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Presently I have a BMW E60 530d which failed it's annual inspection due to corrosion around the rear jacking point (which is near the rear subframe mounting point).

Typically ahead of welding (MIG or ARC) I would disconnect the battery to prevent damaging any electrical systems on the vehicle. This particular vehicle has quite a large number of electrical systems including an alarm system which seems able to activate even when the battery is disconnected. Therefore, either specifically to the BMW E60 on generally on modern, computer controlled cars, is there anything else that ought to be disconnected ahead of welding?

As a side thought, I wonder how much would need to be disconnected when welding up a Tesla...

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  • How much is there to weld on a Tesla?
    – Solar Mike
    Mar 14 '19 at 12:00
  • How does disconnecting the battery prevent damage to the electrical systems?
    – HandyHowie
    Mar 14 '19 at 13:27
  • @SolarMike Quite a bit I reckon. They're a boron steel chassis draped with either titanium or aluminium panels (dependent on model). I've worked on Lancia Fulvia HF's that have dissimilar metals in their construction. I can only imagine what the effects of galvanic corrosion added to years of salty road spray will do to one.
    – Steve Matthews
    Mar 14 '19 at 14:42
  • I'm not saying this is the right answer, but.... Current flows through a completed circuit. This is why you can touch the end of your welder to ungrounded metal without arcing. If the ground is close the the point you're welding, and assuming the point you're welding is a beefy hunk of metal, the current flowing through it won't directly affect the rest of the system. The biggest possible issue would be inducing a current in something else. If this weld is close to a delicate system, such as a computer, you might have problems, but odds are very high that nothing bad would happen.
    – the_storyteller
    Mar 15 '19 at 15:25
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I usually disconnect both sides of the battery and also disconnect the alternator as well.

I also make sure that the welding earth is as close as possible to the weld area to minimize the return path...

Never had an issue yet...

Edit based on comments: This is the advice for the Jaguar X type, page 48 of the pdf version of the workshop manual: enter image description here

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  • I never disconnect anything and have never had an issue. As long as I stay away from wires, I cant see how there will be a problem. What does disconnecting the battery achieve? Also why the alternator and not all the ECUs?
    – HandyHowie
    Mar 14 '19 at 13:20
  • Cars tprically use negative earth which means that the bodyshell is the return path to the battery for all electrical systems. That is to say that the bodyshell effectively becomes the negative terminal and part of the electrical circuit. Unplugging the alternator makes sense.
    – Steve Matthews
    Mar 14 '19 at 14:05
  • It is possible for eddy currents to be set up that can damage things... Murphy's law reigns supreme - may not happen etc.
    – Solar Mike
    Mar 14 '19 at 14:09
  • Eddy currents flow in closed loops within conductors -en.wikipedia.org/wiki/Eddy_current I can't see how that will damage anything. Can you explain more?
    – HandyHowie
    Mar 14 '19 at 14:18
  • 1
    outbackjoe.com/macho-divertissement/macho-articles/… this link goes some way to demonstrate the risks to electrical components of welding with the battery in place.
    – Steve Matthews
    Mar 14 '19 at 14:34

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