The secret to compression

Gazoo

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I feel that it is most effective to simply tighten the frame so the cells are firm to each other.
I agree but at what SOC? I have mine lightly compressed with a full SOC. When I discharge the cells the frame (plywood and threaded rods) is loose. Not concerned and not going to get into the weeds here. People using cells in mobile environments need to take this into consideration.

I will add too much compression can reduce cycle life according to EVE. I have said many times the cells will most likely age before capacity reaches 80% due to cycling. IMO, keeping the cells between the knees is the best and easiest way to increase cycle life.
 

Short_Shot

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I thought the point of a hydraulic controlled mechanism was to keep constant pressure as the cells expand. It keeps a constant 11 psi just like a piece of foam would. Using a rigid system torqued to 11 psi will lead to more pressure as the cells try to expand.

I'm not a mechanical engineer but I think that 11psi is 11 psi
Mech E student here. Can confirm 11 psi is 11 psi. Perhaps we can get a PE to confirm my confirmation.


And that your statement about increasing pressure is also correct, but you need to factor in the existing compression limiting expansion as well.

So there is a condition where you have a sort of diminishing returns. As they are already under pressure, expansion is reduced. Further expansion results in increased pressure but it won't expand as much as it would without any at all.

So the system limits itself somewhat.

Since the cell will expand more in the middle it will also lead to increased pressure in that area which should further limit movement while allowing the pressure to be distributed a bit more evenly to the outside.

This may or may not result in excess pressure in the center area.

As this foams properties are known, one could simply measure the end result and estimate the approximate final compression force with a full charge. A tiny hole in the foam and end plate could allow a measurement to be made at full charge if it really needed to be tested.


Personally I'm just going to slap it in some foam, get the right measurement, and send it.

If I get 25% more life out of it then great. If I get 75% of the "ideal" increase then even better. I won't cry if I don't maximize it.
 

bobdelso

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Mech E student here. Can confirm 11 psi is 11 psi. Perhaps we can get a PE to confirm my confirmation.


And that your statement about increasing pressure is also correct, but you need to factor in the existing compression limiting expansion as well.

So there is a condition where you have a sort of diminishing returns. As they are already under pressure, expansion is reduced. Further expansion results in increased pressure but it won't expand as much as it would without any at all.

So the system limits itself somewhat.

Since the cell will expand more in the middle it will also lead to increased pressure in that area which should further limit movement while allowing the pressure to be distributed a bit more evenly to the outside.

This may or may not result in excess pressure in the center area.

As this foams properties are known, one could simply measure the end result and estimate the approximate final compression force with a full charge. A tiny hole in the foam and end plate could allow a measurement to be made at full charge if it really needed to be tested.


Personally I'm just going to slap it in some foam, get the right measurement, and send it.

If I get 25% more life out of it then great. If I get 75% of the "ideal" increase then even better. I won't cry if I don't maximize it.
Thats what I'm thinking of doing too.

Over compression is bad. So even if the foam gives and doesn't provide a solid wall, it will provide a scaleable PSI. EG it starts are 11 @ 50% charge. Gets up to 18 psi at full charge and drops down to a few psi when discharged. Seems good enough
 

bobdelso

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Do you have a source that states cycle life can be increased to 6000 - 14000 cycles if using compression? The only source I have is the spec sheet for the newer EVE LF280K which states cycle life can be increased to 6000 cycles if using perfect compression. EVE does have a hydraulic jig for testing compression.
There was an eve testing graph that showed you could get up to 12-14k cycles with around 11-13 psi. I don't have the link handy but it was posted in one of the compression mega threads
 

jmole

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Mech E student here. Can confirm 11 psi is 11 psi. Perhaps we can get a PE to confirm my confirmation.
I wish I had access to some of the high resolution 3D scanners I had before the pandemic started. Take one of those to the side of an EVE cell and you’ll see that the “flat” sides are really very hilly when you look microscopically.

When discharged, the highest ridges are probably at the edges of the cell, meaning the center is concave-ish. When charged, you’ll have the characteristic swelling of a lithium cell.

clamp a cell between two hard, flat surfaces, ok - *maybe* you can get a reasonable pressure distribution that way. Definitely not as consistent as you could with a compression pad.

but clamp multiple cells together under that same 11 psi and you have a very different pressure profile between cells - one that imparts high pressure (>11 psi) at the cell edges when discharged and high pressure at the center when charged (again, larger than the 11psi design value).

adding a pad between cells with the right composition can ensure that you have that 11psi of pressure across the entire face of the cell.
 

