Best practice for 300kgf ‘fixture’ 280Ah cells

[Gazoo]

And here it is in all its glory:

 

[zorlig]

Wow this went to studs fast. I didn't have any problem with the normal screws hand tight.

For compression the easiest is threaded rods with wood or something else at each end. They are the only way to get up to the force specified, all my other attempts failed, but they made it really easy.

You can go for small rods from the hardware store, which had all the hardware and was really cheap and easy.

The range of force to get good results is pretty wide, 6-17 psi.

I actually used a scale to figure out how tight the bolts had to be so I figure I'm in the range, but using a torque wrench will get better results.

Some other people talked about springs, but I've not seen that in practice nor does it seem necessary.

If someone's cells are still expanding in a fixture then I doubt they have it nearly tight enough.

 

[fafrd]

Wow this went to studs fast. I didn't have any problem with the normal screws hand tight.

For compression the easiest is threaded rods with wood or something else at each end. They are the only way to get up to the force specified, all my other attempts failed, but they made it really easy.

You can go for small rods from the hardware store, which had all the hardware and was really cheap and easy.

The range of force to get good results is pretty wide, 6-17 psi.

I actually used a scale to figure out how tight the bolts had to be so I figure I'm in the range, but using a torque wrench will get better results.

Some other people talked about springs, but I've not seen that in practice nor does it seem necessary.

If someone's cells are still expanding in a fixture then I doubt they have it nearly tight enough.

Yeah, this is pretty much what I was planning (using 1/2” black ABS plastic for the ends).

I’m still struggling wit deciding whether to put the cells / battery in a case or not. With a hard-side case, it seems as though you could just squeeze the right thickness of plastic on either end between the rigid walls of the case and the empty cells and call it a day.

But since a 280Ah 8S/24V battery is not going to be moved much, just building a simple plastic enclosure to protect the busbars/terminals is another option.

From what I read in several of the threads others pointed to here, I’m no longer really concerned about having ‘too much’ force (unless it proves to degrade capacity).

 

[Off-Grid-Garage]

Apparently a compression is not necessary as per EVE but only a fixture (yes, I read the long 25page thread...)

 

[upnorthandpersonal]

@Off-Grid-Garage Notice he says it's 'not needed' but you do need a fixation so they don't deform. Once you prevent the deformation in your fixture, you are applying a compression. I have some calculations somewhere on how much force that really is... but in any case, there is a compression that is pretty close to the 300kgf mentioned in the datasheet. The enclosure or fixture should be rigid enough to work against the swelling of the cells - so not just anything will do. It all also depends if you want the max. number of potential cycles out of the cells.

 

[Off-Grid-Garage]

@Off-Grid-Garage Notice he says it's 'not needed' but you do need a fixation so they don't deform. Once you prevent the deformation in your fixture, you are applying a compression. I have some calculations somewhere on how much force that really is... but in any case, there is a compression that is pretty close to the 300kgf mentioned in the datasheet. The enclosure or fixture should be rigid enough to work against the swelling of the cells - so not just anything will do. It all also depends if you want the max. number of potential cycles out of the cells.

Yeah, good point... thanks.

 

[Solarfun4jim]

Just to throw another issue into the mix...the compression across the cell face is not evenly distributed, since the bulge is mainly at the positive anode end (imo). Thus a paralleled force(to the main cell wall) applied through end plates can apply more pressure to the bulge, than the less expanded cathode end. This is mitigated somewhat when putting the cells in series, since the positive and negative ends are alternated, but nevertheless, rigid spacers between each cell would apply disproportionate pressure across the cell face....imo. I have used a compressible foam (3mm) to help even out the applied pressure, including at the end cap surfaces.
What i would love to know, is what would be the ideal nm on a torque wrench, to tighten the nuts on the threaded rods, to give the required 17psi @ full charge. That way, you don't overdo the compression and even if it does fall below 11psi at maximum discharge, you will still have some compression, which is better than none.

