuse if not some toasted wires within the dashboard. > > eb comes with a sort of box with some led lights, when the green turns on you try and start the car, supposed to take 10 min or so. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 14, 2010, 6:02am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/24 "2010-01-14T06:02:19Z")
Wow lots of strange opinions here > [@Superhal](#): > > Highly likely. Average cars are not grounded properly to give jumps, so even correctly jumping can damage a car, especially today when nearly all cars have an onboard computer. > > If the donor car is a pickup truck or work vehicle, it may have proper grounding. Not properly grounded? Waaaa? A pickup, a semi and a KIA are grounded exactly the same way. This statement of yours makes no sense what so ever. > [@capeo](#): > > You’re lucky the car getting jumped wasn’t damaged. The charging car really shouldn’t be a problem. Crossing jumper cables can destroy radios, amps, blow fuses and in extreme cases even ruin an alternator, or worse, your car’s EFI or PCM (although they are usually protected with a relay that opens if reverse current is applied to it). Not mention you can blow up your battery. Assuming that you make the last connection with the dead car with the ignition switch OFF, the big assed spark would be your warning that something was not Kosher and assuming you A) did not turn on the key, and B) removed the cables and got them right it is a no harm no foul situation. > [@awldune](#): > > I would be worried about the friend’s car. Do you know that it is OK? > > From a similar stupid act (I installed a battery backwards) the following items had to be replaced: ($$$) > > Alternator > ECU (engine computer) > Several expensive relays > Ignition module > Just about every fuse in the fusebox > > Not recommended, especially if you drive a european car. Fortunately I was able to obtain junkyard replacements for many of these, and avoided spending $1200 on a new ECU, for example. Big difference between a jumper cable that sparked and a battery that is capable of delivering about 600A instantaneously. Last battery I saw installed backwards was about $2500. It was ugly. > [@Critical1](#): > > idiot proof jumper cables are great, > I picked up a pair of interesting ones from think geek recently that charge via the cigarette lighters. they also have some that you connect the clips to the batteries then connect the cables in the middle. Except for one little detail. On some cars the power socket is switched via the ignition switch. You have to have the ignition switch on. Those cable can transfer about 10A. My car draws 15A with the key on. How long do I have to leave the cables attached before I can jump start my car? :smack: [QUOTE=smoke] After we were done, we pushed his car back into its spot. Not that it would take much juice or anything, but the remote trunk release still worked, and his headlights still came on. Today 06:06 PM [/QUOTE] And now Ladies and Gentlemen we have the reason the cables sparked. The battery on the “dead” car was not dead, it was only mostly dead. Not strong enough to start the car, but strong enough to cause a spark and melt the cables. If I had to venture a guess, I would bet that those cables are the very cheap consumer grade jumper cables. They will probably melt at between 50-100Amps. The starter in the car might pull as much as 200A (or more) It does not take much to destroy cheap cables. I have had them melt just from the starter draw. Now I believe that there probably is no damage done to either car. Assuming that you did not leave the cables connected for more than a moment or two. If the dead car had a stone flat battery and no spark resulted and you tried to start it, then then very well could be damage. Depending on the year and model lots of damage. But based on what I have read here, I would not be too concerned. **Gary T** What do you think? ---
**Author:** ![kombatminipig](https://avatars.discourse-cdn.com/v4/letter/k/b5ac83/32.png) [@kombatminipig](https://boards.straightdope.com/u/kombatminipig)\ **Post date:** [January 14, 2010, 8:55am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/25 "2010-01-14T08:55:24Z")
> [@Rick](#): > > Wow lots of strange opinions here > Not properly grounded? Waaaa? A pickup, a semi and a KIA are grounded exactly the same way. This statement of yours makes no sense what so ever. Yeah, I was wondering about that myself. What, do pickups have ground cables dragging beneath the chassis? In Sweden, where we have an ancient car pool and generally long winters, jump starting cars is common practice. As long as you’re aware of the difference between a parallel and a serial connection, you’re generally fine. ---