Short_Shot

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You can measure that with a surface plate and indicator fairly easily without any fancy 3d tech.

You'll probably need to fixture the cell though.
 

Bob B

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Yes, it makes sense, but doesn't change my view. Foam allows the cell to expand slightly, and internal components to move. That movement (I believe) is what leads to shortening cell life.
My post was actually in support of rigid since that is what the manufacturer is using.
 

Bob B

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I thought the point of a hydraulic controlled mechanism was to keep constant pressure as the cells expand. It keeps a constant 11 psi just like a piece of foam would. Using a rigid system torqued to 11 psi will lead to more pressure as the cells try to expand.

I'm not a mechanical engineer but I think that 11psi is 11 psi
My point was that utilizing a constant pressure with a rigid mechanism like EVE uses .... is quite different than exerting that pressure evenly over the face of the cell like foam does.
EVE sets the spec as certain KG of force .... we are the ones that translated that to 11 or 12 PSI. It's actually not the same thing, but gives us a basis for doing our calculations.
 

Bob B

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I wish I had access to some of the high resolution 3D scanners I had before the pandemic started. Take one of those to the side of an EVE cell and you’ll see that the “flat” sides are really very hilly when you look microscopically.

When discharged, the highest ridges are probably at the edges of the cell, meaning the center is concave-ish. When charged, you’ll have the characteristic swelling of a lithium cell.

clamp a cell between two hard, flat surfaces, ok - *maybe* you can get a reasonable pressure distribution that way. Definitely not as consistent as you could with a compression pad.

but clamp multiple cells together under that same 11 psi and you have a very different pressure profile between cells - one that imparts high pressure (>11 psi) at the cell edges when discharged and high pressure at the center when charged (again, larger than the 11psi design value).

adding a pad between cells with the right composition can ensure that you have that 11psi of pressure across the entire face of the cell.

I agree that the foam would even out the pressure across the face of the cell better than a rigid plate ..... What I think is being missed is that ..... this is NOT what EVE does in their testing.
Maybe even pressure across the face of the cell is not as good a rigid frame at a controlled pressure like EVE uses. We don't really know ... and likely there is no way for us to know.
I am just postulating that trying to emulate the way EVE does it during their testing is probably the best way.

Edit ... maybe we could use the foam to control the PSI ... and put a rigid plate between the foam and the cell?
 

jmole

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I agree that the foam would even out the pressure across the face of the cell better than a rigid plate ..... What I think is being missed is that ..... this is NOT what EVE does in their testing.
Maybe even pressure across the face of the cell is not as good a rigid frame at a controlled pressure like EVE uses. We don't really know ... and likely there is no way for us to know.
I am just postulating that trying to emulate the way EVE does it during their testing is probably the best way.

Edit ... maybe we could use the foam to control the PSI ... and put a rigid plate between the foam and the cell?
Characterization is not the same thing as application. The issue is that prismatic and pouch cells can't self-compress like cylindrical cells do (I.e. when they expand, all that energy is spread equally around the circumference, like an arch.). what we're doing is supporting the long side of the cells with pressure to avoid premature aging.

My take on this is that resilient pads are the right way to do it, but maybe I'm biased because mine is going into an RV, right below where me and my family sleep, and safety is my #1 concern.
 

bobdelso

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I agree that the foam would even out the pressure across the face of the cell better than a rigid plate ..... What I think is being missed is that ..... this is NOT what EVE does in their testing.
Maybe even pressure across the face of the cell is not as good a rigid frame at a controlled pressure like EVE uses. We don't really know ... and likely there is no way for us to know.
I am just postulating that trying to emulate the way EVE does it during their testing is probably the best way.

Edit ... maybe we could use the foam to control the PSI ... and put a rigid plate between the foam and the cell?
Doesn't EVE test 1 cell with this machine?

I would think that 1 cell in a hydraulic machine with a flat clamping face would have a pretty consistent pressure across the face. Or at least along the parts that are in contact.

