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/Previous-Sympathy801 Dec 31 '23

Batteries are DC supplies, current flows from the negative terminal to the positive.

The order only matters because, if you connect the last clip to the battery terminal it can arc. Car batteries off put a flammable gas, which can be ignited by the arc. It is unlikely but that is the reason.

(I’m and Electrical/Computer engineer)

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u/chickenCabbage Dec 31 '23

I'm an electrical engineer as well, I know the polarity is important, I was wondering about the order of connection. Reading up on it, lead-acid batteries can release hydrogen, so you'd want to create the spark away from the battery. The engine block or the chassis next to the battery wouldn't do any good, you want to hook that up below the battery (hydrogen rises up quickly) and not too close.