When a jumpstart goes wrong? Safety Question

Neighbor tried jumping my wife’s ‘06 Nissan Altima, we left it for 10 minutes and came back and the cables had melted through the headlight of both cars and some of the bumper. I wasn’t there but thankfully they stopped their car and were able to disconnect the cables without incident. We noticed after there had been mice living in around her engine from the mouse poop, minimum the last two weeks. What causes jumper cables to do this? Something a rodent may have chewed? Definitely an issue with my wife’s car. Our poor neighbors have a newish midsized suv. My wife has also had constant issues starting her car, even with a new battery I got a year or two ago. Anyone seen this before?

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u/f0rcedinducti0n Dec 31 '23

that's why you do;

Donor positive to recipient positive, donor negative to recipient ground.

If there is an internal short of the bad battery, it's isolated by using engine/chassis ground instead of the negative terminal of the battery.

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u/Pimp_Daddy_Patty Dec 31 '23

A short is a short. Ground is connected to negative. They're not isolated.

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u/f0rcedinducti0n Dec 31 '23

You'll be unable to pull the same amount of current through the ground strap as you can if you connected the jumper directly to the battery. It is also possible that the ground strap is a fusible link.

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u/Pimp_Daddy_Patty Dec 31 '23

I've never seen a ground strap being used as a fusible link. If it is, then it's a really shitty ground.

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u/f0rcedinducti0n Dec 31 '23

I bet your ground strap is a thinner gauge than most jumper cables.

That in and of itself would make it the "fusable link". 🙃

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u/Pimp_Daddy_Patty Dec 31 '23

It is, but there are also 2 paths back to the battery negative.

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u/f0rcedinducti0n Dec 31 '23

When charging a deeply discharged battery, it releases hydrogen gas, and using a chassis ground, away from the battery, reduces the chance of a spark near the source of said gas.

/Shrug

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u/Pimp_Daddy_Patty Dec 31 '23

And that's the actual reason why you use chassis ground.

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u/f0rcedinducti0n Dec 31 '23

And that's the actual reason why you use chassis ground.

you say that like you weren't arguing against it all along

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u/Pimp_Daddy_Patty Dec 31 '23

I wasn't. I was arguing about it magically being isolated from a short. Which it is not.

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u/f0rcedinducti0n Dec 31 '23

when you have multiple paths and a dead short, which path melts first?

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u/Pimp_Daddy_Patty Dec 31 '23

The one with the most resistance.

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u/f0rcedinducti0n Dec 31 '23

The one with the most resistance will have the least current flowing through it.