diy solar

diy solar

Best practice for 300kgf ‘fixture’ 280Ah cells

fafrd

Solar Wizard
Joined
Aug 11, 2020
Messages
4,188
I know the subject of the 300kgf ‘fixture’ (compression clamp) to extend cycle life of 280Ah LiFePO4 cells manufactured by EVE from 2500 cycles without fixture to 3500 cycles with fixture has been discussed in various threads on other subjects, but I could not find any single thread dedicated to that subject and now that I have my 280Ah cells and am beginning to think about how to assemble my battery, I am interested in current ‘best practices’ on this subject.

I have run my first charge and discharge cycle without any clamp/fixture and the cells do swell quite a bit.

So if the fixture is calibrated to 300kgf when the cells are empty, force is likely to far exceed that level once the cells are full.

And if the cells are filled before the clamp/fixture is calibrated, already-established swelling is likely to distort the battery far more than if a constant force of 300kgf had been applied during charging.

So my current thinking is to calibrate the clamp/fixture when close to empty (at least not yet bulging) and then to recalibrate every 0.1C or so as the battery charges to full.

There will still be less than 300kgf as the full battery dischargres but when bulging gets most extreme, there should be 300kgf in counter force and overall stability of cell position should be good.

Anyone else who has already faced this issues and solved these problems care to chime in?

My current plan is to use two 1/2” ABS plastic endcaps and two threaded rods per side to create the clamp/fixture.

If anyone has found a good solution to putting an entire 8S 24V 280Ah battery into a stock case with a 300kgf clamping fixture solution, I’m also interested in that..,
 
Manufacturer's spec was a particular compressive force, I think 12 psi, at a particular state of charge. Do you want to second guess them?
They seemed to mean a fixed fixture, an immovable object. But we discussed either springs or a stack of weights as an alternative.
 
Yeah, mine swelled too when I parallel top balanced. The swelling reduced quite a bit when I hooked the cells in series and and fully discharged them. I charged and discharged several times before I mounted them in a fixture and noticed they do like to "breathe".

My fixture consists of 1/2 inch plywood and six 5/16th threaded rods for my 8S pack. What I discovered in the end was hand tightening the nuts with no lock washers when fully discharged, the cells still swelled when charged. Someone pointed out the threaded rods act like springs. I would not have believed that had I not seen it for myself.

I am going to redo my pack and see if I can't get it to be more square. A couple of my cells came undulated as @AussieSim pointed out in this thread. The gap between my cells widens at the bottom. I am also concerned about this putting stress on the terminals. I think the only way around that is to use braided bus bars. Even short flexible cables would be better than stressing the terminals using the solid busbars I received with the cells.

Yes there are several threads discussing this and I have read through all of them including your springs idea thread @Bob B . :) I like your idea but I am open to other ideas as well. And I think the only way to prevent stress on the terminals is to use braided busbars or flexible cables with lugs no matter what method is used? I have to think through about the best way to deal with my problem and I can't proceed until I receive the replacement cell I need because I stripped one of the terminals on mine....stupid me.
 
So you haven't read the thread on repairing threads?
Helicoil, or tap oversize, or clean out and JB-weld a stud in.
And use studs not bolts next time, Dumb Dumb!
 
Yeah, mine swelled too when I parallel top balanced. The swelling reduced quite a bit when I hooked the cells in series and and fully discharged them. I charged and discharged several times before I mounted them in a fixture and noticed they do like to "breathe".

My fixture consists of 1/2 inch plywood and six 5/16th threaded rods for my 8S pack. What I discovered in the end was hand tightening the nuts with no lock washers when fully discharged, the cells still swelled when charged. Someone pointed out the threaded rods act like springs. I would not have believed that had I not seen it for myself.

I am going to redo my pack and see if I can't get it to be more square. A couple of my cells came undulated as @AussieSim pointed out in this thread. The gap between my cells widens at the bottom. I am also concerned about this putting stress on the terminals. I think the only way around that is to use braided bus bars. Even short flexible cables would be better than stressing the terminals using the solid busbars I received with the cells.

