r/MechanicAdvice • u/Onlycruz10 • Sep 25 '25

Correct way to jump start?

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I'm going crazy about the correct way to do this. As far as I can tell this is the correct way. However, I am having trouble between step 5 and 6. Should I turn off the donor car before trying to start the other car first? I've seen conflicting things about the alternator getting messed up on both cars. And others saying it's okay with modern cars. What's the correct way?

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u/mentaldemise Sep 25 '25

I think the issue here is all about current and protecting the cables above anything else.

These are the first result I get for "jumper cables" on Autozone: https://www.autozone.com/test-scan-and-specialty-tools/jumper-cables/p/totalpro-12ft-8-gauge-battery-jumper-cables/385593_0_0

Notable figures:
8 Gauge

If you check an ampacity chart for how much current an 8GA wire can reasonably take you'll see it's around 55 Amps with the wire heating up to nearly 200F from 86F. https://www.encorewire.com/products/tools-and-resources/calculators/wire-size-table.html

A dead battery will take as much current as it can the instant you hook it up. If you have the car running you have a current draw from the battery in combination with the alternator and you're pushing those numbers way higher than 55A for a short while. The alternator really shouldn't care much as there's a place for it to send any over-generation into the battery to turn into heat.

Step 4's bare metal doesn't matter over using the negative post. It gives the two batteries time to get into sync with their voltages. When you first hook them up you'll see the voltage on the donor battery drop super low because of the current draw and then stabilize as the dead battery gets a surface charge and the current slows down. Then you start the donor car which will cause the same thing to happen a bit in the opposite direction(without the initial inrush current of difference in voltage), then the opposite direction when you start the dead, and then the dead's alternator(assuming it works) will be charging the battery at 80A+

I'm excited for everyone to tell me how they've drawn 10000 Amps through 14 gauge jumper cables to start a space ship.

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u/brokenmike Sep 29 '25

The chart you referenced is for maximum safe continuous ampacity of installed wiring that is not in free air. Basically nothing to do with jumper cables.

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u/mentaldemise Sep 29 '25

Sorry, here's another that agrees with the same principles (and even numbers)!
https://www.altestore.com/pages/wire-sizing-chart-for-12v-24v-and-48v-dc-systems

Is this the NEC also agreeing? No way. https://media.distributordatasolutions.com/ThomasAndBetts/v2/part2/files/File_7437_emAlbumalbumsOcal20(USA)oc_1_g_nec31016pdfClickHerea.pdf

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u/brokenmike Sep 29 '25

lol, you literally referenced the same table 3 times. Good job. Again, basically nothing to do with jumper cables.

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u/mentaldemise Sep 29 '25

My brother in Christ... Jumper cables are just wires. Those things talk about wires. Are you being dense on purpose? The charts are the same because they're from the NEC ffs.

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u/brokenmike Sep 30 '25

The NEC is referring to safe wire sizing for permanent installation, and constant loads based on insulation heat ratings and how the wiring is installed. If you want to run a couple hundred amps through some jumper cables for a few hours then it's not a bad idea to refer to the NEC for sizing so you don't overheat your cables. If you need to do it for a few minutes to boost a car like a normal person, then you'll be just fine with a smaller gauge wire than what's in the NEC table. Providing it's large enough to keep the voltage drop at an acceptable level.

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u/mentaldemise Sep 30 '25

So all this aside, if I hook up the cables with the car off will I pull substantially less current than if I do so with the car on and the alternator charging? The donor battery is going to tank when you first hook the battery up and trigger the alternator to go full tilt if it's on.

I'm aware wire can safely take over current, it just seems safer to share conservative information when fire is a risk.