**Author:** ![EvilTOJ](https://avatars.discourse-cdn.com/v4/letter/e/ee59a6/32.png) [@EvilTOJ](https://boards.straightdope.com/u/EvilTOJ)\ **Post date:** [January 14, 2010, 9:16am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/26 "2010-01-14T09:16:34Z")
The way I’ve always jumped a car is positive to positive, negative to negative. I was taught that it was positive to positive, negative to the body of the dead car, but the first way works fine for me. I also have some homemade jumper cables made with 4 AWG wire that’ll reach behind the van, since I always seem to leave my lights on in a place where I can’t get another vehicle alongside the other car. And to answer the OP, if you screwed up the car, you’d know it immediately. it’s not like cancer or emphesema where it slowly creeps up on you. I should know! I’ve done everything wrong when it comes to jumping cars. I’m surprised I’m not dead. The first time I tried jumping a car on my own, I don’t know what I did but there was a blinding flash and the battery posts were completely melted. That sucked since it was winter. Luckily they were early 80’s cars so they had very little in the way of electronics in them. I wonder how many people have used that cigarette lighter trickle charger on their own vehicle. 😃 ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [January 14, 2010, 10:16am UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/27 "2010-01-14T10:16:04Z")
> [@EvilTOJ](#): > > The way I’ve always jumped a car is positive to positive, negative to negative. I was taught that it was positive to positive, negative to the body of the dead car, but the first way works fine for me. Yea, positive to positive and negative to negative. But the last connection makes a spark. You don’t want a spark around a battery. So the last connection should be negative, and should be made to the frame of the car. It doesn’t matter if it’s the donor car or receiver car… just make sure the last connection is negative and it’s made to a sturdy, non-painted, metal bracket. I usually use the alternator bracket. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [January 14, 2010, 12:34pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/28 "2010-01-14T12:34:42Z")
> [@Rick](#): > > Except for one little detail. On some cars the power socket is switched via the ignition switch. You have to have the ignition switch on. Those cable can transfer about 10A. My car draws 15A with the key on. How long do I have to leave the cables attached before I can jump start my car? :smack: I don’t understand this. Why is your car drawing 15 amps with the key on, but the radio, headlights, interior lights turned off? What’s using all that power? ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 14, 2010, 1:46pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/29 "2010-01-14T13:46:12Z")
> [@EvilTOJ](#): > > The way I’ve always jumped a car is positive to positive, negative to negative. I was taught that it was positive to positive, negative to the body of the dead car, but the first way works fine for me. I also have some homemade jumper cables made with 4 AWG wire that’ll reach behind the van, since I always seem to leave my lights on in a place where I can’t get another vehicle alongside the other car. > 😃 Yeah, the more I think about it, the more I hav it backward. Connecting ground last, to the body nowhere near the dead battery, ensures that if the battery has produced hydrogen you are less likely to spark and set it off. Of course nowadays, I don’t know if the voltage regulator etc. are better technology than 20 years ago; would they stop the alternator from producing excess current through the rectifiers in the event of a voltage-drop from a short? Certainly the other technology is better. IIRC replacing the alternator was a pretty regular occurrence in the 1970’s, usually because it no longer charged, usually because the rectifier bank was dead (sometimes the bearings were gone…). A friend of mine with mechanical experience remarked that the alternator rectifiers on his small Japanese car back then looked like they were sized for a big truck… ---
**Author:** ![Ferret\_Herder](https://avatars.discourse-cdn.com/v4/letter/f/e47774/32.png) [@Ferret\_Herder](https://boards.straightdope.com/u/Ferret_Herder)\ **Post date:** [January 14, 2010, 1:48pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/30 "2010-01-14T13:48:44Z")
As long as we’re on the topic - how effective/safe are those portable units that you can use to jump start a car? I assume they’d at least be tougher to screw up since you only have a couple cables to deal with. (Example [found here](http://www.amazon.com/gp/product/B0012MYI8U/ref=pd_lpo_k2_dp_sr_3?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00062I2XE&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1TNAQRNN3SZGCCM3T3A7). And is there anything in particular one should be looking for in one of these units?) ---