Yes there are several threads discussing this and I have read through all of them including your springs idea thread @Bob B . :) I like your idea but I am open to other ideas as well. And I think the only way to prevent stress on the terminals is to use braided busbars or flexible cables with lugs no matter what method is used? I have to think through about the best way to deal with my problem and I can't proceed until I receive the replacement cell I need because I stripped one of the terminals on mine....stupid me.

Good to know I’m not alone (as I knew I would’t be ;).

Swelled cells and an undulating row which was originally straight are my recent experience.

One of those threads included a message from EVE so it’s good to understand that ‘overseelling’ causing delaminaton and/or electrolyte leakage is the reason cycle life is degraded with no fixture/clamping force (so the cell degrades/falls apart before delivering a full maximum 3500 cycles).

And it also sounds like ‘too much force’ near the end of the charge cycle is not an issue - you just want at least 300kgf(14-15psi) near the end of the charge cycle when the bulging gets worst.

Reading all of this, I’m more comfortable clamping with 300kgf near the middle of the charge cycle or near empty and just letting it ride to higher levels approaching 100% SOC.

The only thing I saw causing me pause was someone claiming that capacity degrades with increasing force - has anyone verified that?

If a capacity test shows reduced capacity with clamping force in excess of 300kgf, you can always reduce clamping force to 300kgf near full, charge to 100%, recalibrate clamping force to 300kgf and repeat until no additional charge is absorbed and/or the force remains 300kgf after charging (meaning there was no additional swelling force with additional charging).

From that reference point, you should get 100% of your charge capacityand the full 3500 cycle lifetime.

The endcaps & threaded rod approach is pretty straightforward. Has anyone found a good metallic case solution for 8 of these 280Ah cells?

Mechanically calibrating a pastic spacer between the first/last cell and the walls of the case seems like another easy way to achieve what is required...
 
So you haven't read the thread on repairing threads?
Helicoil, or tap oversize, or clean out and JB-weld a stud in.
And use studs not bolts next time, Dumb Dumb!
Yes I have. All of them. Have you read my posts describing my experience using studs and nuts? And using JB Weld? Perhaps you should before you start making false accusations!
 
Hey, I only asked a question, didn't make any accusations. (well, a statement with a question mark was intended as a question)
And I quoted YOUR tag line, all in fun :)

I see you did write, "Anyways, I got some JB Weld, reinserted the stud and am going to let it cure for a couple days", so how did that work out?
That was a few days ago, but you have to wait for new cells?

JB weld has worked for the things I've needed to do. I think it has enough shear strength for this application.
 
Reading all of this, I’m more comfortable clamping with 300kgf near the middle of the charge cycle or near empty and just letting it ride to higher levels approaching 100% SOC.
That is what I did the first time after I mounted them in my fixture. They still swelled mostly at the bottom.
The only thing I saw causing me pause was someone claiming that capacity degrades with increasing force - has anyone verified that?
I think using springs would solve that problem. At this point I am lot looking for 3500 cycles. If I had the thousands of dollars worth of equipment EVE does to have a perfect compression at all SOC's, then I would go for it.

2000 cycles is a lot, and considering many don't do full cycles and stay between the knees, the life time of the cells before they reach 80% capacity is greatly increased. By the time I get there the cells will most likely be starting to age.
 
Hey, I only asked a question, didn't make any accusations. (well, a statement with a question mark was intended as a question)
And I quoted YOUR tag line, all in fun :)

I see you did write, "Anyways, I got some JB Weld, reinserted the stud and am going to let it cure for a couple days", so how did that work out?
That was a few days ago, but you have to wait for new cells?

JB weld has worked for the things I've needed to do. I think it has enough shear strength for this application.
Sorry. It's the holidays, I am slightly stressed. That and having to put up with my own stupidity of stripping the threads and all. :p Sorry I took your post the wrong way.

