Stud broken, looking for help/insight.

Rich585

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Mar 16, 2021
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Ontario Canada 🇨🇦, somewhere near Ottawa
I guess I am in MurphyGuy age bracket. High school in early 80s, they had auto shop where they actually fixed cars, and shop where you learned how to weld, use a drill press, lathe, band saw, milling machine, measure things to thousand of an inch.

Back then, everyone when turned 16 got driver’s license as soon as possible and bought an old wreck and wrenched it themselves to keep it running.

Last 10-20 years how many high school kids or even those in their 20s knows what the above do or how to even check the oil on a car unless they went to a trade school to become a technician. The general population does not think to fix anything, you buy something, if it breaks you buy a new one.

Why would you spend time trying to fix it????

My neighbours look at me funny when I have my car jacked up in driveway changing out the brakes. Don’t understand why I just don’t take to garage.

World has changed and left me behind…..
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,109
Yeah I'm considering milling it flat in the near future. Highschool machine shop? I've been out of highschool twenty plus years now and we didn't have one of those then. Useful sh*t isn't taught in America anymore.

"A college diploma in every pot"

My high school (Skyline, in Oakland), removed its metal/wood/auto/electronics shops and used them for classrooms.
No one is diverted to the trades, everyone is prepared to move on to college (and college freshmen have to be taught what previous generations learned in high school, because the bar was lowered to improve high school graduation rate.)

Of course not all children fit on the college track. But our government believes in one-size-fits-all, and by paying schools for test results they get "teach the test" (and drug the laggards, because if diagnosed with learning disabilities their performance isn't held against the school.)

But don't try to do it by hand no matter how desperate you get.. it needs to be razor flat..

I think, but haven't demonstrated by measurement, that flatness and area aren't important. Force and pressure are.

I think contact resistance is determined almost entirely by force. Too large an area and resistance for a given force will be higher. But start with an uneven surface and as force is increased, pressure reaches the point where material is mashed and area increases. I think that will give good contact resistance.

Think of what happens when you put stranded wire (including aluminum) in a lug and tighten a set screw onto it.
Initially there is little contact area, but that increases with force.
The one modification I've made to following torque instructions is I wiggle (ideally rotate) wire back and forth while tightening, because that helps the strands settle flatter and I can turn screw further before reaching torque setting. Otherwise, wire becomes loose due to wiggling later.

iu
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,109
Thanks Joe. I’m glad you got it fixed! I’ll be going through this process as well soon. Any reason you didn’t go with an M8 bolt size? Might reduce that gap.

Did you guys break these studs off by torquing the stud relative to the cell terminal?
Or maybe by tiling force, due to offset force vectors?

As a nut tightens on a bolt that goes through a hole, sometimes you need to hold the bolt head with a wrench, sometimes with your fingers, sometimes nothing is needed and you can just hold the object it goes through. There is friction between bolt head and object that resists rotation.

If you have a heavy pack and pull on a wrench, 100% of torque may be taken by the stud weld. If a cable is coming off the last cell, this is likely the case. If a busbar is between two cells, the busbar would have to rotate and shear stud off next cell, which is much less likely.

I think if the cable terminal or busbar can be restrained so wrench is working against it, rather than against cell body, that should avoid torque on weld. For instance, if a wrench or vice grips held the cable terminal and you put a box end wrench on nut and squeezed between the two with thumb and fingers. That way you're not just pushing on the wrench.

Tilting - Except for a box end, if you use a socket wrench there is torsion on the stud when you push on wrench.
If you use a 6" extension between socket and torque wrench, you can use palm of one hand to keep it from flopping while turning the wrench. This eliminates torsion on the stud, so it won't be pried off. It also eliminates torsion on the thread between nut and stud, reducing friction so you get more clamping force for a given torque. (A longer handle wrench gives same torque with less radial load, also reducing friction.)
But with two hands in use, you need a 3rd hand to restrain cable terminal.

A torque screwdriver would provide same benefit as extension; you have to use it in a manner that keeps it aligned, doesn't apply torsion.


It appears these welds aren't of suitable quality, have insufficient penetration.
The process or each weld ought to be tested during manufacturing process, like by restraining cell terminal and pull-testing the stud.
But what's available to DIY'ers usually doesn't get that sort of care and documentation.
 

MurphyGuy

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May 20, 2020
Messages
1,034
I think, but haven't demonstrated by measurement, that flatness and area aren't important. Force and pressure are.

