The secret to compression

Tdevery

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Getting new 280ah prismatic cells. Should I compress them when charged or discharged?
Using the aluminum plate with end springs.
 

Tdevery

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So when I recieve the new cells, I will compress them, then top balance.
 

noenegdod

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PSI vs kgf is irrelevant however (as it’s the same).
This is not correct. The PSI that the force was converted into was done so by taking the force they were using and dividing it by the surface area of the wide side of a 280AH cell.

They are not the same.

This is why using a bladder such as an airbag directly on the side of a cell is not how it should be done:

Ever sleep on a water bed? Try to lift yourself up with one hand and your hand sinks into the bed. It offers little resistance to your hand. The cell is the same thing. The expansion of the cell is not symmetrical. The majority of the deformity is in the center of the flat surface of the cell. With a bladder there is a uniform pressure applied to the surface however the majority of the force from the cell is in an isolated area (the middle). The cell is able to freely expand into the air bag in this area resisted by a measily 12 or so PSI.

I believe misunderstanding has resulted from converting the force used in China to PSI for general consumption in NA.

It really isnt PSI you want. It is a compression force applied to the side of 1 280ah cell, I believe through a ridged flat surface. This will result in higher pressure (PSI) in the middle of the side of the cell where we see the highest amount of deformation. I believe the pressure difference on the surface of the cell is irrelevant. It is maintaining the geometry of the cell. Everyone seems to forget that cylindrical cells do not change their shape or expand to any significant degree. Why would these cells require being able to swell?
 

noenegdod

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Getting new 280ah prismatic cells. Should I compress them when charged or discharged?
Using the aluminum plate with end springs.
If you want to be crazy about it, after have done everything and are ready to put them into service, set your spring length for your desired force at 50% SOC and if you have no compressible foams in the system, forget about it. If you have any compressible foams anywhere in your assembly, reset it again in a month or so and then forget about it.
 

noenegdod

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Mine bulge not in the center, but on the two off center areas where the jelly roll is folded to make the 180 turn.
It really doesnt matter where or how many bulges there are, they do not expand and contract uniformly which is the point.
 

Bob B

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I did some research a while back trying to find a suitable bladder to maintain 12PSI but couldn't find anything I liked.
I do agree that EVE is probably testing in a rigid fixture ... but this can be corrected by putting a piece of rigid material between the bladder and the cell .... then, the Kgf and 12PSI WOULD be the same thing.
 

noenegdod

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I did some research a while back trying to find a suitable bladder to maintain 12PSI but couldn't find anything I liked.
I do agree that EVE is probably testing in a rigid fixture ... but this can be corrected by putting a piece of rigid material between the bladder and the cell .... then, the Kgf and 12PSI WOULD be the same thing.
100%. I said exactly that in post #156

I do like pneumatics or foam behind a rigid board in the context of a temperature stable, stationary application. I think hydraulics would also be a very easy to accomplish, rock solid pressure regulation method. Better than springs from a consistency standpoint. With a little electronics if you knew what you are doing I think it would be easy. But for dead simple, relatively consist regulation in a mobile environment that has big temperature swings IMO springs are the way to go.
 

Just John

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It really doesnt matter where or how many bulges there are, they do not expand and contract uniformly which is the point.
I just thought I'd point that out, since you seemed to state it as fact to back up your statement that air pressure can't deliver.
Personally, from my point of view, using an air bladder has drawbacks, but uniform pressure isn't one of them.
 

cinergi

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I did some research a while back trying to find a suitable bladder to maintain 12PSI but couldn't find anything I liked.
I do agree that EVE is probably testing in a rigid fixture ... but this can be corrected by putting a piece of rigid material between the bladder and the cell .... then, the Kgf and 12PSI WOULD be the same thing.

That’s exactly what I meant. For the given surface area it’s the same. As to what’s happening between the rigid material and the cell at various locations, that’s a different issue.
 

noenegdod

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I just thought I'd point that out, since you seemed to state it as fact to back up your statement that air pressure can't deliver.
Personally, from my point of view, using an air bladder has drawbacks, but uniform pressure isn't one of them.
A bladder will absolutely deliver uniform pressure to the surface of the cell. What I am trying to say is that uniform pressure on every square millimeter of surface on the cell is not what you want (based on what I read from that report/study). What I interpreted from that paper is you want to apply a force to the surface (through a flat plate) to prevent any area of the cell from swelling. This will result in higher pressures in some locations and lower in others as the swelling is not uniform.

If china has wanted uniform pressure applied to the surface of the cell, they would have used a units of pressure, not "kg of force"
 

Bob B

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That’s exactly what I meant. For the given surface area it’s the same. As to what’s happening between the rigid material and the cell at various locations, that’s a different issue.
Agree.
 

