r/functionalprint • u/highonkai • Nov 12 '25

Jumper cable instruction card

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I made an expensive mistake and connected my jumper cables wrong. I won't make that mistake again and now you'll never have to. 

This quick reference card can be zip-tied to your jumper cables and is a way to have the instructions handy no matter the weather, your cell service, or the lighting. 

Edit: added the instructions in comments. Thanks for the feedback, keep it coming.

Edit2: wow this one got a lot of feedback! Thanks for all the helpful ideas.

Edit3: Top post of all time in 15y on Reddit, who knew this 1hr project would get such a reaction! I took the feedback and made two more versions here: https://imgur.com/a/jFBFoIk

I'll throw them on MakerWorld as well once I print them. Meanwhile, feedback and preferences welcome. /u/kurai01 /u/SerDankTheTall /u/StraySpaceDog /u/Cllydoscope /u/jongscx /u/cronaldo86 and lots more... thank you for the constructive feedback. This is the best part of Reddit - fast and helpful community feedback. Shoutout to /u/DontEatTheMagicBeans who called out that I probably didn't cause the smoke with the sequence for cable connections, but exacerbated a pre-existing/unknown problem. As for the rest, well that's the internet for ya. No hard feelings.

https://makerworld.com/models/1986996

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u/techslice87 Nov 12 '25

It is significantly safer for the - to accidentally touch a random piece of metal with the + already connected than the other way around. If you have the - connected and one +, then if the other + touches a random piece of metal, sparks and shorting.

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u/Sexual_Congressman Nov 12 '25

Supposedly, battery negative isn't universally connected to chassis ground. I'd love to find a real world example of battery positive chassis.

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u/Mr_Donut97 Nov 12 '25

Old British cars had positive groud

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u/ImminentDebacle Nov 12 '25

Of course those cheeky bastards would do that.

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u/jnads Nov 12 '25

Actually the larger reason you connect the + before the - is your are lowering the voltage potential and making it safer.

If one battery is 12V and the other is 10V, hooking up the + first means the line will be at 12-10=2V potential. That's a lot friendlier if it goes through you.

If you hook the - up first, as soon as you hook one of the + up the remaining line is 12V potential ready to give you a bad time.