I think contact resistance is determined almost entirely by force. Too large an area and resistance for a given force will be higher. But start with an uneven surface and as force is increased, pressure reaches the point where material is mashed and area increases. I think that will give good contact resistance.

Think of what happens when you put stranded wire (including aluminum) in a lug and tighten a set screw onto it.
Initially there is little contact area, but that increases with force.
The one modification I've made to following torque instructions is I wiggle (ideally rotate) wire back and forth while tightening, because that helps the strands settle flatter and I can turn screw further before reaching torque setting. Otherwise, wire becomes loose due to wiggling later.

iu

Like many other things in life involving multiple characteristics which affect each other, I'm pretty sure both flatness and pressure are both important..

If you have the flatness, you don't need as much pressure, and if you have the pressure, then you don't need as much flatness.

In this particular situation, pressure is a problem because of the crappy Chinese metal and design.. So to reduce the importance of needing lots of pressure, we work to flatten the area. Six (or so) small #6 screws aren't going to provide a lot of pressure, but if that aluminum is machined nice and flat, that requirement won't be as critical.

It also depends on how many amps the cells will be delivering.. if those cells are working to deliver 160 amps all the time, that's going to be a close call.. but if his battery bank spends most of its time in the 30 or 50 amp range with only brief spikes, then it shouldn't be an issue.

The poster might also want to consider using some carbon contact glue when making the connection.

And make sure to bolt the contact plate down within minutes or seconds of sanding the aluminum after its been flattened.. this will help stop the aluminum oxide layer from forming, further decreasing contact resistance.

He'll be fine.
 

Hedges

I See Electromagnetic Fields!
Joined
Mar 28, 2020
Messages
9,109
The paper I linked here has a graph "Figure 69 – The effect of pressure on the contact resistance of a joint"
I think the main issue for these terminals is aluminum and its native oxide, which begins to form instantly after cleaning. I think sufficient pressure (smaller area for a given force) helps break through that.



If we just had tin-plated aluminum like all the UL listed terminals, wouldn't be nearly as much of a problem.
 

corporate.joe

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Jan 25, 2021
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Vocational Tech school was available here for the high school aged, but it was not suggested nor advertised. To this day I'm not sure how children are put in that program. From my assumptions it's the kids that are either doing poorly or dropped out and are going the G.E.D. route with some useful hands on work force education. I don't know any of this for fact, it's just my assumption mostly based on negative stigma I remember being associated with it when I was younger. Heck, the back yard of the Vo Tech school here intersects with the yard of the Juvenile Detention center. How's that for association?

While I was top balancing those cells I had used a screw driver with 1/2 inch driver on it. When I pulled that stud from the weld I was using a 1/4" ratchet. There's a big difference in the ease of applying the same amount of force with each of those tools. (Levers and all that). I will stick to using my new torque screw driver that was designed with the intention of installing rifle scope rings even if it's not as accurate as a torque wrench/ratchet.

I'll be glad once my battery boxes are complete and there will be no more disassembly of the packs further inviting more to go wrong. I'd order these same batteries again but I would prefer the untapped terminals I think.

It also depends on how many amps the cells will be delivering.. if those cells are working to deliver 160 amps all the time, that's going to be a close call.. but if his battery bank spends most of its time in the 30 or 50 amp range with only brief spikes, then it shouldn't be an issue.

Three individual 8s packs connected to a Growatt 3k all-in-one unit. Each pack fused with 125a ANL fuse on the negative side to Neg busbar. All positives go through a busbar to a 300a t-class, then the thornwave shunt, then a 150a breaker to Pos busbar. Currently only have two packs installed and running. Third pack is sitting in parallel until I have everything to build the boxes I have envisioned. Most I have pulled so far from the inverter is 66.8 amps running the hot air gun for a heat shrinking session. Highest planned sustained draw is right in that same range of 1,600 watts for a rooftop AC. Highest charge rate so far I have seen is 48.5 amps(surge after cloud cover cleared and panels cooled) with 1830 watts of panels, 3s pointing towards morning sun and the other three pointing south.

They are practically being babied. I will add another charge controller in the near future. I already have some used panels(960W total) that are on the sideline waiting. 18ish kWH is great but the most I have put into them since service(Aug 27th) is 6kWH in a single day. Need a bit more for Winter months I think. And I might be a little addicted to this stuff. Most expensive hobby for me so far.

Joe-
 
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