# Car jump-starting and vulnerable electronics
**URL:** https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654
**Category:** Factual Questions
**Created:** [September 24, 2024, 12:27pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654 "2024-09-24T12:27:25Z")
**Posts on this page:** 13
**Page:** 1
### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [September 24, 2024, 12:27pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/1 "2024-09-24T12:27:25Z")
For the purposes of this thread, the “conventional” way of jump-starting a car is:
\*connect positive cable to positive terminal of dead battery
\*connect positive cable to positive terminal of good battery
\*connect negative cable to suitable ground point on dead car (away from battery)
\*connect negative cable to suitable ground point on good car (away from battery)
Inspired by this thread:
> [@How long will my car battery last setting at the airport](https://boards.straightdope.com/t/how-long-will-my-car-battery-last-setting-at-the-airport/1007602/42):
>
> While we’re on the subject… If an in-situ car battery is depleted or dead, “jumping” is always Plan B. Plan A is always to charge the depleted/dead battery using a battery charger. (And when doing so, it’s best to electrically isolate the battery from the vehicle.) Plan B should only be performed if Plan A is not feasible.
It was noted that certain cars can be damaged by jumpstarting in the conventional way. Can someone explain how this damage could happen, and why it’s prevented by connecting to “special” jump-start connection points?
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### Author:  [@burpo\_the\_wonder\_mutt](https://boards.straightdope.com/u/burpo_the_wonder_mutt)
#### Post date: [September 24, 2024, 1:47pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/2 "2024-09-24T13:47:42Z")
Being a machine elf, you probably already know this: the battery is hooked up as you described to prevent a spark from igniting whatever batt fumes are hovering around, sparing you an acid bath when/if the sucker explodes.
I assume everyone in the known universe knows this (if I know it, everybody does). There is no condescension implied.
Now what you need to watch for are those tire pumps that plug into your “cigarette lighter.” The one I have blows 20 amp fuses if it’s plugged into the wrong one.
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### Author:  [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)
#### Post date: [September 24, 2024, 2:05pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/3 "2024-09-24T14:05:21Z")
> [@Machine\_Elf](#):
>
> \*connect negative cable to suitable ground point on dead car (away from battery)
> \*connect negative cable to suitable ground point on good car (away from battery)
I was always taught that negative on the dead car was last and that you can connect to the neg terminal on the good battery because a) there is no spark risk since the circuit is not complete and b) discharged batteries give off hydrogen gas which is the explosion risk.
We had an Audi years ago that had jump starting terminals under the hood, but the battery was actually under the back seat.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [September 24, 2024, 2:09pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/4 "2024-09-24T14:09:50Z")
> [@burpo\_the\_wonder\_mutt](#):
>
> Being a machine elf, you probably already know this: the battery is hooked up as you described to prevent a spark from igniting whatever batt fumes are hovering around, sparing you an acid bath when/if the sucker explodes.
> [@FinsToTheLeft](#):
>
> I was always taught that negative on the dead car was last and that you can connect to the neg terminal on the good battery because a) there is no spark risk since the circuit is not complete and b) discharged batteries give off hydrogen gas which is the explosion risk.
Yeah, the sparks-near-the-dead-battery explosion hazard isn’t the issue I’m thinking of here. Here’s the issue, quoted from that other thread:
> [@How long will my car battery last setting at the airport](https://boards.straightdope.com/t/how-long-will-my-car-battery-last-setting-at-the-airport/1007602/36):
>
> As long as we’re talking about jump starting cars, be sure to check how to jump start your particular car. Don’t just assume you can hook up battery to battery. Some cars should not be jumped by hooking up the batteries directly. Instead, there may be special posts under the hood that should be used when attaching jumper cables. I know some BMWs are like this. If you jump start a BMW from the battery, it may fry some of the computers (learned from personal experience). Instead, they have a special positive post under the hood that can be used with jumper cables.
How does jump-starting via the battery damage onboard computers in certain cars, and how do special jump-start connection points prevent such damage?
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### Author:  [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)
#### Post date: [September 24, 2024, 2:13pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/5 "2024-09-24T14:13:39Z")
> [@Machine\_Elf](#):
>
> How does jump-starting via the battery damage onboard computers in certain cars, and how do special jump-start connection points prevent such damage?
My Bimmers never had this warning, but I’m guessing that there must be some voltage regulation or current limiting on these connections. Given that the highest voltage you can get off the donor car’s alternator is about 15V which is the same as your own car, I’m not sure how it could actually cause damage.
