Stripped threads on aluminum cell

I suspect, that it will progress to m8.
Everything improves over time.

Actually, the new 'norm' is welded M6 Studs.

True

Threaded holes in aluminium seem to cause grief sooner or later.

The "correct" engineering solution is to fit some threaded studs, the very first thing you do.
Put them in finger tight + 1/4 turn, and if you use proper thread locking compound, they are in there pretty firmly, but are still removable.

Then use a plain nut with a washer, and you can heave down on that nut, screw and unscrew it a thousand times, and you will never have any trouble. The chances of cross threading a nut are just about zero. And if you do wreck the stud, a new one is a quick simple fix.

Warpspeed said:

Threaded holes in aluminium seem to cause grief sooner or later.

The "correct" engineering solution is to fit some threaded studs, the very first thing you do.
Put them in finger tight + 1/4 turn, and if you use proper thread locking compound, they are in there pretty firmly, but are still removable.

Then use a plain nut with a washer, and you can heave down on that nut, screw and unscrew it a thousand times, and you will never have any trouble. The chances of cross threading a nut are just about zero. And if you do wreck the stud, a new one is a quick simple fix.

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I read this as a stud threaded into the aluminum threads and 'heave down' on the nut. I see the aluminum treads pulling right out.

I would stick to 3nm or 25 in-lbs for that heave.

Studs work very well in the vehicle industry, and for aviation, and aerospace too, where aluminium is used a lot.
It will probably work for you too with your battery terminals.

Its not that aluminium is particularly weak, its that if you have a blind threaded hole, the bolt needs to be the exact right length.
If its too long, it will bottom out in the hole and then strip.
If the bolt is too short, only a fraction of the threads will be engaged, and the thread will probably then strip.
If you cross thread a bolt going into an aluminium hole, all bets are off.....
And if that bolt is frequently removed and replaced, wear in the thread, if nothing else will eventually lead to disaster.

So the proper engineering solution is to use a stud that goes all the way down, using all the available threads in the hole, and you will get all the strength that is possible with an absolute minimum risk of damage. Once the stud is in there, LEAVE IT ALONE.
Every time you screw something in and out of a tapped hole in aluminium the thread wears very slightly.

Its a crime against nature that the battery manufacturers usually supply bolts with these batteries. They cannot possibly know what you are going to bolt down to that terminal.
If its a really thin lug, the bolt must not be supplied too long.
If its a quarter inch thick copper busbar you are bolting down, the bolt will probably be a quarter of an inch too short, and probably strip sooner rather than later.

If the battery supplier provided reasonably long studs in their kit, you can bolt down just about anything using all the threads in the battery, no matter how thick what you are bolting down. And that stud will probably last the life of the battery without damage.

Of course "Bubba" after drinking a crate of beer with his massive long spanner can strip out just about any bolt.

We are talking here about applying sensible engineering, and doing the job properly.

Aluminum engine blocks also have much longer threaded holes than these cells do, so I still wouldn't heave on them too much.

Torque specifications should always be followed, no matter what you are working on.

Warpspeed said:

Its a crime against nature that the battery manufacturers usually supply bolts with these batteries. They cannot possibly know what you are going to bolt down to that terminal.
If its a really thin lug, the bolt must not be supplied too long.
If its a quarter inch thick copper busbar you are bolting down, the bolt will probably be a quarter of an inch too short, and probably strip sooner rather than later.

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I would think the bolts are for the included bus bars only. Any change to that needs to be engineered by the user. Even a single thin ring terminal will reduce the rating.

I would think the bolts are for the included bus bars only.

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You and I know that, but the average person may not have thought the whole situation right through.

Torque specifications should always be followed, no matter what you are working on.

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You are quite right of course, but how many of us can hold our hand over our heart, and can honestly say that we use a torque wrench on every single nut and bolt and screw we assemble.