As far as the JB Weld, I am not sure yet. I was able to tighten down to 3nm using a beam torque wrench, but the nut kept turning like it was getting ready to break the bond. Maybe it did. But the nut is remaining torqued down as far as I can tell.

Initially I used Red Loctite but I didn't use the primer. So it is possible it didn't cure properly or I applied too much torque. The stud was coming up and I didn't notice it until it was too late. I totally stripped the threads.

Even if the JB Weld does prove to work out, I am getting a replacement cell just in case. I know about the heli coils and tapping a larger hole. Knowing me, I would mess that up. There is only 6mm of depth in that hole. I found some 6mm depth heli coils on ebay and I also know about cutting off the tap to get maximum thread depth.

Anyways, all of the other studs as far as I can tell are ok. And I will definitely be using my beam torque wrench from now on. I didn't want to have to go through a top balance again and I was able to charge the pack to almost full after using the JB Weld. So if the JB Weld doesn't pan out, when I receive the replacement cell I will fully charge the pack and then fully charge the replacement cell.

I won't be able to test anything further until after the holidays. JB Weld is really good stuff. I think the long cure original is the best and I let it cure for a couple of days. Back in the day I was trying to fly RC helicopters. I managed to hover them but that's as far as I got and I still broke the landing struts and other things. I used the quick cure JB Weld to repair them and the copter never broke again where I used it....lol.
 
So you haven't read the thread on repairing threads?
Helicoil, or tap oversize, or clean out and JB-weld a stud in.
And use studs not bolts next time, Dumb Dumb!
Using studs needs to be an item that has a lot more prominence on this site. It's almost as important as top balancing, IMO.
 
Oh...BTW @Hedges thanks for your suggestion to try JB Weld. I saw that in the thread where you were suggesting it to @SherylinRM . I had thought about it and when I saw your suggestion I thought...why not? Then I started researching it...lol. As I recall someone posted a youtube video and he managed to get a 4.5nm torque on a 5/16 bolt before the bond broke.
 
Using studs needs to be an item that has a lot more prominence on this site. It's almost as important as top balancing, IMO.
And making clear 8nm is way too much torque. Personally I think 4nm is enough.
 
If the nut kept turning, probably pulling the stud out. Best not to use it and have a high resistance contact.
Just as a SWAG I expected that much surface area to have sufficient shear strength for the pull on the bolt, given low torque spec, but I haven't done the math.
Would have to be completely clean, of course.

"4.5nm torque on a 5/16 bolt before the bond broke."

Is your stud is maybe half that? So you confirmed his results?
Unless his length of engagement was shorter.

Torque of threads is dominated by friction. The clamping force isn't nearly as much as you'd think based on Archimedes and ramps.
Radial loading applied to a bolt by the torque wrench affects it. Better to use an extension, which forces you to counterbalance with the other hand.
 
If the nut kept turning, probably pulling the stud out. Best not to use it and have a high resistance contact.
That cell had the lowest voltage when I charged the pack. So you are right. But it's hard to tell if I can torque it down more and I won't know for sure until I redo the pack. The reason is the BMS terminal is under the nut. I have been wondering if having the BMS terminal under the nut has any effect on the torque? Because the BMS terminal turns when tightening down the nut.

I am thinking it might be best to use double nuts and put the BMS terminal between the two nuts. The voltage readings should still be accurate even though the nuts are stainless steel. And the balancing current is so little on my BMS it should not be a problem. This would also prevent the nut connected directly to the busbar from coming loose.

I am using the flanged serrated type nuts. As far as the cells terminal, I did make sure the cells terminal hole was completely clean. Used acetone in it and made sure it was completely dry before using the JB Weld.

Is your stud is maybe half that? So you confirmed his results?
Unless his length of engagement was shorter.
My studs are 6mm. Same as in all the EVE and Lishen cells. I think the Lishen cells have a 10mm depth in the terminal. I am sure his bolt was longer than the 6mm's we have to work with. Plus I had literally no threads left in the hole which doesn't help.

