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/voucher420]

I could see a cheap set of jumper cables doing this.

[u/IHaveNoAlibi → u/voucher420]

If they're trying to actively start the car with them, yes.

If they're just letting the battery charge through them, no.

It seems reasonable to assume that when they left it for a few minutes, and came back to it melted, it means they weren't actively trying to start it.

[u/voucher420 → u/IHaveNoAlibi]

Depends on how dead the battery was or if it was internally shorted. When you hook up two batteries together, they will transfer energy from the one with higher voltage to the one with lower voltage until they equal out. With lead acid batteries, the more they are discharged, the more current they will take. As they come closer to a full charge, they will not be able to draw as much current.