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/chickenCabbage]

The spark doesn't change based upon the order of connection, it's the same voltage :)

[u/MarsRocks97 → u/chickenCabbage]

It really does change. This has been studied extensively and is one of the reasons for the order of connection of cables.

[u/Breadmash → u/MarsRocks97]

I believe faulty car batteries can also cause flammable gas releases when charging - so removing the spark from the battery area by connecting the final lead to the chassis removed the potential for ignition.

[u/JFrankParnell64 → u/Breadmash]

This is the correct reason that the order matters.