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This post explains why it's a potential hazard to clip to the negative post on the dead battery when giving a boost. But I've also been told that I could damage the electronics in my own car if I give someone a boost or do it "wrong".

Is this really the case? What are the circumstances that this could end badly for my car?

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4 Answers 4

up vote 3 down vote accepted

I can't think of any reason why you would damage the electronics in your car, unless you touched one of your jump leads somewhere you shouldn't and short something delicate or you connect the leads back to front.

Connecting them back to front shouldn't damage electronics, as your car should have protection against this sort of thing, but it is the key thing to check - just in case.

An example I can think of might be - an aftermarket car stereo may not have surge/voltage protection and it may be connected directly to the battery, not through the ignition switch, so connecting 12v across it the wrong way may damage it. More realistically it may just blow a fuse or a diode.

As commented below, this could damage batteries - I was purely talking about electronics. Look out for your batteries :-)

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Connecting jumper cables in reverse will probably damage one or both of your batteries, most likely the battery in the dead car as the good car will have it's alternator running. Other components may or may not have diode protection, but if they do, fuses will not blow. Fuses blow due to high current across the fuse. Reversing 12v and Gnd could cause this in circuits lacking reverse bias protection. Circuits that are protected by diodes should cause no current, meaning no fuses blown. So if you do blow a fuse after reversing +/-, you've probably ruined something else, or gotten very lucky. –  bobpaul Jan 25 '12 at 15:51
@bobpaul - yep - updated. Thanks –  Rory Alsop Jan 25 '12 at 16:25
With the information posted below I want to improve the answer to the question. There's a lot of hypotheticals in the various answers. So assuming I have connected the cars properly, under what circumstances would I damage the battery or electronics in my car (the car giving the boost). –  Gregory Bell Jan 31 '12 at 23:00

You can get transient spikes from the other car while it's trying to start. Any battery or alternator issues that affect power quality will be transmitted to your car. The good news is that large 12v batteries make excellent buffers against power issues that might otherwise cause damage. However, it is still possible to suffer damage to your car from jumping someone elses. Pretty rare though, doesn't make my list of things I worry about. :-)

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Yes, it is possible to damage either or both of the cars. There's a risk of serious overvoltage when jump-starting and that can damage any electronic equipment and even headlights that are on during the procedure.

That's why, for example, Ford Focus C-Max manual says that all electronics and headlights should be turned off during the jump start.

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I don't really understand most of that article. But I get that there's a possibility of something bad happening to the electronics in my car. But it doesn't explain under what circumstances it could happen. Assuming I connect cables properly can this over voltage situation occur? –  Gregory Bell Jan 31 '12 at 22:57
@Gregory Bell: Yes, it can, that's why the C-Max manual says all electronics and headlights should be turned off. –  sharptooth Feb 1 '12 at 7:01
@sharptooth, but, say, on my 2008 jetta, bought in the US, it's not possible to turn off the headlights, since they also double as DRL; similarly, I think Canada requires all cars to have DRL; what would you do if your car is fitted with DRL? –  cnst Jun 21 '13 at 21:42

It is very possible to harm yourself if you arch the terminal over a dead battery. Charging batteries are putting off very flammable hydrogen gas and can explode very easily. Make sure to use the engine block to ground the cable vs. the terminal.

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The original poster has already stated this in his question. Re-posting parts of the question as an answer isn't all that helpful! –  Nick C Jan 27 '12 at 13:49

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