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Pack / Cell compression Optimized By Using Springs.

If each pack has independent plates on either end, you will get expansion/contraction on both ends even if the springs are only one end (so the center cell or two will be stationary with a small amount of expansion / contraction on either end of the pack).

With a common plate on one end, both packs will be forced to expand/contract by the same amount on that one end.

That’s probably going to be the case anyway and should be fine, but in the extreme case that one pack expands/contracts much more than the other (including the initial phase while the cells are settling over the first few cycles), the common plate will force the far plate and all the cells in between to move more than it would if it had an independent plate on both ends.

Again, should be fine, but what are you really gaining by having a single double-width plates versus two singles? (Cost, number of springs, etc… is all the same, right?).
My goal would be to build a full enclosure so the shared fixed side would also be screwed to the bottom where the cells rest and sides. I'd also make the shared fix plate and connected sides a bit taller with a groove to slide a plexiglass plate in and out.

Essentially I'd have a completely enclosed and connected box with the only exception being the two plates with 4 springs each staying free to compress and decompress as needed.
 
My goal would be to build a full enclosure so the shared fixed side would also be screwed to the bottom where the cells rest and sides. I'd also make the shared fix plate and connected sides a bit taller with a groove to slide a plexiglass plate in and out.

Essentially I'd have a completely enclosed and connected box with the only exception being the two plates with 4 springs each staying free to compress and decompress as needed.
Well first, with a design like that there will be no difference between a single fixed plate or a split fixed plate.

The potential issue with a fixed-end design is that you are forcing all of the expansion to translate through the full pack.

Rather than having the central 1-2 cells fixed and the remaining cells expand / contract towards either end, it’s the last cell that will be fixed in your design and the first cell will move ~twice as much as it would in a double-floating-plate design.

On the other hand, if you made a single long fixture will all cells in a single row and with two floating plates, the movement you’d have in that double-floating plate design is identical to what you get on your double-row single-floating-plate design.

So I think you’ll be fine (I’m using 16-cells in a single-row double-floating-plate design and the movement of the pack through the settling period has been fine).
 
That makes sense. My main challenge is space requirements as it’s going to be in a campervan so trying to minimize footprint for a specific build and it makes the most sense for the rest of the van build to have it closer to square rather than longer but narrower.

Alternatively I could stick the original plan and have them be completely separate and non fixed with springs still only on one side. But just build a box / carrier for it so both ends are free too move.

Would that solve the issue?
 
That makes sense. My main challenge is space requirements as it’s going to be in a campervan so trying to minimize footprint for a specific build and it makes the most sense for the rest of the van build to have it closer to square rather than longer but narrower.

Alternatively I could stick the original plan and have them be completely separate and non fixed with springs still only on one side. But just build a box / carrier for it so both ends are free too move.

Would that solve the issue?
You would have the biggest movement occurring along the side with springs, with the last cell (against the "fixed" side) moving much less. You could even anchor the non-sprung side of your pack to your outer box.
 
That makes sense. My main challenge is space requirements as it’s going to be in a campervan so trying to minimize footprint for a specific build and it makes the most sense for the rest of the van build to have it closer to square rather than longer but narrower.

Alternatively I could stick the original plan and have them be completely separate and non fixed with springs still only on one side. But just build a box / carrier for it so both ends are free too move.

Would that solve the issue?
My friend, the only sense it made was it went in a circle. You dont have a problem regardless of how you approach, either is fine. Having one solid side will prevent the two batteries from crashing into each other. I would probably go that way.
 
My friend, the only sense it made was it went in a circle. You dont have a problem regardless of how you approach, either is fine. Having one solid side will prevent the two batteries from crashing into each other. I would probably go that way.
Thanks I gathered that although it took me a moment. Lots of learning diving into this project. I will post pics of my build once its complete.
 
One thing that confuses me about the whole 'springs' usage for compression is that I thought the point of putting these cells under 12 psi compression was to keep them from expanding which harms the cells? If they don't expand at 12 psi pressure, then the springs should not compress at all, otherwise, if the springs compress then the batteries are expanding and if they are expanding, then what is the point of putting them under compression? The batteries would be damaged from swelling. The only point I can see for springs is that if the cells are having a catastrophic failure for some reason (over charging, too hot, too high of a C-rate discharge, physical damage to the cell, etc. ) then the springs would allow the fixture setup to expand beyond the rated compression to keep the pack from exploding by pressure build up (like in a non-spring fixture setup). The cells would be ruined but the fixture would not explode. Please check my logic on this and tell me what I am missing.
 
Just thought I'd share my implementation of a spring based fixture. I have separated each cell with a thin sheet of silicone. I like that material because it is heat resistant, provides some chafe protection, electrical insulation, and it "clings" to the cells with no adhesive, so it is easy to work with.

IMG_0121 2.jpegIMG_0016 2.jpegIMG_0014.jpegIMG_0041.jpegIMG_0018.jpeg
 
One thing that confuses me about the whole 'springs' usage for compression is that I thought the point of putting these cells under 12 psi compression was to keep them from expanding which harms the cells? If they don't expand at 12 psi pressure, then the springs should not compress at all, otherwise, if the springs compress then the batteries are expanding and if they are expanding, then what is the point of putting them under compression? The batteries would be damaged from swelling. The only point I can see for springs is that if the cells are having a catastrophic failure for some reason (over charging, too hot, too high of a C-rate discharge, physical damage to the cell, etc. ) then the springs would allow the fixture setup to expand beyond the rated compression to keep the pack from exploding by pressure build up (like in a non-spring fixture setup). The cells would be ruined but the fixture would not explode. Please check my logic on this and tell me what I am missing.
I would suggest reading this thread from the beginning.

The basis for compression to 12 PSI is from EVE's spec sheet where they indicate significantly improved cycle life when the cells are compressed .... but within a specified range.

The springs will have a constant pressure on the cells .... and the cells do still expand and contract somewhat even under compression. The springs tend to keep the pressure more constant instead of the unknown pressure of a rigid fixture.
 
Those look interesting .... the pressure rating is a little odd though. They rate at 125 PSI closed and 200 open. Did you do any measurements to determine for sure what the pressure was?
Its not odd for the application. Its a valve spring. The only thing that matters is pressure when the valve is open (compressed) and pressure when the valve is closed (still compressed but not as much as when the valve is open).
 
am waiting to buy any specific spring until my homework is done dividing between all the spots they’ll go and stuff and spring rate and such.

have secured four 200kg rated S type load cells which will go on the middle of each of the four M12 threaded rods to compress my pack of 302Ah CATL cells.

by combining end plates, threaded rod, load cell per threaded rod, and Springs, i hope to have something that will allow me a good chance of maintaining the equivalent of 12psi across each end face.

1634937621289.png

work in progress…
 
EVE dont mess with springs, initial compressive force in the jig and then weld the sides to the end plates.


Mike
 
thank you for this insight?

still kind of waffling on the springs. maybe omit them for now and look at the equivalent pressure as seen by the load cells. if it doesn’t exceed 15 psi equivalent force then maybe keep it that way. as long as the psi equivalent does not drop below 10 psi equivalent cycle to cycle.

with pre compression and some skim neoprene between the cells i am hoping it might not need the springs.

only way to figure out is make it :) will try to get on that..

cheers
 
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