The secret to compression

jmole

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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.


editing to add my later post here:

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:



View attachment 59677
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My battery:
IMG_2376.jpeg
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Bob B

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The one thing I have wondered about foam is how long will it maintain that amount of compression?

In my experience, even memory foam will become permanently compressed over time.... but I don't know the details.
 

jmole

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Interesting build all together. Do you have something to measure that 11psi and confirm it?
I don't, in this case datasheet + manufacturer reputation was good enough for me. I thought about incorporating a calibrated force sensor in the stack (pressure = force/area), but ultimately decided that I didn't care *that* much.

Dimensional stability WRT the busbars was the most important to me, second was to ensure a constant pressure profile between the cells without any hotspots (cells don't expand at the same rate along the entire face, they swell more in the center). Foam served both of those considerations quite well.
 

jmole

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The one thing I have wondered about foam is how long will it maintain that amount of compression?

In my experience, even memory foam will become permanently compressed over time.... but I don't know the details.
Poron is spec'd at 5% max compression set when tested w ASTM D 3574-95. Don't know the details of the test TBH, but if it's good enough for EV manufacturers it's good enough for me.

Edit: not Tesla, they use cylindrical cells which have different constraints.
 
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Bob B

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Poron is spec'd at 5% max compression set when tested w ASTM D 3574-95. Don't know the details of the test TBH, but if it's good enough for Tesla it's good enough for me.
Is Tesla trying to maintain a long term specific amount of compression .... or just keep things from moving around?
 

jmole

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Correct, Tesla doesn't use compression on they're batteries, or atleast not for the reason we do. Our prismatic cells are weak and need compression.
Is Tesla trying to maintain a long term specific amount of compression .... or just keep things from moving around?
Shouldn't have said that - Tesla was the wrong EV manufacturer to pick.

Rogers recommends these for use with pouch cells that *do* expand, not cylindrical cells which mostly don't.

PORON® Polyurethane Battery Pad Technology
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.
 

shadowsteve

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Your pack is 8p4s? Does each cell have a BMS connection? Will be starting my pack builds soon as the batteries seem to have left the boat and wanted to make sure I had it right.

I plan each pack to have 4 280ah cells in series with a single 150A BMS and then two packs in parallel for 560AH total.
 

A.Justice

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Also known as Poron:

Foam compresses 25% at 11psi.
.
They have samples of the material available. I ordered some and I'm going to leave it stuck under something heavy for a while and see how it holds up. I'll report back.
 

Zwy

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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.
Great minds think alike. https://diysolarforum.com/threads/compression-material-thoughts.20909/post-245041

My very first thread I started here was about cell compression and foam. It is the way to go, no doubt in my mind. https://diysolarforum.com/threads/a...hread-this-time-about-foam.16537/#post-187793
 

Short_Shot

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Jul 13, 2021
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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.
That's a nice looking pack. Nice job.

I'll be saving this thread.
 

jmole

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Great minds think alike. https://diysolarforum.com/threads/compression-material-thoughts.20909/post-245041

My very first thread I started here was about cell compression and foam. It is the way to go, no doubt in my mind. https://diysolarforum.com/threads/a...hread-this-time-about-foam.16537/#post-187793

Yep, your post from the other thread nails it here:
None of the cells are perfectly flat plus anode damage could occur. There also reports of continuity between the cell face and terminals, possible short condition.

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.
 
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jmole

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Your pack is 8p4s? Does each cell have a BMS connection? Will be starting my pack builds soon as the batteries seem to have left the boat and wanted to make sure I had it right.

I plan each pack to have 4 280ah cells in series with a single 150A BMS and then two packs in parallel for 560AH total.
4p4s, using REC BMS. Each busbar has its own cell balance wire. The busbars are tapered to distribute current equally to all cells even at max load.
 

Draconius

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And I presume this foam is electricly inhibitive in the event of abrasion? I have 4 (Blue film style) cells with plastic cutting boards between all cells, ends, top and bottom to prevent abrasion (in vehicle) to try and make shorting impossible if film wears. Wonder if I can take this foam instead as it will likely also create friction and hold cells together better...hmmmm
 

jmole

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Polyurethane is an insulator unless you add something like Carbon to make it conductive.
When compressed, there’s a lot of friction between the cell and the foam, I don’t think you’d be able to displace the cells in any significant way.

And I presume this foam is electricly inhibitive in the event of abrasion? I have 4 (Blue film style) cells with plastic cutting boards between all cells, ends, top and bottom to prevent abrasion (in vehicle) to try and make shorting impossible if film wears. Wonder if I can take this foam instead as it will likely also create friction and hold cells together better...hmmmm
 

Draconius

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Polyurethane is an insulator unless you add something like Carbon to make it conductive.
When compressed, there’s a lot of friction between the cell and the foam, I don’t think you’d be able to displace the cells in any significant way.
Having just read fhorst's fire, it's got me thinking...I put the cutting boards in and tightened my 4 corner nuts to decent tightness, not too extreme, but since these cells bulge in the middle, there will always be gaps at the corners..

Might allow me to re wire/fix my BMS also as I can ground my pack safely (no risk of spark) and THEN connect the balance leads.

Thanks for getting me thinking!
 

Q-Dog

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I recently bought a prebuilt pack because I wanted more capacity and hurricane season could be over by the time I receive cells direct from China. (plus a warraty) These new cells are stuck together with double sided foam tape. They made no mention of compressing the cells.

Made me feel better about the foam I put between the cells in my first pack. Also, no compression on the first pack, just constrained so there is no room for the cells to expand.
 

bobdelso

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I recently bought a prebuilt pack because I wanted more capacity and hurricane season could be over by the time I receive cells direct from China. (plus a warraty) These new cells are stuck together with double sided foam tape. They made no mention of compressing the cells.

Made me feel better about the foam I put between the cells in my first pack. Also, no compression on the first pack, just constrained so there is no room for the cells to expand.
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
 
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