I remember the report saying over compression is worse than no compression.

If you put 4 cells in a row and clamp them with 2 boards, the outside ones get the most even pressure. And the inside ones only get pressure where the cells contact. Without a board pressing on the cells, they have room to expand into the gaps of other cells.

I don't see how applying a consistent pressure along the face would be a bad thing
 

Bob B

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I don't see how applying a consistent pressure along the face would be a bad thing
I didn't say it was a bad thing .... I said it is an unknown since this is not the way EVE does their testing and we don't have any other evidence that compression improves cycle life other than their spec sheet.

It is curious if cells in series will get the same benefit ... maybe those cells really should have a thin piece of hardboard between them.
 

Zwy

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I agree that the foam would even out the pressure across the face of the cell better than a rigid plate ..... What I think is being missed is that ..... this is NOT what EVE does in their testing.
Maybe even pressure across the face of the cell is not as good a rigid frame at a controlled pressure like EVE uses. We don't really know ... and likely there is no way for us to know.
I am just postulating that trying to emulate the way EVE does it during their testing is probably the best way.

Edit ... maybe we could use the foam to control the PSI ... and put a rigid plate between the foam and the cell?
I just received 40 brand new EVE cells yesterday, these are perfectly flat.

And yes, I will be using the same foam as I did previously on these packs. I'll get back to you in a few years if I ever tear them down and let you know if the foam didn't work. :)

I explained in detail how I did my original pack. All compression was figured under full charge. I measured the base length of the pack before and after fully charging the cells when top balancing (I did have these slightly clamped with a bar clamp and plywood). I saw no relative difference. I guess I could have if I found a micrometer long enough and measured the pack. Then it was simply matter of bolting together with the foam and tightening it all down after I set my base plate and rods stops to the correct length.

I have yet to see any loose cells after discharge. Now if you are building a 16S pack with all the cells in one straight line, you just might see a difference of less than 0.03125" in the total length from full charge to discharge, I'll wager even less.
 

Zwy

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I recall some older threads where some went to pretty great lengths to measure how much the cells actually expanded during charge and discharge ..... will have to see if I can find them.

Here is a very short one that is more recent. https://diysolarforum.com/threads/eve-280s-expansion-per-cell.17487/
Good info, if that was accurate. On a 4 cell line, that amounts to 2mm or less. 8 cell line (48V for example) then 4mm or less. And if the cells were initially charged with some compression, it appears according to that information that it will be less. Also, reading that post about the 4 cells in compression and 4 not in compression, the 4 in compression did not expand.
 

Bob B

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Good info, if that was accurate. On a 4 cell line, that amounts to 2mm or less. 8 cell line (48V for example) then 4mm or less. And if the cells were initially charged with some compression, it appears according to that information that it will be less. Also, reading that post about the 4 cells in compression and 4 not in compression, the 4 in compression did not expand.
HMMM ... that doesn't fit with my memory of the discussion ..... Cinergi did use compression on his pack and I'm pretty sure he measured a couple mm of expansion even under compression .... will have to find that those old posts.

In that post, he just indicated they stayed square and tight under compression .... not that they didn't expand.
 

Bob B

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I was looking for a post that summed it all up, but didn't locate it.

Cinergi did a LOT of experimentation during his build .. and part of that was studying the effects of compression.
Just going from memory, I think he measured about 2 MM of movement even under 300 KG of compression. I think he had more movement than that during the initial testing and it decreased to 2 MM after a few cycles.

He had a LOT more movement in the cells that weren't compressed.

Having the cells compressed during the initial top balance is probably an important thing most of us are overlooking.

That part of the discussion starts around page 15 in this thread .... https://diysolarforum.com/threads/cinergis-28-kwh-4-kw-solar-10-kw-inverter-rv-build.13786/page-15
 
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Zwy

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Yes EVE cells will expand even under compression. I measured this on my pack.
That means that simply putting cells together without any foam in between could cause excessive pressure on the cells. Makes the case for using foam that much more important.
 

Turponieminen

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That means that simply putting cells together without any foam in between could cause excessive pressure on the cells. Makes the case for using foam that much more important

Or use threaded rod and springs at the end. Much more cheap than some specialty foam.
 
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