The secret to compression

RobertGreen

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jmole, THANK YOU for this extremely helpful tip! I'm assembling a few packs soon and I have been pondering about the best way to deal with the compression question. This seems like it's just the thing!
 

~Yuri~

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My personal opinion is that I don't think this method is effective. It looks like a desire to do something, but the effectiveness of such an action tends to zero. Especially for longer periods of time, a year/two/three...
Foam cannot provide stability.
 

Diysolar123

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My personal opinion is that I don't think this method is effective. It looks like a desire to do something, but the effectiveness of such an action tends to zero. Especially for longer periods of time, a year/two/three...
Foam cannot provide stability.
foam can, if, and this is the really important part...it is an engineered product that is designed to stay in its elastic state under the designed load without transitioning into its plastic state(where it deforms and does not recover).

It is interesting that poron evextend 4701-43 is designed to be "two-sided" so its design purpose is between the battery and vehicle not between cells(maybe??):
"These materials are firmer at the front, to maximize their impact-absorbing effects in the direction most impacts come from. Then, the back is softer. The back expands slowly to precisely cushion the battery. "

(update: I looked at their battery pad application note which talks about poron polyurethane battery pad technology between cells, which I guess is the same as evextend 4701 but there is a 4701-43HBF and a 4701-43RL and they have different specs so be careful)


as to lifespan, well..."PORON EVExtend® is a durable polymer. It will last a long time." sound a bit like marketing speech to me as I am not familiar with the temporal units of measure associated with "long time" hehe
 
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wholybee

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I don't think foam is a good idea. The problem is that if you compress foam to 25%, the cells can still expand and bulge, and the whole point of compression is to prevent that.
Consider, the 11psi figure is given at a specific state of charge. As the cell charges and attempts to expand, that figure will naturally go up, as the compression frame should not have any give within it. And at full discharge, the figure should go down. The mechanisms that lead to expansion I don't think are that simple, but that is the general idea.
It (IMHO) isn't the 11psi that is important. What is important is that you apply enough pressure to stabilize the aluminum case, so that the case cannot distort as internal pressure in the case rises. But not to apply too much initial pressure to distort the case.
Efforts to maintain exactly 11psi, with foam, springs, or whatever, defeat the whole purpose of compression, as they allow the cases to distort as internal pressure rises.
I feel that it is most effective to simply tighten the frame so the cells are firm to each other.
 

Q-Dog

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Most prebuilt packs are sold with a 2000 cycle life after which it will have 80% capacity. Without compression you can get 2k cycles. With compression you can get 6k-14k
Allegedly. ;) It's going to be a few years before this is proven.
 

Turponieminen

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Most prebuilt packs are sold with a 2000 cycle life after which it will have 80% capacity. Without compression you can get 2k cycles. With compression you can get 6k-14k

Which test/data you are referring to? I have seen a datasheet that shows 2500 cycles uncompressed vs 3500 compressed. And even then, capacity curve showed faster degradation to over 1000 cycles with compression vs uncompressed.
 

Bubba1

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How much does it cost. Their website doesn't have an obvious way to order. Fast shipping?
 

Turponieminen

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Yep, your post from the other thread nails it here:


I physically cringe when I see that people are just compressing raw cells together with no foam or anything. You can’t stop the swell, or contain it with a hard flat surface. If you try, your cells are going to rupture.

Even with the perfect compression spring setup, the center of your cells will bear 95% of the force if you don’t have foam between cells to spread it out. Do the math and it’s like 100+ psi, in the same place, cycle after cycle. Recipe for disaster.