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### Author:  [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)
#### Post date: [September 24, 2024, 2:25pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/6 "2024-09-24T14:25:14Z")
> [@FinsToTheLeft](#):
>
> I was always taught that negative on the dead car was last and that you can connect to the neg terminal on the good battery because a) there is no spark risk since the circuit is not complete and b) discharged batteries give off hydrogen gas which is the explosion risk.
Yeah, you don’t have to connect to the frame on the dead car, as long as you connect the negative on the good car to the frame and you do that one last.
Hydrogen gas is one danger, but another danger is that if you accidentally reverse the cables, you can draw too much current which can potentially overheat either battery and cause its electrolyte to boil, resulting in the battery exploding. It’s a good idea to keep your face away from the battery in either case.
As for damaging your car, I don’t see that happening as long as you connect the cables correctly. But that is where the danger is. If you accidentally reverse the cables, you can fry the ECU, radio, traction control system, all sorts of stuff.
The only way I know to damage a car if you jump it “correctly” is that if you are jumping an antique car that runs off of 6 volts. Jumping it with 12 volts can damage all of the electrical components since they aren’t designed for that voltage.
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### Author:  [@filmore](https://boards.straightdope.com/u/filmore)
#### Post date: [September 24, 2024, 2:29pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/7 "2024-09-24T14:29:13Z")
In my case, it was a 2000-era BMW. I had a dead battery and jump started it using the regular way of positive to battery, negative to body. It started up, but the transmission stayed in safe mode. I was thinking it was related to the dead battery and looked around on the web for solutions. I found mention of other people having the same problem after a jump and people saying that the computer likely got fried from jumping from the battery rather than posts under the hood. I replaced the transmission control module and everything was fine.
Here’s a video where a guy mentions the reason at this 2:19 timestamp:
[ ](https://www.youtube.com/watch?v=J26NxIAtS38&t=139)
Paraphrased:
> [@](#):
>
> When you jump from the rear, it sends the power down the wrong way to the modules and it can’t actually calculate the voltage is coming via the car when you jump start it. When you start your car, it gives a voltage spike. The jump on the battery bypasses the voltage sensor.
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### Author:  [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)
#### Post date: [September 24, 2024, 3:10pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/8 "2024-09-24T15:10:32Z")
> [@engineer\_comp\_geek](#):
>
> As for damaging your car, I don’t see that happening as long as you connect the cables correctly.
If the battery is dead or very depleted, I believe the “jumped” car _and_ the donor car can be damaged.
Anecdotal, but here is a first hand-experience:
Around 20 years ago I was having lunch with my buddy at a restaurant. When we went to leave, his car wouldn’t start. His battery was obviously dead. I gave him a jump. Three days later my alternator failed. Coincidence? Maybe. But here’s my hypothesis: 1) an alternator is not designed to be connected to a dead battery, and 2) an alternator is not designed to charge a dead battery. I was the donor car, and I think #1 happened to me; my alternator was connected to a dead battery, which puts a large amount of stress on the rectifier diodes. if this is a valid assumption, then it’s also true that the diodes in an alternator can be damaged if it tries to completely charge a dead battery.
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### Author:  [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)
#### Post date: [September 24, 2024, 3:58pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/9 "2024-09-24T15:58:38Z")
> [@Crafter\_Man](#):
>
> But here’s my hypothesis: 1) an alternator is not designed to be connected to a dead battery, and 2) an alternator is not designed to charge a dead battery. I was the donor car, and I think #1 happened to me; my alternator was connected to a dead battery, which puts a large amount of stress on the rectifier diodes. if this is a valid assumption, then it’s also true that the diodes in an alternator can be damaged if it tries to completely charge a dead battery.
Except for the fact that jump starting a car has been manufacturer approved probably since shortly after the alternator was invented. I’ve been driving for 40 odd years and the vehicle manual always details the procedure. If the alternator could be damaged by this and the manufacturer’s warranty is liable, they would fix it.
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### Author:  [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)
#### Post date: [September 24, 2024, 4:42pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/10 "2024-09-24T16:42:19Z")
> [@Machine\_Elf](#):
>
> How does jump-starting via the battery damage onboard computers in certain cars, and how do special jump-start connection points prevent such damage?
I would imagine due to inductance issues when he circuit is first completed. Kind of like why an old lightbulb will tend to blow when you first turn it on.
ETA: does using a jump box avoid frying your electronics.