**Author:** ![smoke](https://avatars.discourse-cdn.com/v4/letter/s/c5a1d2/32.png) [@smoke](https://boards.straightdope.com/u/smoke)\ **Post date:** [January 14, 2010, 2:27pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/31 "2010-01-14T14:27:43Z")
> [@Rick](#): > > If I had to venture a guess, I would bet that those cables are the very cheap consumer grade jumper cables. Yeah, they were definitely the cheap ones. But they would have worked fine I’m sure had they been hooked up correctly. Thanks for chiming in, **Rick** - I don’t like to page people by name, but you were one of the people I hoped would drop in. I will definitely let everyone one know the outcome of this whole situation once it’s resolved. ---
**Author:** ![cantara](https://avatars.discourse-cdn.com/v4/letter/c/6f9a4e/32.png) [@cantara](https://boards.straightdope.com/u/cantara)\ **Post date:** [January 14, 2010, 3:28pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/32 "2010-01-14T15:28:29Z")
I did this to my mothers 1983 Honda Accord. I got a spark when I hooked it up, but I was expecting something…it was a little bigger than I expected. When I tried to start her car is when the cables melted. It’s tough to try to quickly get the cables off when they are covered in molten plastic. There was no apparent damage to the jumping car. I blew the voltage regulator (I think, this happened 20+ years ago). Unfortunately the part was internal to the alternator and a rebuilt alternator was $150+. I bought the (part that I blew) and took apart the alternator myself. I had to unsolder and resolder the new piece in, but it worked and for only $10 (+new jumper cables)! I’ve heard that jumping cars is not good anymore due to the computers, but have had to do so with my minivan and my sister-in-laws car. Her PT Cruiser has a labled -ve jump point, so it must be expected sometimes. Seems to me that all cars should have jump points in a separate box somewhere easily accessible ([see bottom](http://www.project33.com/article.cfm?ID=508)). ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 14, 2010, 4:31pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/33 "2010-01-14T16:31:58Z")
> [@ZenBeam](#): > > I don’t understand this. Why is your car drawing 15 amps with the key on, but the radio, headlights, interior lights turned off? What’s using all that power? twenty four computers on a high speed data network draw a metric butt load of power. About 11 amps to be exact. Add to that the interior lights, instrument cluster lights, day time running lights and you are at 15 amps. I have measured it and know this to be a valid number. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 14, 2010, 4:34pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/34 "2010-01-14T16:34:46Z")
> [@Ferret\_Herder](#): > > As long as we’re on the topic - how effective/safe are those portable units that you can use to jump start a car? I assume they’d at least be tougher to screw up since you only have a couple cables to deal with. > > (Example [found here](http://www.amazon.com/gp/product/B0012MYI8U/ref=pd_lpo_k2_dp_sr_3?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B00062I2XE&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=1TNAQRNN3SZGCCM3T3A7). And is there anything in particular one should be looking for in one of these units?) As long as you hook them up corectly they are very effective. We have two of them in my shop. ---
**Author:** ![muldoonthief](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muldoonthief/32/3188_2.png) [@muldoonthief](https://boards.straightdope.com/u/muldoonthief)\ **Post date:** [January 14, 2010, 5:09pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/35 "2010-01-14T17:09:27Z")
To hijack - how dangerous is it really to make the last connection directly to the negative terminal? The conventional wisdom seems to say it’s because batteries generate hydrogen gas, which can explode. While this is true, hydrogen is pretty light stuff, so any gas generated would immediately float away, and certainly wouldn’t pool over the surface of the battery after the hood has been opened. Whenever I do a jump, I follow the instructions, even though it can be tricky to get a good connection between the cable and the metal chassis of my car. And I’ve had to call AAA twice for a jump in my life, and both times the AAA tech hooked up the cables directly to the battery, complete with sparking, and nothing bad occurred. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [January 14, 2010, 6:09pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/36 "2010-01-14T18:09:09Z")
