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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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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
    Commented Jan 25, 2012 at 15:51
  • 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
    Commented Jan 31, 2012 at 23:00
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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
    Commented Jan 31, 2012 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
    Commented Feb 1, 2012 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
    Commented Jun 21, 2013 at 21:42
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Answer is yes yes yes to damage. Some guys borrowed my car to jump-start another yesterday. They mixed up the pos/neg connection and my car died a sudden death. I have just phoned my mechanic who said it will have damaged any number of things/fuses/electric systems. I am currently waiting for the AA to come and jump-start the car so I can drive it to the mechanic. I am not at all looking forward to the bill and really annoyed @ the guys. :(

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  • Unfortunately, if the car is not starting, a boost/jump is not going to do much good. More than likely a fuse or relay was fried, which will need to be replaced before you can see if there was any more damage, like an alternator.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Oct 27, 2015 at 21:03
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    How about updating to list the actual damage you discovered? Anything other than fuses?
    – JDługosz
    Commented May 11, 2016 at 5:13
  • Please do tell us what damage you found out
    – Peter M. - stands for Monica
    Commented Jul 6, 2020 at 2:15
  • To be more specific, damage can occur when done improperly. That's usually the case for most things. But not when done properly.
    – Justin Emlay
    Commented Jul 16, 2021 at 6:20
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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 ♦
    Commented Jan 27, 2012 at 13:49

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