I don't think cells rupture, if they are assembled with hard surface against them or in between them. Many have done exactly that and i have never heard of an example of cells rupturing. Also cylindrical cells do not need compression due to their shape. They can not swell much and yet they won't rupture

Also foam does not remove the fact that cells swell mostly from middle, foam does not spread the force, it only allows cells to swell from middle.
Now i don't know if there is a sweetspot of how much swelling vs no swelling. That is again another matter...🙃🙂
 

Bob B

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I don't think foam is a good idea. The problem is that if you compress foam to 25%, the cells can still expand and bulge, and the whole point of compression is to prevent that.
Consider, the 11psi figure is given at a specific state of charge. As the cell charges and attempts to expand, that figure will naturally go up, as the compression frame should not have any give within it. And at full discharge, the figure should go down. The mechanisms that lead to expansion I don't think are that simple, but that is the general idea.
It (IMHO) isn't the 11psi that is important. What is important is that you apply enough pressure to stabilize the aluminum case, so that the case cannot distort as internal pressure in the case rises. But not to apply too much initial pressure to distort the case.
Efforts to maintain exactly 11psi, with foam, springs, or whatever, defeat the whole purpose of compression, as they allow the cases to distort as internal pressure rises.
I feel that it is most effective to simply tighten the frame so the cells are firm to each other.

I'm pretty sure that EVE's test mechanism is a hydraulic controlled mechanism that maintains a constant pressure ..... Their spec for compression is what most of the discussion stems from.

I'm also pretty sure this test mechanism is rigid .... I'm not 100% sure how the introduction of a flexible membrane is going to affect things.
With a rigid mechanism exerting X amount of force .... that force could be greater in one area than another as SOC and temperature varies and maybe the middle of the cell has more pressure than the outside once the cells try to expand ... just thinking out loud here, but foam that compresses at X PSI will not necessarily exert the same force on a cell a rigid frame.

Did that make any sense?
 

jmole

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I'm pretty sure that EVE's test mechanism is a hydraulic controlled mechanism that maintains a constant pressure ..... Their spec for compression is what most of the discussion stems from.

I'm also pretty sure this test mechanism is rigid .... I'm not 100% sure how the introduction of a flexible membrane is going to affect things.
With a rigid mechanism exerting X amount of force .... that force could be greater in one area than another as SOC and temperature varies and maybe the middle of the cell has more pressure than the outside once the cells try to expand ... just thinking out loud here, but foam that compresses at X PSI will not necessarily exert the same force on a cell a rigid frame.

Did that make any sense?
I don't think cells rupture, if they are assembled with hard surface against them or in between them. Many have done exactly that and i have never heard of an example of cells rupturing. Also cylindrical cells do not need compression due to their shape. They can not swell much and yet they won't rupture

Also foam does not remove the fact that cells swell mostly from middle, foam does not spread the force, it only allows cells to swell from middle.
Now i don't know if there is a sweetspot of how much swelling vs no swelling. That is again another matter...🙃🙂
I don't think foam is a good idea. The problem is that if you compress foam to 25%, the cells can still expand and bulge, and the whole point of compression is to prevent that.
Consider, the 11psi figure is given at a specific state of charge. As the cell charges and attempts to expand, that figure will naturally go up, as the compression frame should not have any give within it. And at full discharge, the figure should go down. The mechanisms that lead to expansion I don't think are that simple, but that is the general idea.
It (IMHO) isn't the 11psi that is important. What is important is that you apply enough pressure to stabilize the aluminum case, so that the case cannot distort as internal pressure in the case rises. But not to apply too much initial pressure to distort the case.
Efforts to maintain exactly 11psi, with foam, springs, or whatever, defeat the whole purpose of compression, as they allow the cases to distort as internal pressure rises.
I feel that it is most effective to simply tighten the frame so the cells are firm to each other.
My personal opinion is that I don't think this method is effective. It looks like a desire to do something, but the effectiveness of such an action tends to zero. Especially for longer periods of time, a year/two/three...
Foam cannot provide stability.

IMO, if you're going to compress the cells, you want to have foam in between them. All these fancy spring rigs would be much safer with foam compression pads.