If you do decide to use studs, the nut will spin very freely down the thread, and once seated with finger pressure, only needs only about a quarter turn to go quite tight. A bolt going into an aluminium thread "feels" very different as it tightens up, it gains tension much more slowly.
Then you get that awful sinking feeling that the last turn did not feel it getting any tighter.....

I just got 16 more cells. In one box, a thread from each was not threaded. Smooth inside, with no threading. Speaking to the seller now. The cells appear fine, just overlooked in the construction.

I can just thread them myself with an M6 tap... right?

Jbbonnette said:

I just got 16 more cells. In one box, a thread from each was not threaded. Smooth inside, with no threading. Speaking to the seller now. The cells appear fine, just overlooked in the construction.

I can just thread them myself with an M6 tap... right?

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Just curious what the hole diameter is. If you have a good drill index you should be able to measure with some accuracy.

Smooth inside, with no threading.

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Sounds like really top notch inspection and quality control.

Warpspeed said:

Sounds like really top notch inspection and quality control.

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For sure.

Roswell Bob said:

Just curious what the hole diameter is. If you have a good drill index you should be able to measure with some accuracy.

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I do not have a drill index. Perhaps I will look into that. I sure don't want to mess it up either, right now I have a valid reason for the company to replace them, sure don't want to make it worst on all fronts.

Jbbonnette said:

I can just thread them myself with an M6 tap... right?

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You will need to modify some taps in order to get the threads deep enough. The holes are so shallow that even the taper on a bottoming tap will be a problem. You will need to start with a bottoming tap and then finish with a bottoming tap that has been ground to near flat on the tip.

FilterGuy said:

You will need to modify some taps in order to get the threads deep enough. The holes are so shallow that even the taper on a bottoming tap will be a problem. You will need to start with a bottoming tap and then finish with a bottoming tap that has been ground to near flat on the tip.

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Probably 3 steps.

I agree the terminals would be nice with an additional 4mm of thread.

I bought a kit from Amazon and then ground down the included drill bit on a grinder wheel. Just measure the hole depth so you don't drill too deep. It seemed to work okay for me. It repaired the Cell thread I stripped out.

FilterGuy said:

I have rethreaded terminal pads to M8.... but would not do it again if I had to.

How to drill and tap aluminum cell terminal to modify from M6 to M8 threads

To get this paper, click on the orange button at the top of the page I stripped out the M6 threads in the terminal hole of an Aluminum Case LiFePO4 Cell. This deck shows what I did to drill out the hole and tap for an M8 hole. Revision 2...

diysolarforum.com

Every time you screw the stainless steel stud in or out, it damages the soft aluminum threads. My advice to people with threaded terminals is to put studs in with some red locktite and primer and never take them out again. (use a minimum length of 20mm on the stud. 25mm is better.

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I'm curious, what would you do if you had stripped M6 threads now? I stripped the M6 terminals in two of my batteries. I had been using studs but I was working on my batteries in cooler temps and I think the aluminum was brittle... anyways, I was going to follow your guide and drill to M8 and epoxy the new studs in. Would you do a different repair now?

Also, should I go back and pull all the other studs from my pack and put red loctite on them? I had planned to put JBWeld Marine Weld on them as in my research the resistance between the stud and the terminal shouldn't matter much... but I know there's a lot of debate here so I don't want to start that debate here.

Just a little background, this pack I built about a year ago and is in my RV. A couple weeks ago I was getting things ready for spring and I noticed on this pack my cell voltages were way out of line from each other. Started looking for bad connections/resistance in the terminals when I pulled off the first buss bar. Didn't find anything wrong so I put my nuts and studs in like normal and tightened up like normal but got that bad feeling at the end as it didn't get tight and the threads were pulling out. I think the battery was at a temp of around 40-50 degrees F at the time.

So, this leads me to question.... with my LFP batteries being in the RV freezing all winter, freezing and thawing, would the tension of the studs in the aluminum caused the threads to weaken during the freezing temps and become brittle and loosen up the tension on the terminals? I'm wondering if I might have other failing threads in my pack and if I should just retap all of them to M8 right away.

Any thoughts appreciated.