Compress or not, flexible busbar or not

[Cheap 4-life]

If it is acceptable for 1 cell it is acceptable for 2, 3 or 20.

It might be acceptable for one cell in a testing rig but most likely not for a long term setup and more so if many cells are put together..

 

[Cheap 4-life]

Have any EV packs used springs?
If no, then it is a non issue

No they do not and as I said before if I had welded busbars like most EVs and good server rack packs then I would not be concerned as much about using springs. Simple compression methods that they use like straps and maybe a thin wall case would be enough because the busbars can’t loosen due to the welded busbars and cause bms issues which could cause cell issues.. I instead will use springs so the pressure/compression does not get to high during high SOC and also flexible busbars since welded busbars are not an option. Also a lot of EVs use pouch cells that sometimes each cell has its own case for compression but that’s a different discussion

 

[Cheap 4-life]

They say they still had at least 80% capacity after 4,000 cycles at 0.5C.

Right that’s a short duration test without years of aging and years of possible over compression. I’ll stick with 17psi tops and anything over is worse than not compressing at all. That info doesn’t just go poof when additional data comes out.. nowhere does it say to no longer go by 17psi or 18psi or whatever. they are simply showing how they tested one cell. It’s not a guide for DIYers building their large battery packs

 

[Cheap 4-life]

How many cells do you suppose they tested in a row before they wrote the older datasheet?

Idk I didn’t ask. How is that relevant

 

[justgary]

It might be acceptable for one cell in a testing rig but most likely not for a long term setup and more so if many cells are put together..

Please go to your local community college and take a physics class.

 

[Cheap 4-life]

Please go to your local community college and take a physics class.

Where on that data sheet does it say it’s ok to have multiple cells or even one cell in a fixed rigid rig like that for the long term.. are you simply forgetting the older information about not exceeding 17psi.. it doesn’t go away because you think it is implied in a new data sheet.. and please stop with your snarky remarks. They are not helping anything

 

[justgary]

I’ll stick with 17psi tops and anything over is worse than not compressing at all.

That statement, if I recall correctly, came from @ghostwriter66 , who admitted that her technical Chinese may not have been quite as good as that used by the people she talked to at EVE. Perhaps she has learned more about it. I am curious how the cells are doing at her business since they apparently buy a lot of them.

A few posts back I noted that under rigid compression and according to the new datasheet, the normal compression would be between 12 and 19 PSI in a rigid frame. If normal use exerts only 19 PSI or so, what is the issue?

 

[Cheap 4-life]

Again, the new EVE datasheet implies that this is no longer a concern. You don't seem to agree, but that's fine with me.

Implications are objective.. as I said before just because one cell is tested that way does not imply that over compression isn’t a concern..

 

[justgary]

please stop with your snarky remarks. They are not helping anything

I'm sorry, I didn't mean to sound snarky. I was trying to be helpful. A physics class might help you. Honest.

 

[Cheap 4-life]

Please point to the part of the datasheet that says the cells will be "overcompressed."

Please point out the part of the data sheet that says the cells won’t be overcompressed and that it’s fine to do this fixed/rigid compression long term..

 

[Cheap 4-life]

But in the real world it does. I just showed you how much it will stretch

Threaded rod might stretch but not by a noticeable amount from the amount of pressure these cells will exert under normal operation. The cells would be over 17psi before any noticeable threaded rod stretching would occur

 

[Cheap 4-life]

Again, please post the part of the datasheet that discusses overcompression.

Again this data sheet is how they tested one of the cells.. not how it should be done long term.. please post the part of the data sheet that says the max amount this fixed rigid rig compresses these cells is perfectly fine long term (not 2 years)? And also show me where they say to no longer go by 17psi for our own DIY packs

 

[Cheap 4-life]

That statement, if I recall correctly, came from @ghostwriter66 , who admitted that her technical Chinese may not have been quite as good as that used by the people she talked to at EVE. Perhaps she has learned more about it. I am curious how the cells are doing at her business since they apparently buy a lot of them.

A few posts back I noted that under rigid compression and according to the new datasheet, the normal compression would be between 12 and 19 PSI in a rigid frame. If normal use exerts only 19 PSI or so, what is the issue?

My understanding is that anything over 17psi (not 19psi) is worse than not compressing at all.. I’ll have to relook to find where it says in a rigid/fixed frame it will not be possible for there to be over 19psi, however that’s still to much.. as I said this new data sheet showing rigid/fixed testing does not make the 17psi go away.... it’s just how they tested a cell

 

[42OhmsPA]

I agree the cutting board is to weak, but at least he has some compression I guess if some compression was his goal

I find it curious that you are worried about springs, foam, and whether steel rod will stretch under force, yet you plan to use HDPE cutting boards as end caps. The datasheet says 8mm steel or aluminum. I cheated that one by using 1/4" aluminum, but then again I built to the old datasheet that said to keep the compression constant. I also used aluminum bar for the sides rather than threaded rods.

