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/t3sl1 Dec 31 '23

Red dead battery to red live battery, black live battery to unpainted metal surface or engine block of dead car battery

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u/chickenCabbage Dec 31 '23

Why does the order matter?

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u/TheOnlyCraz Dec 31 '23

I believe it's because electricity flows in one direction? (Correct me if Im wrong) and also so you don't dead short either battery/electrical system when you've got 2 jumper cable clamps on one battery and 2 loose clamps in your hand.

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u/chickenCabbage Dec 31 '23

Avoiding the 2 loose clamps is a good idea and that's probably the reason, but the electricity flows in a circuit - in both directions.

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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.

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u/TheOnlyCraz Dec 31 '23

You may be correct, but it was my understanding that's one of the differences between AC and DC currents, where in DC the direction stays the same, and in AC (such as house voltage) switches direction multiple times a second (hence 60hz in the US)

Like I said I could be wrong, it's happened before.

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u/chickenCabbage Dec 31 '23

Ah, yes. The polarity (red to red, black to black) does matter, the order in which it is done does not.

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u/Previous-Sympathy801 Dec 31 '23

You are correct.

AC is alternating current.

DC is direct current.

DC current goes negative to positive. AC the current switch direction X number of times per second. In the US it is a 60hz system, meaning the current switched direction 60 times a second.

Cars are DC

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u/TheOnlyCraz Dec 31 '23

Thanks for the confirmation!

I believe the 60hz could be visible with my flux core arc welder from harbor freight, about 60 times a second you get cool splatter!

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u/Z3400 Dec 31 '23 edited Dec 31 '23

The battery is DC, but cars are AC. The battery is only used to start the engine, once running the alternator is running the car and recharging the battery for the next start.

Edit: after a quick google because I questioned myself, it appears alternators are 3phase ac, but rectified so the car is actually still running on dc. Neato.

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u/81optimus Dec 31 '23

The alternator produces AC but this is then rectified back to DC in order to power the car and recharge the battery. Predominantly the car is DC

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u/Z3400 Dec 31 '23

You were probably typing that while I was editing my post lol. I typed my comment, then remembered how many times I have seen people argue about it and googled it, then added the edit.

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u/porchprovider Jan 01 '24

This argument is as old as Tesla vs Edison.

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u/SquishyBaps4me Jan 01 '24

You are correct, dc only flows in one direction. It's the only difference.

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u/Pinksquirlninja Jan 02 '24

DC -> direct current

AC -> alternating current

So yeah something along those lines. AC current can move back and forth, which is why people with solar panels can “sell” any of their excess electricity production to the power grid.

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u/SquishyBaps4me Jan 01 '24

DC does not flow in both directions.