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I tried to understand what is going on when one jump starts a car with a dead battery and reading through a lot of explanations, one question remained:

You start by connecting the the + poles with the first (red) cable. After that, you take the second (black) cable and connect it to the - pole of the charged battery.

For safety reasons, you apparently should not connect it to the - pole of the dead battery though, but rather to some bare metal away from the battery. Here is where I am confused. While it is explained everywhere why we do not want to connect it to the battery directly and I think I understood the issue here (risk of explosion), I could not find an explanation for the following:

How does connecting the cable to some arbitrary piece of bare metal charge up the battery? Does that mean the negative pole of the battery is connected to all of the metal in the car, and thus the current flows through the whole car body during the process?

Maybe I also have a fundamental physical misunderstanding here, I hope someone can enlighten me.

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  • \$\begingroup\$ The current flows through the car chassis not just during the jumpstart process but all the time the car is running. All of the loads in the car use the chassis to connect to the negative terminal of the battery. As DKNguyen it is designed that way to be more cost-effective and avoid requiring negative wires running all around the car. \$\endgroup\$
    – Kevin White
    Commented Jul 22, 2020 at 21:00
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    \$\begingroup\$ Fibreglass or wooden cars may differ. \$\endgroup\$
    – user16324
    Commented Jul 23, 2020 at 9:15

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Does that mean the negative pole of the battery is connected to all of the metal in the car, and thus the current flows through the whole car body during the process?

Yes. It's cheaper and more convenient than running a copper ground wire everywhere.

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  • \$\begingroup\$ Doesn't that seem unsafe somehow, so could you not get a shock when touching the car? Or is everything outside insulated from the inside? \$\endgroup\$
    – Staki42
    Commented Jul 22, 2020 at 20:58
  • \$\begingroup\$ It's a matter of degree. There's only 12V across the battery terminals and you can touch both terminals without getting shocked. It is too low to be hazardous. Remember that your skin has resistance too. Plus the frame itself is probably somewhere around the same potential as the soil you are standing on so the difference between you, the soil, and the frame is even less than 12V. \$\endgroup\$
    – DKNguyen
    Commented Jul 22, 2020 at 20:58
  • \$\begingroup\$ Thanks, you very well explained what I wanted to know! :) \$\endgroup\$
    – Staki42
    Commented Jul 22, 2020 at 21:26
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The negative terminal on a battery connect directly to the body of the car, usually the frame or engine. So when you connect your jumper cables to some arbitrary piece of bare metal, you're in fact hooking it to the negative terminal of the battery.

It's so much easier hooking up jumper cables when you can just hook up the negative to any bolt on the engines.

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  • \$\begingroup\$ Thanks! Precisely what I wanted to know. \$\endgroup\$
    – Staki42
    Commented Jul 22, 2020 at 21:26

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