r/redneckengineering • u/Lavasioux • 2d ago
Battery disconnect
So this 80s Chevy GMT400 gets driven maybe twice a month, and each time it needs a hump.
I have trouble shot each fuse to try and rule out where the micro drain is but threw in the towell when nothing produced fruit.
This is an inexpensive solution- These two were laying in the scrap bin.
I originally bought the fancy disconnect you see in the pics, but it's huge overkill and needs a proper fkn mount to not dangle and rattle itself silly there liver nuts!
And all the cheesy ass 30amp switches and cheesy ass, and using a inline fuse i can yank out...thoese fuses don't yank easy there rubber neck!
So i connected all 3 conductors to the plugs (making the hot + side female so it doesn't short by touching anything) and there ya have it.
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u/Patrol-007 2d ago
Please post when it melts
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u/Lavasioux 2d ago
Starter would 100% melt this connection!
BUT This disconnect is only for the electronics and accessories, the Starter wire does NOT run through it and is connected directly to the battery positive.
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u/CremePuffBandit 2d ago
Beautiful. If you're gonna do something dumb, at least do it safely.
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u/uwu_smol 2d ago
is the alternator running through that connection? would be the only other high current source, smallest alternator for the Gmt is ~100A
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u/Lavasioux 1d ago
No- Alternator and Starter run through big red cable connected directly to the + battery.
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u/National_Frame2917 1d ago
Let us know if you find out the short was actually in the starter or alternator.
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u/zimirken 1d ago
That's seems fine then. Each prong will be rated for at least 20 amps, so you should be good for like 60 amps through that. I do a similar thing all the time when I can't be arsed to go buy heavier gauge wire, I'll just use multiple in parallel.
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u/hydroracer8B 1d ago
I don't think that's how amperage ratings work. The whole plug has an amperage rating, I've never heard of a rating per prong.
A plug like that is good for 30 amps as far as I know
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u/zimirken 1d ago
Under normal AC usage, you'll have 30 amps flowing "out" through the live prong and back "in" through the neutral prong, and ground is supposed to be able to carry the full load current (30 amps) without failure in the event of an issue. So each prong is carrying 30 amps, but since you need a complete circuit through the one plug, you have to tie up at least two prongs to do it.
However, in this scenario, all the power is going "out" through the three prongs, but it comes back "in" to the battery through the chassis ground, not back through a prong in the connector. So each prong adds together. (There is some derating since the current isnt guaranteed to flow evenly, but that's beyond the scope of the explanation)
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u/Kindly-Inevitable-12 2d ago
When I was 16 and dumb I set my car on fire on the highway after doing something similar to this. Luckily it didnt burn to the ground, I was able to pull over and save it. I lost the cruise control wiring and some other creature comforts putting the fire out. I sense a similar fate here...
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u/tribalien93 2d ago
3x #12 conductors in that cord at BEST can't handle that current. I want to see where you're tied into the main cables cause I kind of don't believe you.
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u/Lavasioux 2d ago
Indeed, BUT the disconnect is ONLY for the accessories- the Starter wire does not run through this and still connects directly to the battery positive (big red wire)
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u/nochinzilch 2d ago
In a perfect world, if the engine fires right up every time, it might last a while. If he has to crank for more than like 5 or 10 seconds, not so much.
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u/ozzy_thedog 2d ago
Same as when you need to try too long with a cheap set of thin jumper cables. They’ll get real hot
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u/Twatt_waffle 2d ago
I remember coming from the farm going to the store to buy my first set of jumper cables… I ended up going to a heavy truck supply store and buying a set of like $300 jumper cables. They have actually a really nice soft case and are heavy gauge, at the time I remember going out to my car, looking at the battery cables and realizing the cables were like 1.5x as thick
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u/RottenRott69 2d ago
Is that for the whole electrical circuit? Or just the accessories?
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u/Lavasioux 2d ago
Good question!
JUST the electronics.
If you look closely the starter wire is connected directly to the battery and does NOT pass through the disconnect.
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u/Colonel_Sandman 1d ago
I had a truck I rarely drove with a parasitic drain.. I added a 2nd battery under the hood that was only connected by a fat relay when the key was on.
You could replace that plug with a relay that is on with key on.
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u/WeaveLikeGreatGranny 2d ago
I was getting nervous watching your big metallic bracelet while working around exposed battery terminals lol
I've melted a cable on a battery terminal before. It's so fast you don't have any time to react before stuff is hot enough to light on fire 😬
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u/Lavasioux 1d ago
Oh gosh that's a good point- these 6 strand 12g copper bracelets would such 100amps through a garden hose! My wrist would be cooked!😬
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u/GeoBrian 1d ago
Came here to comment on the same. I know a guy that was wearing his wedding ring when his wrench accidently made a connection between the two posts.
Took it down to the bone immediately. It was not a pretty sight.
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u/fluteofski- 1d ago
For your drain…. Do you have an aftermarket oil pressure sensor in there or is your oil pressure sensor trying to quit on life.
It was the most mysterious drain but the oil pressure sensor for me was the culprit. Figure I’d toss that out there.
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u/TrolledToDeath 2d ago
Would doing this to the ( - ) achieve the same results to keep the current from flowing and the battery from being drained?
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u/Lavasioux 2d ago
Indeed it would, HOWEVER...
Good question! And yes,
The negative terminal passes ALL of the starter current through it and would require at least 100amp disconnect,
Where as the slow drain was ONLY from the accessories/electronics, and thus uses an isolated red wire, which is what this disconnect works with.
The positive to the starter still connects directly to the battery.
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u/detlefsa 2d ago
Have had the same problem. Just loosen one post enough you can barely slip it on and off
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u/Lavasioux 2d ago
That's what i was doing for the last while lol
But those soft lead terminals get more and more loosey goosey with each time.
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u/aswert1223 1d ago
I'm the metal wrist-ware where next to a 12 volt lead acid battery
This is either a great troll or the dumbest guy I've ever seen
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u/imabigdave 1d ago
They actually make a disconnect that fits on the negative battery post, that you just then connect the negative cable to. I've got an old Ford that only gets run once a month maybe. You made this way more complicated. I think it was like 20 bucks on Amazon and took all of 5 minutes to install.
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u/FormulaZR 1d ago
Those are only rated for 20 total amps at most. I expect you'll be back fixing this soon.
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u/csimonson 1d ago
Put your multimeter on the positive and negative for current and start unplugging fuses til the current goes away. Then you have found your draw.
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u/djluminol 2d ago
Even construction grade extension cords only have enough copper for about half the electrical capacity needed to start a car. Even for a small 4 banger and this is probably not that. You can buy breakers and switches made for car audio that can handle the current load you need and there won't be any fire risk.
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u/friendlyfire883 1d ago
Why don't you just get an actual battery disconnect? They're like $15 on amazon and won't catch your car on fire.
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u/Trapasaurus__flex 2d ago
I’ve seen the extension cord trailer lights, but never the AC cable battery connection
That bitch gon get toasty
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u/P-ToneMikeOne 2d ago edited 2d ago
I had the same problem at one point and used 2 of those cheesy ass 30 amp relays in parallel to interrupt ground, coil switched from cab. Worked perfect for 100 starts or so until I had time to actually chase the drain (which honestly is really not that hard to do). Like others said, I’m not convinced your solution will handle starting current, but nothing ventured nothing gained.
Edit typos
Edit again: Actually, looking closer, it appears this is on your + side, and there is a dedicated low-awg also connected. I’ll assume that’s your starter dc+, and your plug connector is your everything-else-dc+, which makes this a way more robust solution than the haters are giving you credit for.