Bob B

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A bladder will absolutely deliver uniform pressure to the surface of the cell. What I am trying to say is that uniform pressure on every square millimeter of surface on the cell is not what you want (based on what I read from that report/study). What I interpreted from that paper is you want to apply a force to the surface (through a flat plate) to prevent any area of the cell from swelling. This will result in higher pressures in some locations and lower in others as the swelling is not uniform.

If china has wanted uniform pressure applied to the surface of the cell, they would have used a units of pressure, not "kg of force"
Do you have a link or reference to the report you are talking about?

Sorry if you've already posted that, but I couldn't find it and am interested to understand this aspect of compression.
 

noenegdod

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Do you have a link or reference to the report you are talking about?

Sorry if you've already posted that, but I couldn't find it and am interested to understand this aspect of compression.
The report/test that was done by EVE ( I think) that started this whole compression thing. I dont have it readily available. I got what I needed and moved on.
 

Bob B

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The report/test that was done by EVE ( I think) that started this whole compression thing. I dont have it readily available. I got what I needed and moved on.
What I remember seeing from EVE is simply the information in their specification that shows the benefits of compression on cycle life ..... and a post from ghostwriter66 who had a conversation with one of the EVE engineers which brought out more details.

I don't remember anything specifically saying that the pressure on the cells is BETTER or only works if the pressure is from a rigid plate. I understand what you are saying about the uneven pressure a plate will provide ... might .... be necessary for the compression to do it's magic.
Don't get me wrong ... I have posted in the past that my assumption would be that is the way EVE is doing their testing with a solid plate .... and therefore that would be the most prudent way to do it.

Beyond that, I would put into the area of theory or speculation.
Is it possible that a soft bladder with 12PSI might be even better .... I don't know .... If there is a document that shows the pressure must be from a rigid plate, I would really like to have it for reference.

Not sure we can make rigid statements regarding this ... pun intended.
 

cinergi

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Beyond that, I would put into the area of theory or speculation.
Is it possible that a soft bladder with 12PSI might be even better .... I don't know .... If there is a document that shows the pressure must be from a rigid plate, I would really like to have it for reference.

+1 Especially before anyone starts claiming that this is fact and is the better way to do it. Making your own decision that one way or another is better is fine (personal choice) but publishing it as such in a public forum is a very different thing. Documented facts first = better for all of us. I get that that's a lot more tedious but anything else just looks like posturing (of which I'm guilty, too).
 

noenegdod

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What I remember seeing from EVE is simply the information in their specification that shows the benefits of compression on cycle life ..... and a post from ghostwriter66 who had a conversation with one of the EVE engineers which brought out more details.

I don't remember anything specifically saying that the pressure on the cells is BETTER or only works if the pressure is from a rigid plate. I understand what you are saying about the uneven pressure a plate will provide ... might .... be necessary for the compression to do it's magic.
Don't get me wrong ... I have posted in the past that my assumption would be that is the way EVE is doing their testing with a solid plate .... and therefore that would be the most prudent way to do it.

Beyond that, I would put into the area of theory or speculation.
Is it possible that a soft bladder with 12PSI might be even better .... I don't know .... If there is a document that shows the pressure must be from a rigid plate, I would really like to have it for reference.

Not sure we can make rigid statements regarding this ... pun intended.
LOL on the pun :)

I agree with everything you said and that is why I have always qualified what I said with "in my opinion". I may have missed qualifying the odd statement but it is all my opinion

The logic I use is based solidly on the fact that cylindrical cells do not expand at all (significantly) while a prismatic cell that is being restrained by a bladder would still be able to expand at its weakest points. 600kg of force restraining a few square inches on the side of a prismatic cell that is trying to bloat is far more pressure than 12 psi. That is where I am coming from.

The fact remains that they published a force. Not a pressure.
 

Just John

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What I remember seeing from EVE is simply the information in their specification that shows the benefits of compression on cycle life ..... and a post from ghostwriter66 who had a conversation with one of the EVE engineers which brought out more details.

I don't remember anything specifically saying that the pressure on the cells is BETTER or only works if the pressure is from a rigid plate. I understand what you are saying about the uneven pressure a plate will provide ... might .... be necessary for the compression to do it's magic.
Don't get me wrong ... I have posted in the past that my assumption would be that is the way EVE is doing their testing with a solid plate .... and therefore that would be the most prudent way to do it.

Beyond that, I would put into the area of theory or speculation.
Is it possible that a soft bladder with 12PSI might be even better .... I don't know .... If there is a document that shows the pressure must be from a rigid plate, I would really like to have it for reference.

Not sure we can make rigid statements regarding this ... pun intended.
I was under the impression this is the most accurate information we have about how EVE accomplishes it:
 

Bob B

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I was under the impression this is the most accurate information we have about how EVE accomplishes it:
I think your impression is correct.
 
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