 

[zorlig]

Just to throw another issue into the mix...the compression across the cell face is not evenly distributed, since the bulge is mainly at the positive anode end (imo). Thus a paralleled force(to the main cell wall) applied through end plates can apply more pressure to the bulge, than the less expanded cathode end. This is mitigated somewhat when putting the cells in series, since the positive and negative ends are alternated, but nevertheless, rigid spacers between each cell would apply disproportionate pressure across the cell face....imo. I have used a compressible foam (3mm) to help even out the applied pressure, including at the end cap surfaces.
What i would love to know, is what would be the ideal nm on a torque wrench, to tighten the nuts on the threaded rods, to give the required 17psi @ full charge. That way, you don't overdo the compression and even if it does fall below 11psi at maximum discharge, you will still have some compression, which is better than none.

View attachment 31401

I examined mine, I don't see this. Also 17psi is not the required pressure at full charge...

 

[Ampster]

since the bulge is mainly at the positive anode end (imo)

The internal construction of the cell is like a jelly roll. The connection to the terminals is done with tabs. Consequently the anode is evenly layered throughout the cell. The swelling of prismatics seems to appear in the middle of the cells in the cases I have observed

 

[Sillyputty]

I've read the corresponding threads and linked materials and don't recall concern for swelling on the narrower sides of each cell. Have any here observed, or is there any documentation from the manufacturer mentioning this? What I'm trying to narrow down is whether a fixture and/or force needs to be applied to the sides of the battery cells, in addition to the larger 'front' and 'back' surfaces.

 

[fafrd]

Just to throw another issue into the mix...the compression across the cell face is not evenly distributed, since the bulge is mainly at the positive anode end (imo). Thus a paralleled force(to the main cell wall) applied through end plates can apply more pressure to the bulge, than the less expanded cathode end. This is mitigated somewhat when putting the cells in series, since the positive and negative ends are alternated, but nevertheless, rigid spacers between each cell would apply disproportionate pressure across the cell face....imo. I have used a compressible foam (3mm) to help even out the applied pressure, including at the end cap surfaces.
What i would love to know, is what would be the ideal nm on a torque wrench, to tighten the nuts on the threaded rods, to give the required 17psi @ full charge. That way, you don't overdo the compression and even if it does fall below 11psi at maximum discharge, you will still have some compression, which is better than none.

View attachment 31401

Why are we back to talking about ‘overdoing’ the compression?

From the exchanges with EVE linked to earlier in this thread, it’s clear that clamping the cells to 300kgf (13psi or whatever) when they are near-empty (or at least not swelled) and then letting forces increase to well beyond that level when the cells are charged to full is fine (and delivers the intended cycle life).

At least when talking about forces counteracting swelling as the cells attempt to expand, there is no thing as ‘too much’ - enclosure in a rigid container with no ability to deform is fine.

What you don't want to do is crank down on already-swollen cells with an unbounded force. The idea is to keep the cells from swelling (mechanically deforming), not trying to force already-swollen cells back to their near-empty profile.

After reading what EVE themselves said, we are all making a much bigger deal out of this than we need to (largely because of EVE’s deficient documentation).

 

[Solarfun4jim]

I'm probably getting the wrong end of the stick...i will bow out gracefully.

 

[Bob B]

@Off-Grid-Garage ....This looks like information from a sales person VS the conversation others have had with an EVE engineer. Who you gonna give more weight? Also .... just because something isn't "necessary" doesn't mean doing it won't be beneficial.
It has been pretty well documented that optimum compression around 12psi will increase the cycle life of the cells.

 

[eranrund]

What i would love to know, is what would be the ideal nm on a torque wrench, to tighten the nuts on the threaded rods, to give the required 17psi @ full charge.

I too would love to know this.

 

[Hedges]

Here's a calculator to convert torque to compressive force.
Inputs are bolt diameter and coefficient of friction (which differs considerably lubricated from dry)
Follow the link it gives for your thread material.

Bolt Torque, Axial Clamp Force, Bolt Diameter Calculator

This fastener bolt torque calculator will Estimate the unknown torque, diameter, axial force applied and coefficient of turning friction for the given conditions.

www.engineersedge.com

 

[Dzl]

Apparently a compression is not necessary as per EVE but only a fixture (yes, I read the long 25page thread...)

@Off-Grid-Garage ....This looks like information from a sales person VS the conversation others have had with an EVE engineer. Who you gonna give more weight?

