The secret to compression

Bob B

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@Bob B Any suggestion for how I could test the foam? Springs seem a lot simpler to test as cinergy did since you have single point of compression whereas with the foam between all cells and sides I am wondering how to better test this.
The only way I could think to do it scientifically would be to actually use some sort of pressure gauge. I briefly looked into buying one for my own testing, but never found one I liked.
HaldorEE is the only one on here I know of who has one of those.

Maybe someone else will know of a DIY way to measure the compression. Maybe using an Arduino or Raspberry?
 

Short_Shot

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The only way I could think to do it scientifically would be to actually use some sort of force gauge. I briefly looked into buying one for my own testing, but never found one I liked.
HaldorEE is the only one on here I know of who has one of those.

Maybe someone else will know of a DIY way to measure the compression.
Known spring rate and measurements.

A couple of small holes through your compression plate and the foam so you can depth mic it at various states of charge.
The only catch is you need to ensure the pack itself doesn't move nor the depth mic.

If you know the starting compression by carefully measuring the compression distance you can calculate the difference in force.


Perfect? No. Good enough? Absolutely. You can test the foam before building the pack to verify the specs.
 

Bob B

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Known spring rate and measurements.

A couple of small holes through your compression plate and the foam so you can depth mic it at various states of charge.
The only catch is you need to ensure the pack itself doesn't move nor the depth mic.

If you know the starting compression by carefully measuring the compression distance you can calculate the difference in force.


Perfect? No. Good enough? Absolutely. You can test the foam before building the pack to verify the specs.
I don't think you can use the foam itself to test the actual pressure being exerted by the foam. Doesn't seem very scientific .... but you are probably just itching to start another argument LOL
 

rhino

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I assume a more accurate way would be to use a "load cell"? I see some on ebay that measure 5 tons of force :ROFLMAO:.. maybe can find one that measures smaller force.
 

Bob B

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I assume a more accurate way would be to use a "load cell"? I see some on ebay that measure 5 tons of force :ROFLMAO:.. maybe can find one that measures smaller force.
I did find some on ebay that seemed to be the right range. but that's been a while ago now. There are probably different names for them ... pressure gauge, strain gauge, etc.

A pack that had cells sitting side by side would require 600 Kgf or more of pressure.
 

Bob B

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I assume a more accurate way would be to use a "load cell"? I see some on ebay that measure 5 tons of force :ROFLMAO:.. maybe can find one that measures smaller force.
I saw a post somewhere else ... can't remember where .... where a guy was actually sandwiching a blood pressure cuff to measure pressure. I don't know if it would be possible to use an air bladder to measure the PSI? They do make those designed to jack up equipment and such.
My problem with them is that in my experience with a flat tire .... the PSI in the tire will remain the same whether there is force on it or not ..... Maybe someone will check in who has more experience with force measurements.
 

Short_Shot

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I don't think you can use the foam itself to test the actual pressure being exerted by the foam. Doesn't seem very scientific .... but you are probably just itching to start another argument LOL
... this is engineering 101.

You measure the change in compression and do some basic math based on the known properties of said material.

It's literally the exact same thing as using a mechanical scale. The difference with the scale however is someone else already did the math to and created a mechanism to translate it to a readout for the user who doesn't understand how the scale works in the first place.
 

Short_Shot

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I saw a post somewhere else ... can't remember where .... where a guy was actually sandwiching a blood pressure cuff to measure pressure. I don't know if it would be possible to use an air bladder to measure the PSI? They do make those designed to jack up equipment and such.
My problem with them is that in my experience with a flat tire .... the PSI in the tire will remain the same whether there is force on it or not ..... Maybe someone will check in who has more experience with force measurements.
That's actually a bit of a myth, but only in the sense that it does increase but the amount is virtually nothing in practical use.

The reason for it is that the tire volume only changes a tiny amount under load and the change in volume is the only way to change the pressure. That they don't change in volume much is by design. When the tire is loaded it'll bulge out a bit to "accommodate" the the space lost to the flatter bottom.

In application (reality) the change is so insignificant that the difference is not measurable by most cheap tire pressure gauges.
The difference between hot and cool air inside the tire is more significant by far.

Blood pressure cuffs don't actually work by measuring the changes in air pressure from blood flow though, interestingly enough, however flattening the whole thing can absolutely change the volume of it enough to actually raise the pressure inside. Though I don't think it'll be 1:1 with parts of it bulging out of the sides. It would need to be totally contained to prevent the rubber from stretching.

Signed: someone with experience with force measurements (though not as much as perhaps others). I use strain gauge sensors, springs, and hydraulic systems daily.
 

