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enter image description hereI made a big mistake today attempting to jump start a car but connected negative clamp (-) to wrong part of the car. Basically there was no negative connection between the two cars. I did not notice until later because the jump start actually did work, and the car with the dead battery started to run again and we got the battery replaced. But I noticed small electrical problems on both cars after the jump start.

First of all, I do not understand how the jump start worked with only positive + connection, and I worry because I am not sure if it is safe to drive either car. I cannot assess how much harm the jump start did to both cars. Should I bring in to the repair shop? The feeder car was 2018 BMW X1 and the empty car was 2015 Nissan Murano.

The green circle was where I connected ‘live’ negative cable. The other end on a bolt in ‘dead’ car. (Ground)

Electrical problems for X1 now I notice include Seat position memory gone (which I could reprogram), and radio channel showing differently on screen. (When I select 1, it will show 2 on screen) Those are minor so I am not worried, but I just don’t know if there were any more damages done to the car that I do not know yet.

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  • Can you add photos of what you connected the cables to?
    – HandyHowie
    Commented Aug 17, 2022 at 10:00
  • Yes I just did!
    – Njk
    Commented Aug 17, 2022 at 12:46
  • Both of these models have chrome trim down low in the front. If the trim is actual chrome plated metal and the vehicles were parked nose to nose and touching each other, the ground connection could have been through the chrome. But if the trim is the usual chrome plated plastic, there must be some other explanation.
    – MTA
    Commented Aug 17, 2022 at 14:26

2 Answers 2

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What you describe is not physically possible.

You claim you did not create a complete circuit. By connecting only (+) terminals and not (-) you didn't make a loop, ergo not a circuit, ergo the fact you connected (+) to (+) means nothing electrically. (I'm an EE, just trust me on this if you're thinking "but but but...")

So either

  1. The battery didn't actually need jumping i.e. something else is the problem or
  2. Your presumption about not connecting (-) is faulty.

Worst case, in which #2 is sorta true, and which I fear is what you may have done, is that one or both of the (-) connections was made to something that's ALMOST (-) Jumping a car involves alot of current. If you made your (-) to a point that was, say, connected with a thin wire to battery (-) or passes through some kind of module, that would possibly damage things. Impossible to say what w/o you identifying exactly where you made your connections and an analysis of the schematics for the cars.

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  • I think you are right and maybe it was #2. I included the picture where I connected ‘live’ negative cable. It did not say ‘-‘ but I took a guess. I never owned BMW before so I didn’t know they could hide it!
    – Njk
    Commented Aug 17, 2022 at 12:47
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Buy a multimeter. Even the cheapest multimeter you can find works, as every multimeter can measure DC voltage with an accuracy that is enough to say "there is voltage" vs "there is no voltage".

Measure voltage between the positive battery terminal and the point where you connected the negative. Do that in the same configuration you jump started -- if the ignition key was on, let it be on during this test.

If it says "0 volts" (or any voltage close enough to be just random noise), you didn't connect the negative. If this is the case, it's possible the battery just needed rest. Lead-acid batteries are slow: they best work at 20 hours discharge. If you take a lot of current, let's say at 5 minute discharge rate, it may exhaust the battery. Just waiting can let the battery provide good current again. So it's possible the rest of the battery is what allowed starting.

If it says "12 volts" or anything close to that (9 - 15 volts), you did in fact jump start.

There's also another possibility, that there was some fuse on the negative side (unlikely, usually they are in the positive side), or something that worked like a fuse, like a very thin electrical wire. In this case it's possible you did have a connection when jump starting but the connecting wire has already melted and is no longer a wire. You can diagnose this only by looking for the wire, to see if there is a wire that no longer functions like a wire, connected to the point that you used as negative for jump starting.

Jump starting with only one wire is not possible. If it worked, you had two wires or didn't need a jump (battery rest was enough). The other wire could however be many things, like unpainted parts of car metal bodies touching each other.

Next time you jump start, connect to some piece of metal that has strong grounding, like the engine.

If the feeder car has electrical problems or lost seat / radio memory, it's very possible that you did in fact do a jump start.

Also, starting from this point on, if you expect the flat battery needs not just charging but also replacing, don't jump start! A dead battery can't act as a buffer for the car electrical systems. The alternator with a dead battery could cause voltage spikes that create expensive electrical trouble, even so expensive that it may be cheapest to just have the car recycled and buy a new car to replace it. Only jump start if you let the lights on and discovered it in few days at most. Few weeks with a totally flat battery may mean it's already dead and can't be salvaged => don't jump start then!

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