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### Author:  [@echoreply](https://boards.straightdope.com/u/echoreply)
#### Post date: [September 24, 2024, 7:46pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/11 "2024-09-24T19:46:25Z")
> [@FinsToTheLeft](#):
>
> jump starting a car has been manufacturer approved
My 2002 Volkswagen’s manual specifically said not to use it to jumpstart other cars. The car was allowed to be jumpstarted, though. I don’t think it had any special instructions to do that, beyond the typical mentioned in the OP.
> [@How long will my car battery last setting at the airport](https://boards.straightdope.com/t/how-long-will-my-car-battery-last-setting-at-the-airport/1007602/36):
>
> I know some BMWs are like this
My BMW motorcycle had attachment points for jump starting it. The positive was (I think) on the transmission, near the foot peg, and the negative was on one of the cylinders. I always assumed that was because getting to the battery required removing the fuel tank, so it would have been impossible to jump start without the remote terminals.
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### Author:  [@Princhester](https://boards.straightdope.com/u/Princhester)
#### Post date: [September 25, 2024, 3:28am UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/12 "2024-09-25T03:28:31Z")
> [@filmore](#):
>
> When you jump from the rear, it sends the power down the wrong way to the modules and it can’t actually calculate the voltage is coming via the car when you jump start it. When you start your car, it gives a voltage spike. The jump on the battery bypasses the voltage sensor.
That guy may well be correct about the appropriate way to jumpstart a BMW but his explanations of why make little sense. There’s just a bunch of strenuous assertions of technobabble.
And I found [this post](https://x3.xbimmers.com/forums/showthread.php?t=636562) on another forum by someone who sounds plausible:
> [@Lotus7](#):
>
> You are correct in assuming that the terminals under the hood are there for convenience. They are certainly easier to access than the battery itself since you don’t have to raise the cargo area floor.
>
> The other issue is the emergency disconnect assembly on top of the battery (the large red plastic assembly). It’s a remotely triggered “kill switch” and covers the positive terminal of the battery. An improperly applied connection to the Emergency disconnect unit will trigger it, requiring replacement. Finally, there is always the issue of hydrogen gas build up near the battery. This should not normally happen with the OEM BMW deep cycle saturated mat battery, but the _possibility_ exists, so BMW doesn’t want anyone generating possible sparks near the battery.
>
> If your charger connections or jumper connections are directly to the _battery_ terminals (especially the positive one) there should be no problem. However if you are accessing the positive terminal through (after) the Emergency Disconnect module, you may be risking inadvertently triggering it which will, of course, render the car undriveable until it’s replaced. Using the front terminals eliminates this potential problem. Accidentally reversing the polarity by connecting a jumper battery improperly is a disaster if you do it directly at the battery OR at the front, under-hood terminals.
If this guy is right there are two reasons for using the specific jump points. Convenience, and to avoid the risk you connect through an emergency disconnect assembly.
> [@Saint\_Cad](#):
>
> I would imagine due to inductance issues when he circuit is first completed. Kind of like why an old lightbulb will tend to blow when you first turn it on.
Yet no one says you can’t replace a battery. And replacing the battery involves connecting a new battery to a dead system, which is little different to connecting an outside battery to a dead system. They only difference is that there may be an inrush from the outside battery to the car’s battery but why would that create a voltage spike?
There is a hellava lot of BS floating around about this subject. I’d like to see a technical justification or explanation of the supposed dangers in terms that someone who knows a little about electronics can evaluate. Because what I see is a bunch of half assed shibboleths.
I’m not saying the shibboleths are necessarily wrong (or don’t have at least have a grain of truth) but I’ve seen so many justifications that either clearly make no sense or are - at the least - too vague to be evaluated, to be sure.
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### Author:  [@Jaydubba77](https://boards.straightdope.com/u/Jaydubba77)
#### Post date: [September 27, 2024, 6:49pm UTC](https://boards.straightdope.com/t/car-jump-starting-and-vulnerable-electronics/1007654/13 "2024-09-27T18:49:09Z")
I have jump started many cars over the years with very few problems. I have even jumped cars that had no battery at all by hooking up to the battery cables. Once the alternator Is running the car will run just fine without the battery as long as the voltage regulater is working properly. I did have a bit of trouble with cars made in the early 1980s around the time they switched from a mechanical regulater (in a metal box under the hood) to a solid state version (built in to the alternator). We had to replace two alternators before we found the problem and it was corrected in later models. When using an older model car to jump the newer ones, the regulater could be overloaded and fail. The work around was to unplug the connector on the alternator, start the car and reconnect the system.