Bob B

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If anyone has any actual helpful information about an inexpensive load cell that might be helpful in making compression measurements .... I'd really like to see some options.
 

rhino

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This is closest I could find of reasonable price but isn't the through hole type which would make it much easier to just put the threaded rod through it. Not sure how could connect this unless add additional interior wall of battery box that this pushed against and the back of it would be against the normal battery box wall. https://www.sparkfun.com/products/13332
 

rhino

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Still not sure this would even work to accurately measure though since as Short_Shot eluded to the foam is going to cushion the pressure some itself as it is designed to do.
 

Bob B

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Still not sure this would even work to accurately measure though since as Short_Shot eluded to the foam is going to cushion the pressure some itself as it is designed to do.
Good find @rhino ..... I had done a little searching, but hadn't figured out how to get the measurement out of the load cell.

I think we could basically build a pack using a standard threaded rod compression fixture with the load cell inserted somewhere in the pack sandwiched between some metal plates. Put the foam on the ends and measure the pressure with different amounts of deflection .... and then fully charge and discharge the pack while monitoring / logging if possible the pressure changes.
It would also be interesting to know how temperature affects the characteristics of the foam.

I'm going to be reconfiguring my pack this winter and would love to have a setup where I could experiment and measure the pressures.
 
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Short_Shot

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Still not sure this would even work to accurately measure though since as Short_Shot eluded to the foam is going to cushion the pressure some itself as it is designed to do.
Yes. But that "cushion" requires x amount of force per y amount of physical compression.
 

Short_Shot

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This is closest I could find of reasonable price but isn't the through hole type which would make it much easier to just put the threaded rod through it. Not sure how could connect this unless add additional interior wall of battery box that this pushed against and the back of it would be against the normal battery box wall. https://www.sparkfun.com/products/13332
You could sandwich that between a stiff plate attached to the other end with thick threaded rod and a loose plate against the cells.

Or you could just trust the math and measure the foam compression depth, apply the same logic that we apply to longer springs to reduce the variation over full compression distance (use thicker foam), and then assemble the pack and call it a day.

Personally I think it makes sense to place a semi rigid sheet (say, 1/4 inch plywood) between the cell and the poron. This will help distribute the higher force at the center of the cell throughout the foam and more closely approximate the flat/rigid plate and fixed force of a test fixture.

You'd then check the spec sheet for the poron you have and select a compression distance equal to the desired load.

One typical chart for the material (varies by grade) shows 11-12psi at roughly 25% compression. Get a half inch of the stuff, charge or discharge your cells till they are about flat, measure, assemble the pack with slightly more than 1/8 inch smaller than the total width including the half inch foam and that spacer sheet and stuff it in.


The reason I say to go slightly more compression is that the stuff does have a couple percent of compression "set" at room temp and slightly more at elevated temps so after a day or so you'd need to account for this. But only slightly.

Then stop worrying about it after it's built because you'll probably get more life increase out of partial charge than the compression anyways.
 

Short_Shot

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Good find @rhino ..... I had done a little searching, but hadn't figured out how to get the measurement out of the load cell.
You use a DAQ device and whatever data sheet the load cell came with to configure your data acquisition software.

There are some inexpensive ones on Amazon.
I've always wanted to try one of these out and I'm trying to talk my employer into paying for it for "research" purposes.

 

Henrysangret

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Also known as Poron:

Foam compresses 25% at 11psi.

If you buy 1/4" foam, then plan for 3/16" between your cells when you design your busbars and your box, and you have 11psi compression with no springs and no hassle.

Worked great for me.

I designed my box to be slightly longer than the uncompressed length, and added a compression plate to push the cells and foam together to achieve the compression I wanted.
Foam and plastics take a set with time, maybe couple days maybe a month, but they will loose their compression guaranteed.
 

rhino

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Foam and plastics take a set with time, maybe couple days maybe a month, but they will loose their compression guaranteed.
Screen Shot 2021-08-29 at 8.29.20 AM.png
Battery Pads
Li-Ion Pouch Cell
PORON® Polyurethane Battery Pad
Whether the battery leverages a single pouch cell or several cells in a series, it is important to maintain a constant stack up pressure on all cells to ensure an optimized electrochemical reaction during the swelling of the battery. But how is constant force maintained on a system that is constantly expanding and contracting?
The answer lies in the use of battery pads (also known as pressure pads). Pouch cell battery pads are typically soft, compressible materials that address changes in compression without becoming overly firm or soft during cell expansion, thereby maintaining a constant pressure on each pouch cell.
PORON® polyurethane battery pads are routinely specified by engineers because they are uniquely formulated to provide stable, long-term elastic properties due to their superior heat-aged compression set resistance (C-Set), which is the material's ability to rebound after being compressed under high heat.

Reference: https://rogerscorp.com/-/media/proj...cation-notes/battery-pad-application-note.pdf
 

Just John

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Foam and plastics take a set with time, maybe couple days maybe a month, but they will loose their compression guaranteed.
Yes, however Chevy, LG, Volkswagen, and Ford seem to like the product, since their battery packs use it.
 
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