Yeah, this is pretty much what I was planning (using 1/2” black ABS plastic for the ends).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, good point... thanks.@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.
I examined mine, I don't see this. Also 17psi is not the required pressure at full charge...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
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 observedsince the bulge is mainly at the positive anode end (imo)
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 too would love to know this.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.
Apparently a compression is not necessary as per EVE but only a fixture (yes, I read the long 25page thread...)
At EVE: at least 2 engineers + The Datasheet.@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?
Dacian said: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 usehere[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
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.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:
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.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.
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' ?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).