I just tried to jump start a friend's car. When I connected the jumper cables, there were a lot of sparks. They were connected for nearly 30 seconds and I noticed a lot of heat and smoke coming from both batteries. I immediately disconnected and didn't try again. I didn't even start the car with the good battery. Why was this, and what can I do to avoid it?

  • Are you sure you had them connected correctly?
    – mikes
    Commented Jan 22, 2015 at 20:34
  • Positive-positive/negative-negative, yes.
    – Mike Cole
    Commented Jan 22, 2015 at 20:36
  • Batteries are made with the positive on the left, negative on the right and the other way around.
    – mikes
    Commented Jan 22, 2015 at 20:47
  • yeah, but there was a red cover on both positives, so unless that was wrong then I'm sure it was correct. I'm 100% sure that it was correct on the dead car. I'll check the other vehicle shortly to make sure.
    – Mike Cole
    Commented Jan 22, 2015 at 20:48
  • 1
    @Mike Cole - never assume that a coloured cap depicts the polarity of the terminal underneath. You MUST look for the + and - symbols on the battery to make sure. If you can't see them, you can probably infer by following the leads - nearly all vehicles these days are negative earth so the negative terminal will have a thick cable going to the car body and the positive terminal a thick cable going to the starter motor. While not definitive, it'd be a 99% good rule of thumb and finally.. Ensure the dead car is OFF. if you see significant sparks when connecting the last lead then abort
    – Caius Jard
    Commented Feb 8, 2017 at 12:00

2 Answers 2


With the description you give, I'd say you connected them backwards with about a 99%+ assurance. Large amounts of sparks followed by heat are both VERY BAD SIGNS (as you soon figured out) and almost always mean you have the jumpers on backwards. Never trust what the covers on the battery says. Check what the battery says to be sure. If there is no way to see what is on the battery (the terminals cover them), follow the cables back to their source. The negative will go to a ground and the positive will go to the starter and to the alternator. If those are correct, ensure that the jumper cables are correctly aligned. Some sets I've seen will have black shielding on both with a thin yellow (or some other color) stripe up them. It would be very easy to mix these up. If you still think it was right, you could ultimately check each side of the jumper cables for continuity to ensure you are matching them up.

  • OK, I'm going to check the good vehicle to see if that's the problem. I recently got my battery replaced so it's possible it's reversed now. Very odd. I'm 100% sure I had it correct on the dead car.
    – Mike Cole
    Commented Jan 22, 2015 at 21:36
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    @MikeCole ... It happens to the best of us. It usually happens when we assume something to be true, which, by your comment, seem like it might be the case. Believe me, I've made simple mistakes which cost dearly. I hope nothing was damaged in the process. Commented Jan 22, 2015 at 21:38
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    OMG, that was it. Wow.
    – Mike Cole
    Commented Jan 22, 2015 at 21:52
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    minor note: It's not 100% guaranteed that negative goes to ground/car body. A minority of positive ground cars exist, probably older/vintage.. In any case you never see "significant" sparking when jumping cars. It's nigh on certain the wires are crossed up/ connected "positive-to-negative" if sparking is significant.
    – Caius Jard
    Commented Feb 8, 2017 at 12:03
  • @CaiusJard - You are correct and great point. There used to be positive grounded vehicles. I believe there may still exist some in some backwards parts of the world too (joking ... they exist, but not in the main stream). As far as sparks go, if you see sparks, it's usually all over but the crying (not every time, but in the vast majority of them). Definitely not a good thing. You really need to know what you're doing when jump starting a vehicle. The possibility of huge consequences is a real thing. Commented Feb 8, 2017 at 17:00

It is probably a short on your positive battery terminal causing it to ground out, which virtually gives you two negatives. I would recommend pulling off your positive cable all the way from the starter and the fuse box and inspect it to make sure there are no cracks, splits or melted/bare spots. Replace if needed and hopefully there is not too much damage from the short.

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