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I am worried about this question - Today I was helped on the road by a random driver (she had Mercedes w2xx, might have been w220) who helped me to jump-start my car. However, after I removed jumper pins from my car, shorty after that, accidentally they were touched each other and they sparkled a bit (it lasted 1 second or less), at that moment her Mercedes engine was on, however, as if nothing happened, we good-byed each other and she went away normally.

Are there some high chances that something could have been damaged in her car (Mercedes) because of this incident?

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  • Almost all cars now have processors spread all through (my 2001 V70 has at least six), connected by a "CANbus". A voltage spike can have far-reaching results (for example, putting the engine management unit into get-you-home mode). Commented Jul 19, 2023 at 9:28

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It depends on where the cables were attached to her car

If the cables were attached to the battery, then it's theoretically possible that you could have damaged the battery — but the odds of that happening with the conditions you state are vanishingly small. The flow of electricity isn't dissimilar to the flow of water — the shortest path is the path the electricity will take. It'll be drawn from the battery before the alternator (it's not quite that simple, but suitable for this explanation as the results won't change). But you'd need to keep those cables connected for a lot longer than a quick spark for damage to occur unless the battery was ready to die anyway. Automotive batteries are necessarily robust and very hard to damage in this way.

Had the cables been connected to the alternator (never a good idea!) then there's a higher chance of a problem because the coils in the alternator can overheat much faster than the battery can. I could be wrong, but I've never personally seen a car that made the alternator that accessible. People either use the battery posts or jumper posts provided by the manufacturer, which are tied to the battery.

In any case, had damage occured her battery or engine light would have come on and she likely wouldn't have driven very far.

No, you're cool. I can't count the number of times I've mistakenly shorted jumper cables. Frankly, if you held them together long enough to seriously raise the odds of causing damage, you'd likely arc weld the cable clamps together — a car battery has that much power.

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Whenever you short jumpers like that, there's always a chance something can be damaged. In this case, however, since the car was still running and drove away, I'd suggest there's no worries. If there was an issue it would have presented itself very quickly.

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  • I once saw someone reverse the jumper cables on one of the cars they were jumping, so if we're talking about car 1 and car 2, it would have been hot1 -> neutral2 and hot2 -> neutral1. This was a short circuit through both batteries in a series connection, meaning twice the voltage and twice the amperage (though I guess one was dead). The connection caused bright arcs and threw off white sparks. He was startled enough to withdraw the last connection, but not understanding he was doing something wrong, tried the connection a few more times. As far as we could tell, no damage to either car.
    – David
    Commented Jul 19, 2023 at 5:58
  • Though now that I think about it, there might have been a blown fuse we had to replace. I don't recall.
    – David
    Commented Jul 19, 2023 at 6:00
  • @David I've done it myself. A Vauxhall (Opel) Corsa I had used a black wire for 12V and brown for 0V to the battery. The black had a red marking on it, but that was in shadow, and by the light of a streetlamp the brown looked red. Luckily the arc was as I approached the terminal with the clip, and warned me.
    – Chris H
    Commented Jul 19, 2023 at 8:34

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