Torque of threads is dominated by friction. The clamping force isn't nearly as much as you'd think based on Archimedes and ramps.
Radial loading applied to a bolt by the torque wrench affects it. Better to use an extension, which forces you to counterbalance with the other hand.
I will get an extension. I have to anyways. That's good advice.
 
Using studs needs to be an item that has a lot more prominence on this site. It's almost as important as top balancing, IMO.
Are all of you using stainless studs or aluminum studs, I would have thought that an aluminum stud would do a pretty good job protecting the terminal threads (stud stripping before terminal). I’ve been just finger-tightening the stainless bolts that came with my cells for top-balancing and capacity testing, but plan to get some studs before final battery assembly.

Does anyone have preferred sources for studs?
 
Just as a SWAG I expected that much surface area to have sufficient shear strength for the pull on the bolt, given low torque spec, but I haven't done the math.
Would have to be completely clean, of course.
Topic was JB weld to hold stud in stripped hole. Also considering the subject of stripped threads.

JB weld is 5000 PSI tensile strength. I'll assume the same for shear.

6mm diameter hole, 6mm deep. 0.175 in^2 surface gives 876 pounds pull-out.

Aluminum M6x1 bolt, 30,000 psi tensile strength (I also find references to shear strength being same figure.)


1 mm pitch 60 degree angle thread leaves 4mm diameter core (and a nice V groove for stress concentration?)
0.0195 in^2, 584 pounds to break.

Aluminum coefficient of friction 1.2 dry, 0.3 lubricated


Torque to 4nm?


Clamping force 556 N dry, 2222 N lubricated


That's 125 pounds dry, 500 pounds lubricated

Torqued to 4 nm with dry threads seems good, lubricated would be in danger of breaking an aluminum bolt.
The JB weld epoxy appears strong enough, holds as much as the aluminum bolts which came with the cells and torque recommendation. (my SWAG wasn't half bad)

Stainless stud of course is stronger, 100,000 PSI or 3.33 times as strong.

As for stripping threads, I thought of 1mm wide thread base, 6mm diameter for female thread, 6 turns.
6 x pi x 6 = 113 mm^2, 0.175 in^2, 5260 lbs if 30,000 psi shear strength.
My calculation is way off. Does something stretch so it shears a little bit at a time?
 
I am using stainless 6mm X 20mm studs.

Sscon 100 PCS M6 x 20mm Grub Screws 201 Stainless Steel Hex Allen Head Socket Set Screw Bolts: Amazon.com: Industrial & Scientific

And nuts like these:

Amazon.com: Orgrimmar Flange Metric Hex Stainless Steel Serrated Lock Nuts M6 (Pack of 50): Home Improvement

As you probably know, aluminum is much softer than stainless steel. The only way either will strip is if applying too much torque.

I just thought aluminum studs would do a better job protecting the threads of the terminals (for the rare event of too much torque being applied) not to mention the fact that there will not be any galvanic corrosion to worry about with aluminum studs (on the terminal threads).

But I appreciate the links and may consider stainless studs if aluminum is too costly or hard to find.
 
My calculation is way off. Does something stretch so it shears a little bit at a time?
Maybe the threads inside the terminal stretched? That's what I think happened in my case. Either that or the loctite didn't fully cure and was acting as a lubricant. It would be nice to have all the specifications of the cells aluminum terminal. The aluminum used seems to be fairly soft to me.

I am no expert on anything and was following everything that had been posted when I assembled my pack with studs. I did neglect buying a torque wrench and it might be a good thing I did at the time. I would have been tightening the nuts to 8nm which I believe to be misunderstood in the EVE spec sheet. I surely would have stripped more of the cells had I tightened to 8nm. I have a feel for it using a beam torque wrench. Judging by the reading of 4nm on the wrench and the force I need to apply to get there, it seems tight enough.

I will try to post a pic of the stud and the threads that came out with it.
 
Back
Top