Another Cell Compression Thread, this time about foam

[justgary]

I'm aiming to do similar to @justgary and I'll be using laminated flexible bus bars. Was going to use springs, but they will protrude from the end of my rack.

That is why I decided to use Poron. I didn't really have room for anything sticking out past the box.
If I were doing it again, I would just hard compress them without the foam. The newest Eve spec says 300 kgf at ~30% charge in a rigid box.

 

[Zwy]

Hard compress with a fiberglass separator I'm assuming? My concern with hard compress is pressure on the cell case at the anode and cathode. Those two spots always seem slightly elevated, I'd rather spread any force from expansion across the entire face. A hard compress would still allow expansion across the face but nothing there to recompress.

 

[Ron-ski]

I actually managed to find this foam in the UK

Just been quoted £96 for a 24" x 24" x 0.25" piece of 50-15250 foam, but £168 shipping!!!!! Oh and a lead time of 5 to 6 weeks. So in total thats £264 or $320 which is just crazy!

I'm looking here https://www.raptorsupplies.co.uk/p/...23763,23763,30141&catKey=116280,116282,412156

Even choosing the 12" x 12" piece which is supposedly in stock gets a £84 shipping charge!

Think I need other options, anywhere in the US that will ship to the UK for more reasonable costs?

 

[Ron-ski]

My PO shows 86375K234 as the SKU I bought.

For some reason I thought you was using 5 PSI foam (50-15250-24X24 is 5 psi), but this shows as 11 psi on McMaster. I've emailed them to see how much they will charge to ship to the UK.

 

[Ron-ski]

That was impressively quick, McMaster have got back to me, total cost including import duty would be £192 + plus whatever handling fee I'm charged for collecting import duty.

Still working out rather expensive, again its the shipping which is rather expensive ($113 plus UK VAT).

I find it hard to believe I can't buy this in the UK.

Any thoughts please?

 

[justgary]

That was impressively quick, McMaster have got back to me, total cost including import duty would be £192 + plus whatever handling fee I'm charged for collecting import duty.

Still working out rather expensive, again its the shipping which is rather expensive ($113 plus UK VAT).

I find it hard to believe I can't buy this in the UK.

Any thoughts please?

Just forget about the foam and use hard compression with hard spacers, especially if you are using Eve cells. The hardest part about that will be knowing when you have 300 kgf on the cells. My experience shows that two Irwin Medium Bar Clamps can just barely apply that amount of force when you squeeze them as hard as you can with two hands. They are rated at 300 pounds each, so two will get you right in the ballpark. If you are handy you might already own them, or maybe a neighbor does.

https://www.irwin.com/tools/clamps/quick-grip-medium-duty-one-handed-bar-clamps

 

[Ron-ski]

Not sure I fancy the idea of a solid enclosure, and I suspect its far esier to get it wrong. The bar clamps I own are made of metal, and have a screw thread, and will be much stronger than those plastic things.

I've found another company in the UK that supplies Rogers foam, but not sure which one I need (no psi listed) - could somebody who understands all the number advise please? I've attached the product sheet. They list EV extend which I'm sure I've seen mentioned in this thread.

@Zwy Didn't you source yours from Rogers?

 

[Zwy]

Look here: https://rogerscorp.com/-/media/proj...sh/data-sheets/poron--evextend-data-sheet.pdf

 

[Ron-ski]

Look here: https://rogerscorp.com/-/media/proj...sh/data-sheets/poron--evextend-data-sheet.pdf

Thanks, although I think I'm missing something, that lists EVExtend 4701-43, but then three different densities, looking at the chart it would appear I want the higher density 192, but what/where are the specific part numbers. Actually is it 4701-43-12???

There's lots of talk on here for Poron, but seems to be lacking specific part numbers, and what numbers I do find impossible to buy in the UK. McCaster only seem to do 0.25" foam, which is too thick for between each cell.

It's so frustrating Sorely tempted to just use some 1.5mm EPDM membrane between the cells in a rigid structure, set at about 30% SOC.

 

[justgary]

Thanks, although I think I'm missing something, that lists EVExtend 4701-43, but then three different densities, looking at the chart it would appear I want the higher density 192, but what/where are the specific part numbers. Actually is it 4701-43-12???

There's lots of talk on here for Poron, but seems to be lacking specific part numbers, and what numbers I do find impossible to buy in the UK. McCaster only seem to do 0.25" foam, which is too thick for between each cell.

It's so frustrating Sorely tempted to just use some 1.5mm EPDM membrane between the cells in a rigid structure, set at about 30% SOC.