The battery can explode for more reasons than just hydrogen (see my earlier post above). However, it’s not likely to explode. I don’t know if there are any actual statistics but I’m betting that somewhere around 99.9 percent of the time the battery won’t explode. But if you just happen to be that 0.1 percenter, do you really want the battery to blow up in your face? It may be rare, but people have been blinded and scarred for life from this sort of thing. I’m sure professional mechanics like Rick and GaryT who do jumps quite often in their shops have actually seen more than one battery explode in their lifetime. It’s rare but it does happen. ---
**Author:** ![old\_joe](https://avatars.discourse-cdn.com/v4/letter/o/58956e/32.png) [@old\_joe](https://boards.straightdope.com/u/old_joe)\ **Post date:** [January 14, 2010, 6:24pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/37 "2010-01-14T18:24:04Z")
> [@capeo](#): > > You’re lucky the car getting jumped wasn’t damaged. The charging car really shouldn’t be a problem. Crossing jumper cables can destroy radios, amps, blow fuses and in extreme cases even ruin an alternator, or worse, your car’s EFI or PCM (although they are usually protected with a relay that opens if reverse current is applied to it). Not mention you can blow up your battery. there was this guy I know (yeah) that managed to connect a brand new battery wrong. Result was a ruined battery, melted cables and wires and ruined alternator. fun day.. this guy was told that in a newer car it would have taken out the computer in all likely hood.:smack: ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 14, 2010, 8:20pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/38 "2010-01-14T20:20:07Z")
> [@engineer\_comp\_geek](#): > > The battery can explode for more reasons than just hydrogen (see my earlier post above). > > However, it’s not likely to explode. I don’t know if there are any actual statistics but I’m betting that somewhere around 99.9 percent of the time the battery won’t explode. But if you just happen to be that 0.1 percenter, do you really want the battery to blow up in your face? It may be rare, but people have been blinded and scarred for life from this sort of thing. > > I’m sure professional mechanics like Rick and GaryT who do jumps quite often in their shops have actually seen more than one battery explode in their lifetime. It’s rare but it does happen. I have seen one battery explode and quite a few others that had exploded. Back in the day California had centralized smog testing with very long lines. A guy a few cars back had opened his hood and was screwing around under the hood. He must have arced the battery because I heard a loud bang (like a gunshot). I looked and saw part of the top of the battery about 25 feet in the air. Guy was stagering arond with his hands to his face. He got sprayed with battery acid (H2SO4). He was taken away by ambulance. While I have made connections direct to the battery I do so with full knowledge of what can happen. If instructing someone I always tell them to use a chassis ground. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [January 14, 2010, 8:56pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/39 "2010-01-14T20:56:40Z")
> [@Rick](#): > > While I have made connections direct to the battery I do so with full knowledge of what can happen. If instructing someone I always tell them to use a chassis ground. Make sure its IS a true chassis ground, or better yet check the manual for where to put the ground when jumping. Over the net, I’ve seen more than one story where somone figured any old piece of metal under the hood was a decent ground, and while it WAS grounded, the path could NOT handle the current for a jump and something somewhere got burned up/melted/whatever. I’ve also done my share of direct battery jumps too, but at the very least I turn my whole face away before I make the connection. A blowing up battery might not be good for the hand and arm, but its nothing compared to getting it on the face/eyes. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [January 14, 2010, 9:02pm UTC](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820/40 "2010-01-14T21:02:54Z")
Yeah, the guy I knew that blew up a battery in his face was doing a charge all night due to old battery, -35 temperature. IIRC charging is what makes hydrogen, so a battery you’ve been chargingf or a while is more dangerous. Disharged-recharged batteries eventually may lose a lot of their water to electrolysis. He got a spark disconnecting the charger. [Previous page](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820.md?page=1) [Next page](https://boards.straightdope.com/t/jump-starting-cars-and-reversing-the-poles/524820.md?page=3)