The datasheet no longer concerns itself with maximum compression.

I've had doubts about the HDPE, it is much weaker than aluminum. https://www.makeitfrom.com/compare/...6061-Aluminum/High-Density-Polyethylene-HDPE/
My thought process was it still has enough strength to displace pressure and it's non conductive, it's the best of both worlds between foam and aluminum ?.
It's quite strong when cut into small sections.
I just wanted some type of fixture to hold the cells so I can get everything up and running. I figure by the time I'm ready to order 32 more cells I'll have learned enough to remake fixtures for all the packs. I'm confident it will be fine to handle 8mm of expansion.
I may order some force pads or other measuring device to try and understand what type of pressure the cells really see.
The monster solid buss bars have me more concerned than anything.

 

[Cheap 4-life]

My friend, your not understanding the linear relationship between 1 cell and the length of the restraint and 19 cells and the length of the restraint for those 19 cells.
Look at the image below really carefully. The cell is clamped between 2 plates and there are 6 rods holding the plates. Those rods are the same length as the width of the cell. Either the rods are stretching or not. If they are not stretching then the answer is that it is irrelevant and you can stack as many as you want to and everything is fine. If they are in fact stretching then it is still irrelevant (as long as you use the same material for the rods) because when you add a second cell, you add length to the rod and exactly double the amount of stretch to match exactly double the cell expansion. Its all relative.

At the end of the day, put springs, dont put springs. Do what you want to but based on the information below, your extrapolation that you can do it with 1 cell but not with 2 or more is incorrect.
View attachment 112721

Bud, I think you are also making implications without facts or proper reasoning to back it up.. is isn’t important whether the rods stretch or not. It is important to know if there’s is to much compression on the cell at any point.. It is simply a way they tested a cell. That data sheet does not say that the max amount of compression that single cell experienced is ok to do to it long term. This new data sheet does not make the 17psi max (or its better to not compress at all) simply vanish.. again this is simply showing how they tested one cell short term...
Does threaded rod and rigid end plates stretch enough to keep the max pressure below 17psi??? I don’t know but why take the chance when springs can be used to make sure 17psi isn’t exceeded

 

[Ellcon123]

This is going nowhere.

 

[justgary]

I don’t know but why take the chance when springs can be used to make sure 17psi isn’t exceeded

Let's back up a bit if you will. Please remind me where the 17 PSI spec came from. You keep quoting it, but honestly I don't remember where it came from.

 

[Cheap 4-life]

Compression is now apparently clarified in the datasheets, so it may be best to not cling to the information in older datasheets. They now mention a compression force range, which (for 230 Ah cells anyway) happens to equate to about 3kN (11.7 PSI) to 5kN (19.6 PSI). The implication is that this compression is the normal range of a cell taken from 30% SOC at 11.7 PSI to 100% SOC in a hard fixture (no springs or foam). Under normal conditions, the force should not exceed 7kN (27.5 PSI). It can exceed that if internal damage or cell leakage occurs.

In other words, when they said 11.7 to 18 PSI before, it seems they didn't mean don't let your compression exceed 18 PSI, they meant that if you clamp the thing rigidly the internal force will not exceed 18 PSI. This is what the new datasheet seems to say (well, now 19.6 PSI). It doesn't matter if they clamped one cell or one hundred in a stack. The force inside will be the same.

I built my batteries using the older information, and I am happy with the results and the foam so I don't plan to change anything.

If it is ok with you I’d like to discuss these implications further because maybe I missed something..
Where are you seeing 3kN to 5kN is a normal range that we can apply to our DIY builds.. All I see is 3kN at 30% SOC.. and how does it imply that amount of compression is ok/normal for the cell long term. I see this data sheet as simply being a way that they tested this cell, not that this is how we should setup our big battery packs.. and yes under normal conditions should not exceed 27.5psi or damage will occur is the implication I get out of it..

 

[42OhmsPA]

Kgf / what unit?

 

[Cheap 4-life]

It doesn't matter if they clamped one cell or one hundred in a stack. The force inside will be the same.

Try to hold two steel plates on each side of one cell and keep it from expanding thru an entire SOC (if that was possible) and now try to do the same thing with 8 cells and you tell me which one requires more pressure/compression to do so. You might get both sides of one cell to stay flat but if you tried to do that with 8 cells you might get the outside cells outside walls flat but not flatten the interior cells at all because it would require more pressure because more cell walls and cell expansion is causing more pressure needed to achieve the same flatness that is seen when only one cell was squeezed.. I’m not saying that if it took12psi to keep one cell flat that it will take 96psi to keep 8 cells flat, but I’d think it would still take more than 12psi to keep 8 cells flat if it only took 12psi to keep one flat. How much more idk.. Meaning that in a rigid fixed case the endplates would see more pressure/psi with 8 cells at 100% SOC than 1 cell at 100% SOC.
I mean when you compressed 4 cells with your clamp for your foam rig, was it easier than when you compressed 8cells with the same clamp?