Don't put foam between each cell if your goal is to meet the compression specification. Use one 1/4" sheet for four cells, and two 1/4" sheets for eight cells. Use hard spacers between the cells.

 

[justgary]

See post #77 in this thread for details on my foam testing.

 

[Zwy]

Thanks, although I think I'm missing something, that lists EVExtend 4701-43, but then three different densities, looking at the chart it would appear I want the higher density 192, but what/where are the specific part numbers. Actually is it 4701-43-12???

There's lots of talk on here for Poron, but seems to be lacking specific part numbers, and what numbers I do find impossible to buy in the UK. McCaster only seem to do 0.25" foam, which is too thick for between each cell.

It's so frustrating Sorely tempted to just use some 1.5mm EPDM membrane between the cells in a rigid structure, set at about 30% SOC.

My first pack I ever built was with 1/4" EPDM, still works fine in my truck camper.

As for EVExtend Poron, I think there is a post where someone tried to buy some but were told they only sell it to manufacturers. As for the Poron I have, I bought everything a seller had on ebay that was drop close to the size I needed and went with the 25% compression like I did for the EPDM. I have compressed it when top balancing, tore the pack down and noted the springback of the Poron was only a short time frame.

I plan on continuing using foam in between cells, I just can't see putting stress on the anode and cathode by a hard liner in between cells. We are all pioneers in this venture, some solar enthusiasts aren't using any compression but then after a few years declare the cells have lost over 5% capacity. I'd rather have some than nothing, it's getting the recipe right that is the challenge.

 

[Ron-ski]

There is a mind bending amount of opinion's on here, but no one really knows, so all I can do is read what people have written, see examples of what manufactures are doing, and form my own opinion.

I plan on continuing using foam in between cells, I just can't see putting stress on the anode and cathode by a hard liner in between cells. We are all pioneers in this venture, some solar enthusiasts aren't using any compression but then after a few years declare the cells have lost over 5% capacity. I'd rather have some than nothing, it's getting the recipe right that is the challenge.

It makes sense to me and feels right that there should be foam between the cells, for the reasons you state above. Using 0.25 foam between my cells I think is possible, my busbars allow 5mm, 0.25 foam compressed 25% is 4.7625mm, but I need to physically check. I need two sheets of https://www.mcmaster.com/86375K234/ which in its self is not expensive, but adding another sheet may well bump up the already rather expensive carriage. Ideally if using a sheet between each cell it needs to be thinner, which will reduce cost.

Actually just doing the calcs, the base of my battery tray is 637mm long, a pack using 0.25 foam, with the 10mm end plates would be 638.9mm so wouldn't actually fit, so I need thinner foam, ideally 3mm / 0.125".

Don't put foam between each cell if your goal is to meet the compression specification. Use one 1/4" sheet for four cells, and two 1/4" sheets for eight cells. Use hard spacers between the cells.

This doesn't make sense to me, if the foam is compressed, and it exerts say 13 psi why would adding foam between each cell not meet the compression requirements, as long as all foam was compressed 25%? If the cells stayed perfectly flat then using hard spacers would makes sense, but they don't so it makes more sense to me to use foam between each cell. I'd be the first to admit that I don;t understand the technical side of things, so this is more a gut feeling.

 

[Zwy]

For my 16S batteries, I have 3mm Poron. I needed it to fit inside this cabinet. https://diysolarforum.com/threads/house-system-battery-box-hoffman.32305/#post-394655

I was able to fit four 16S batteries in it. Still building it. Left it in one piece, sidewall makes one end of compression fixture with threaded rod coming out the side.

 

[justgary]

This doesn't make sense to me, if the foam is compressed, and it exerts say 13 psi why would adding foam between each cell not meet the compression requirements, as long as all foam was compressed 25%? If the cells stayed perfectly flat then using hard spacers would makes sense, but they don't so it makes more sense to me to use foam between each cell. I'd be the first to admit that I don;t understand the technical side of things, so this is more a gut feeling.

The reason is that the foam acts like a spring. If you build a frame using springs on rods around an end cap, the springs are in parallel. If you build a frame using foam, the foam is in series. Each performs differently.

For a parallel spring system, simply add the spring coefficients together to get the total spring constant:

The total spring coefficient is

. Force applied by the spring is then f = (k1+k2) * x, where x is the distance the spring is compressed. In other words, two (same constant) springs in parallel are equal to a spring of the same length with double the coefficient.

For a series spring system, one must add the reciprocal of the spring coefficients, then take the reciprocal to get the total spring constant:

The total spring coefficient is now

. Force applied by the spring is then f = [1 / (1/k1+1/k2)] * x. We see that two (same constant) springs in series are equal to a spring with double the length but half the spring coefficient.