At EVE: at least 2 engineers + The Datasheet.
Add at least 2 more if we include Dacian (designer of the SBMS0, and the engineer(s) who wrote the A123 datasheet referenced in other threads.

I understand not wanting to go through the trouble of compression or going to the extent of calculating pressure and using springs (and to be clear no one--including the sources mentioned above--is saying you must, only that it can increase cycle life by a significant amount).

I think Dacian explains the general idea fairly well:

The cells will change volume slightly based on state of charge so compression is helpful for prismatic type cells. A spring will ensure a constant pressure but even a fixed compression should be better than none.

Springs are the easiest DIY solution

I happen to have proper spec for the A123 System cells that I use here [link is broken]
On page 32 you will find life cycle vs pressure level and there is a fairly large range 4psi up to 18psi you get about the same great results

All cells need compression unless they are cylindrical cells. Compression will improve cycle life by 2 to 3x so is worth doing.
Not sure about the exact compression needed for those cells but there is a fairly large range that is good

 

[fafrd]

At EVE: at least 2 engineers + The Datasheet.
Add at least 2 more if we include Dacian (designer of the SBMS0, and the engineer(s) who wrote the A123 datasheet referenced in other threads.

I understand not wanting to go through the trouble of compression or going to the extent of calculating pressure and using springs (and to be clear no one--including the sources mentioned above--is saying you must, only that it can increase cycle life by a significant amount).

I think Dacian explains the general idea fairly well:

After seeing how much individual cells swell when charged to 3.65V, I’ve decided to rig up a simple jig to clamp cells while charging.

In general, I believe it’s now understood that minimizing mechanical deformation increases cell life.

And I think it’s also pretty clear from a couple of the communications that EVE has provided that (within limits), there is no such thing as too much clamping force - if it’s clamped snug when near-empty, you don’t need to worry about too much pressure when the cell is charged to 100% (do really little reason to go to the trouble of springs).

The only question mark for me surrounds capacity and clamping force - if additional clamping force can reduce effective capacity, that’s a potential reason to back off or go to the trouble of springs.

I have a reference point for the capacity of my cells tested without any fixture and am now repeating the test with my jig in place - will report back if I see any meaningful reduction in capacity...

 

[rhino]

The only question mark for me surrounds capacity and clamping force - if additional clamping force can reduce effective capacity, that’s a potential reason to back off or go to the trouble of springs.

In one of the threads it was stated (from a technical source) that it was like a bell curve. The 13 psi was the optimum but if you have considerably less than that or considerably more than that then the cycle life is decreased. So too much pressure does negatively affect cycle life from the ideal compression amount.

 

[Hedges]

Checking torque/compression calculations and strength of threads isn't too difficult.
But if the swelling cells are an irresistible force and the bolts an immovable object, might strip the threads if too small diameter or to few of them.
Would be good to know how much force will occur.

Anybody with these cells have an Instron or similar?

5900 Series Universal Testing Systems up to 50 kN;

The 5940 single column and 5960 dual column universal testing systems are designed to perform tensile, compression, flexure/bend, peel, tear, shear, and cyclic tests, and range in capacities from 0.02 N up to 50 kN.

www.instron.us

 

[Dzl]

In one of the threads it was stated (from a technical source) that it was like a bell curve.

This chart shows that bell curve, symmetrical to a point, but too much pressure would damage the cell so there is a limit on the high end unlike the low end (or that was my takeaway). The above graph shows the curve for A123 cells, but the info given by the EVE engineers matched what is shown here for the most part.

The top line takeaway for me was: shoot for ~12psi, but if you can get in the ballpark of 6-17 you are in the right ballpark, and if you are going to err one way or the other, definitely err on the side of less pressure. Too much pressure = possible damage

And I think it’s also pretty clear from a couple of the communications that EVE has provided that (within limits), there is no such thing as too much clamping force - if it’s clamped snug when near-empty, you don’t need to worry about too much pressure when the cell is charged to 100% (do really little reason to go to the trouble of springs).

I thought it was pretty clear they were saying that limit was ~17 PSI (maybe we are saying the same thing differently?). To use the phrase ghostwriter used, 'above 17psi things start getting pretty crushy' ?