When a jumpstart goes wrong?

[u/jhwalk09]

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?

[u/MarsRocks97]

Tight isn’t the issue. Each clamp MUST be clamped to the correct polarity. Mismatching will cause the cable to overheat and quite likely also ruin the weaker battery possible both batteries.

[u/jhwalk09 → u/MarsRocks97]

That’s what I meant he insists he clamped em to the right ones, black to black red to red

[u/MarsRocks97 → u/[deleted]]

Wrong. Connecting to the chassis is effectively the negative so the exact same circuit is achieved. The car has no true ground. Only two poles and the negative terminal is “grounded to the chassis”. But again this effectively makes the chassis one big - negative pole. The only reason connecting to the chassis is recommended is that any spark produced at connection or disconnection is distanced from the battery. Batteries vent off the top and this is a risk because of the flammability of the hydrogen/oxygen gasses being released while charging. On dead batteries it’s not an issue since you should just be jump starting and immediately disconnecting the cables before gasses begin to form.

Lastly, connecting to the chassis on modern cars is also becoming much less practical as the prevalence of plastic coverings and fully painted engine compartments means there is rarely a good connection or access to chassis point.