Our goal with any compression frame is twofold: 1) we want to apply ~300 kgf (660 lbf) to the broad faces of the cells, and 2) we want the force to remain roughly constant over a small range of motion of our cells (generally less than 0.5mm (0.020") per cell). I think that the parallel spring case has been well discussed on this site, so let's look at the series case, and we'll use foam as the spring (since that is the topic of this thread).

I tested 1/4" 50-15250 Poron foam using a spring scale and a CNC mill to gradually compress the foam (Post #77 of this thread). What I found was that two layers of foam are indeed the same as a series spring. The problem then was to find a combination of layers of foam that would apply the correct force (condition 1), and allow the correct amount of expansion (condition 2). What I found was that one 1/4" layer of foam per four cells was ideal, assuming one compressed the foam 50% of the total thickness.

If we roughly linearize the results I got, we see that Poron 50-15250 foam has a spring coefficient of 18#/0.15" = 120 pounds per inch of compression (per square inch of foam). Two sheets then have [1/(1/120 + 1/120)] = 60 pounds per inch, so I need to compress them twice as much as a single sheet. Since each sheet of foam has a usable range of about 2mm (0.080") of deflection, each sheet is good for compressing four cells when each one is compressed to about 40% of its original thickness.

What happens if we add more sheets? It seems like a lot of folks want to use the foam as cell spacers, so let's try nine sheets (one at each end cap plus seven more between eight cells). The total spring coefficient of nine sheets is [1/(9*1/120)] = 13.3 pounds per inch (per square inch). In order to get the 660# I want over the 56 square inches of cell face, I'll need to compress my stack of foam 660/56/13.3 = ~0.9" from an original thickness of 9*0.25" = 2.25". Note that this is still a 40% compression, but our stack of foam could now handle a total deflection of 18mm (0.70"), which is way more than we need. Granted, the force applied would be much more consistent, but with the expense and total thickness of the foam, why use more than you need?

Note: Spring images and formatted equations borrowed from https://en.wikipedia.org/wiki/Series_and_parallel_springs.

 

[Ron-ski]

@justgary Thank you for the detailed reply. What are your thoughts on having solid spacers between the cells? Your obviously happy with this as its what you've done, but what's you're reasoning that this is acceptable, and won't cause damage?

One thing I've thought of is the friction between the base of the cell and what its stood on, with foam only at the ends your reliant on half the pack being moved along its base so the foam can take up the expansion, I wonder how this affects things as well?

 

[Ron-ski]

I have 3mm Poron.

Have you got a manufactures part number for the Poron please?

 

[curiouscarbon]

The reason is that the foam acts like a spring. If you build a frame using springs on rods around an end cap, the springs are in parallel. If you build a frame using foam, the foam is in series. Each performs differently.

For a parallel spring system, simply add the spring coefficients together to get the total spring constant:

The total spring coefficient is

. Force applied by the spring is then f = (k1+k2) * x, where x is the distance the spring is compressed. In other words, two (same constant) springs in parallel are equal to a spring of the same length with double the coefficient.

For a series spring system, one must add the reciprocal of the spring coefficients, then take the reciprocal to get the total spring constant:

The total spring coefficient is now

. Force applied by the spring is then f = [1 / (1/k1+1/k2)] * x. We see that two (same constant) springs in series are equal to a spring with double the length but half the spring coefficient.

Our goal with any compression frame is twofold: 1) we want to apply ~300 kgf (660 lbf) to the broad faces of the cells, and 2) we want the force to remain roughly constant over a small range of motion of our cells (generally less than 0.5mm (0.020") per cell). I think that the parallel spring case has been well discussed on this site, so let's look at the series case, and we'll use foam as the spring (since that is the topic of this thread).

I tested 1/4" 50-15250 Poron foam using a spring scale and a CNC mill to gradually compress the foam (Post #77 of this thread). What I found was that two layers of foam are indeed the same as a series spring. The problem then was to find a combination of layers of foam that would apply the correct force (condition 1), and allow the correct amount of expansion (condition 2). What I found was that one 1/4" layer of foam per four cells was ideal, assuming one compressed the foam 50% of the total thickness.

If we roughly linearize the results I got, we see that Poron 50-15250 foam has a spring coefficient of 18#/0.15" = 120 pounds per inch of compression (per square inch of foam). Two sheets then have [1/(1/120 + 1/120)] = 60 pounds per inch, so I need to compress them twice as much as a single sheet. Since each sheet of foam has a usable range of about 2mm (0.080") of deflection, each sheet is good for compressing four cells when each one is compressed to about 40% of its original thickness.