When in doubt, look at what the pros are doing:


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1628657658189.png
 

wholybee

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Most of those links seem to be referencing pouch cells, or very thin prismatic cells that are almost like sheets. Isn't the point of compression to prevent movement of the internal components? No other purpose makes any sense. We are not applying enough compression to distort the aluminum case, so compression, other than preventing expansion and movement, doesn't do anything that will affect the cell inside it. And the cell spiral or plates are already compressed to fit in the case.
With that in mind, a rigid structure is most effective.
 

wholybee

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I'm pretty sure that EVE's test mechanism is a hydraulic controlled mechanism that maintains a constant pressure ..... Their spec for compression is what most of the discussion stems from.

I'm also pretty sure this test mechanism is rigid .... I'm not 100% sure how the introduction of a flexible membrane is going to affect things.
With a rigid mechanism exerting X amount of force .... that force could be greater in one area than another as SOC and temperature varies and maybe the middle of the cell has more pressure than the outside once the cells try to expand ... just thinking out loud here, but foam that compresses at X PSI will not necessarily exert the same force on a cell a rigid frame.

Did that make any sense?
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.
 

Mcgivor

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Jul 24, 2021
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Without having an opinion one way or another, my research revealed two related takes on the subject. First being, it's better to use smaller cells, 100Ah or less, in parallel to create cell blocks, rather than using large capacity individual cells, due to structural integrity. Second being clamping or binding is recommended only in mobile applications to prevent movement inherent to the application.

Personally I wonder how much the average operator understands the immense pressure the common threaded rod can exert, a simple screw jack can lift a car without much effort. Many cells arrive slightly swollen, is this nessersary a bad sign? Or is it the nature of the design allowing some movement, this is something I'm reluctant to voice an opinion on, but I certainly wouldn't base my actions on a single study. The move to ever larger capacity cells to maintain series only connection may be where the problems arise.

Furthermore using automotive examples of different cell chemistries in conjunction with different cell structure is a weak argument, apples to oranges, they need to be constrained because they're in a mobile application, in a stationary installation, earthquakes not withstanding, the cells are not subject to the same conditions.

The beauty of a personal opinion is that nobody can disagree with it, the truth will however reveal itself one way or another, given enough time.
 

shadowsteve

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Feb 11, 2021
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If the foam is intended to exert even pressure across the face of the cells it is positioned between how is it expected to do that given a non-flat surface of the cells? Are those using foam expecting the compression of the pack to bring the cell faces to a fully flat position? I can see that happening with a non-resilient media but not with the foam
 

bobdelso

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I'm pretty sure that EVE's test mechanism is a hydraulic controlled mechanism that maintains a constant pressure ..... Their spec for compression is what most of the discussion stems from.

I'm also pretty sure this test mechanism is rigid .... I'm not 100% sure how the introduction of a flexible membrane is going to affect things.
With a rigid mechanism exerting X amount of force .... that force could be greater in one area than another as SOC and temperature varies and maybe the middle of the cell has more pressure than the outside once the cells try to expand ... just thinking out loud here, but foam that compresses at X PSI will not necessarily exert the same force on a cell a rigid frame.

Did that make any sense?
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
 

Turponieminen

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I don't think foam keeps constant pressure. More likely it acts like a spring, gradually increasing pressure when it contracts.
 

bobdelso

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I don't think foam keeps constant pressure. More likely it acts like a spring, gradually increasing pressure when it contracts.
That's true, but the cells don't expand that much and I doubt the compression would get beyond 15-18 psi (which is the higher end of the recommended compression)
 

Gazoo

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Most prebuilt packs are sold with a 2000 cycle life after which it will have 80% capacity. Without compression you can get 2k cycles. With compression you can get 6k-14k
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.
 

Short_Shot

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My personal opinion is that I don't think this method is effective. It looks like a desire to do something, but the effectiveness of such an action tends to zero. Especially for longer periods of time, a year/two/three...
Foam cannot provide stability.
It can if it's explicitly designed for this application.

It's a matter of material properties. The fault in many folks thinking is in the association of the word foam.

We are all very used to cheap foam compressing and staying that way forever. However, if the material properties are known and the deformation stresses are kept within design specs then it can be said with confidence that the system will last as desired.

At any rate, with limited investigation into this material I'm confident enough to try it myself, and will be ordering some when my cells arrive.
 
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