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I understand that, if charging a car battery without removing it from the car, you should attach the negative crocodile clip not to the negative of the battery but to a metal part of the car; most cars, in fact, have a specific metal thingy to attach clips to for this very reason. There are questions on this, eg here, but I have a few more detailed questions I couldn't find anywhere (I believe the same questions apply to jump-starting, too):

  • Why exactly is this? Is it because the clips can always generate a spark, a spark near the battery is dangerous, so you place the negative clip farther from the battery? But couldn't the positive clip generate a spark, too? You should attach positive first, but, also, you should only plug in the mains after you have connected both positive and negative, so does the negative have a higher risk of sparks?
  • Is the risk of a spark real only with chargers that use more than 7-8ish ampere? Is it safe to attach a negative clip to the negative battery if the charger is 4A? I'm not saying I will do it, I am just trying to understand the theory behind it.
  • How about eyelets and connectors, like the one in the picture below? These can be attached to the negative and positive of the battery, so that, when you want to charge it, you just plug a cable to the connector. But isn't this the same as connecting a negative clip to the negative side of the battery? Or is this safer? If so, why?
  • As far as I know the same precaution (not attaching negative to negative) is never mentioned when it comes to motorcycle batteries. Why? Is it because those are smaller (6 to 16 Ah vs 60-90ish Ah for a car) and are typically charged at 0.8A?
  • Also, I believe the same precaution does not apply to boats. Is it because the voltage potential through the water would cause electrolytic corrosion? But how is the risk of sparks handled on a boat, then?

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You are right, the procedure is to prevent sparks next to the battery, since the battery can produce a highly explosive hydrogen oxygen mixture while charging.

Sparks are generated when you open or close a circuit while it allows current to flow. That is, if the charger is off, you will not get any sparks. If it is on, you will get sparks on the first clip you remove, or on the last clip you attach1. So it does not matter if you place the positive or negative clip last, as long as you connect it with some distance to the battery. It also does not matter if you have a big or small charger. The big one creates big sparks, the small one small sparks.
Now... in a car, the motor and chassis is connected to the negative terminal of the battery, giving plenty of places to connect the black clip to. But the positive terminal is about the only piece of bare metal to connect to. Therefore, connect positive to the terminal and negative somewhere to a metallic part.

The cable in your picture can be connected directly to the terminals, because it will not cause sparks, unless it's connected to the charger while you mount it.

I don't know why this procedure is not mentioned for boats or motorcycles. I definitely get sparks with my small 4Ah battery, too.


1) Though, battery terminals are prone to corrosion, and a placed clip will not always have a good electrical connection. It is possible that you get a spark there, too when you connect the other clip

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  • Thank you. However, I don't understand why connecting a croc clip to the negative might cause a spark, while connecting the eyelet of the connector in my picture to the negative wouldn't. Shouldn't what matter be the fact that the charger is connected to the negative? What is the difference between a croc clip and the eyelet? – Pythonista anonymous Apr 15 at 22:26
  • "You will get sparks on the last clip you attach" - but doesn't this apply if I attach clips which are already connected to a charger plugged in the mains? If I attach the clips first, and only later I plug the charger in the socket, isn't it different, would I still get sparks? – Pythonista anonymous Apr 15 at 22:27

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