What happens if we add more sheets? It seems like a lot of folks want to use the foam as cell spacers, so let's try nine sheets (one at each end cap plus seven more between eight cells). The total spring coefficient of nine sheets is [1/(9*1/120)] = 13.3 pounds per inch (per square inch). In order to get the 660# I want over the 56 square inches of cell face, I'll need to compress my stack of foam 660/56/13.3 = ~0.9" from an original thickness of 9*0.25" = 2.25". Note that this is still a 40% compression, but our stack of foam could now handle a total deflection of 18mm (0.70"), which is way more than we need. Granted, the force applied would be much more consistent, but with the expense and total thickness of the foam, why use more than you need?

Note: Spring images and formatted equations borrowed from https://en.wikipedia.org/wiki/Series_and_parallel_springs.

thank you for taking the time to write this out! reminds me of calculating resistance value for resistors in parallel and series (or capacitors)

i want to use poron or some foam (spring) as cell separator for extra electrical insulation as well as reducing variation of compression force over the expansion/contraction range.

admittedly sourcing the EV rated type is still beyond my skills. i found some affordable foam claiming to be poron which is light blue in varying thicknesses, and will likely use that for a test build (1/32", 1/16", 1/8")

thank you again for the detailed explanation

 

[Ron-ski]

@curiouscarbon The trouble is there is literally loads of different grades of Poron foam, some which would exert too much pressure others not enough.

 

[curiouscarbon]

@curiouscarbon The trouble is there is literally loads of different grades of Poron foam, some which would exert too much pressure others not enough.

indeed. i prefer to simply find affordable poron sheets, characterize performance, and implement in a test pack.

sourcing proper ev type is too annoying to me and i personally feel as though the normal poron sheets will perform adequately for my needs. much better than closed cell neoprene which loses restitution force when compressed for extended time.

poron's restitution force comes from the material itself as i understand, as compared to air pockets with closed cell neoprene.

thank you for your insights!

 

[Ron-ski]

@curiouscarbon not sure where you are, but for me in the UK sourcing any type of Poron is being very annoying, or expensive.

I thought I'd found some foam that might do the job, but its pressure increased too rapidly - the 0.5mm expansion would push it well past 18 PSI.

Just emailed https://cbfrost-rubber.com/ who are in the UK and can supply Rogers Poron, lets see what they come back with.

 

[Ron-ski]

I've found this foam on McMaster 7503N23 they state 11 psi at 25%, but no indication of the pressure at higher rates, compressing a further 0.5mm would mean 41% compression.

So I asked for a data sheet, and they supplied the attached, which is for Rogers Poron 4701-41, but unfortunately no graph or specification for pressures other than at 25% (15 psi), and that differs to whats on McMaster's website (11 psi). The other confusing thing is the sheet states 4701-41, but under the value column there is clearly 3 different products/densities which again causes me confusion with a specific product number.

PS. As they are selling Rogers Poron, I've asked if they can supply EVExtend.

 

[justgary]

the 0.5mm expansion would push it well past 18 PSI.

The current EVE data sheets no longer specify 18 PSI maximum, but nobody seems willing to just hard clamp their cells and move on. It would sure be nice if we could clear the specification up with manufacturers.

 

[Bob B]

The current EVE data sheets no longer specify 18 PSI maximum, but nobody seems willing to just hard clamp their cells and move on. It would sure be nice if we could clear the specification up with manufacturers.

I'm pretty sure that 18 PSI number was the result of a conversation between a forum member (@ghostwriter66 ) and an EVE technician. They said that going to 18 PSI or above was worse than no compression.
I don't know if it was ever part of their spec .... but I could be wrong that it wasn't ever in the spec.

I think the majority of people are just snugging them up and calling it a day .... only those trying to do everything they can to maximize battery are doing springs, foam, or even Belleville washers. The Belleville washers are an option for those concerned with pack length ..... or, it would be possible to use expansion springs instead of compression springs that could be put on the side of the pack.

 

[justgary]

@justgary Thank you for the detailed reply. What are your thoughts on having solid spacers between the cells? Your obviously happy with this as its what you've done, but what's you're reasoning that this is acceptable, and won't cause damage?

One thing I've thought of is the friction between the base of the cell and what its stood on, with foam only at the ends your reliant on half the pack being moved along its base so the foam can take up the expansion, I wonder how this affects things as well?

The solid spacers are basically PC board material without any copper. I'm not sure what damage they could cause.

I did put a layer of kapton tape on the sides and bottom of each cell to help insulate and reduce friction. I really don't expect much motion at all since I am not using a high charge or discharge rate, and I am not trying to stuff the cells full on each charge.