Another Cell Compression Thread, this time about foam

[Q-Dog]

I bought my batteries before I found this forum based solely on the capacity I needed to run my fridge and the fact they were in the US. If I had found the forum first I would probably still be trying to decide what to get. I'd probably also still be waiting on cells to ship from china.

My cells came with a couple of long zip ties and a small roll of double sided squishy tape to stick between them. I thought the foam was an odd choice until I saw the cells beginning to swell and the pack got over 1/4" wider the first time I charged them up. I want to add capacity down the road so may have to reconfigure and I didn't want to damage the cells trying to pull them apart, so I instead slipped some 1mm thick foam sheets between my cells. I cut the foam from some packing envelopes that got away from someone and floated into my yard after a thunderstorm. I cut them a little bit long and used slits in the tops to keep the bms wires organized. After seeing a couple of drop-in LiFePo4 battery teardown and reading a couple of posts here on the forum I modified my cell holder to fit snug around the cells and constrain them, and I haven't noticed the pack moving any more. These are 200ah cells. I imagine the larger cells expand more.

Also, before I found the foam envelopes in the yard, I was going to experiment with paperboard or cardboard or construction paper between the cells to allow them to move a little bit. I was also considering something like felt or fiberglass matt, anything to let them flex a little bit but remain constrained.

 

[Maast]

So are you guys talking about giving way to the batteries to expand ? I thought that we compress them so they won't expand, to prolong life..

We are talking about allowing them to expand, but just a wee little bit so the anode/cathodes doesnt move so much they're damaged. There is a small volumetric change when charging/discharging so we want to allow for that but not allow the cells to bloat and damage. (Really, this is a "how many angels can fit on the head of a pin" type of discussion.) It's perfectly fine just clamping them.

I would allow for cooling airflow by putting something like popcicle sticks between the cells though.

Along that lines I'm thinking strips of 1" wide by 1/8" thick VHB double-sided tape with 1.5"ish separation between the strips to allow cooling airflow, then compress with plywood/threaded rod, etc. The VHB tape has a high density foam core.

 

[lance]

Along that lines I'm thinking strips of 1" wide by 1/8" thick VHB double-sided tape with 1.5"ish separation between the strips to allow cooling airflow, then compress with plywood/threaded rod, etc. The VHB tape has a high density foam core.

This, from my limited understanding here, seems an excellent approach..... 3 birds, one stone. Reinforcing the structure so as to prevent movement/damage at the terminals/bus bars (especially important to me in that mine will be in an off-road vehicle), gaining some expansion room but, not too much, and allowing for cooling/heating air flow around the cells.

 

[Ironman]

I received an email today from Richmond in which he said:

I have checked inventory. I do not have the PF100 available. I can offer you PF40 at 4mm thick 7” x 24ft.
https://www.tapesolutions.saint-gobain.com/solutions/ev-battery/compression-tolerance-pads
Our minimum order is $300
If you would like to proceed, let me know and I will send you the credit card order form.

So that is the state of things on the PF100

 

[Honuz]

That what I said in an other thread, I use pieces of foam between each cells

I got a call today from my contact at St Gobain regarding the PF100.
His name is Richmond Powers and he has given permission to post his name and help the community.
He can be reached at 949-238-0482. We discussed thicknesses and he is getting back to me on this in a couple days. He said the big manufacturers St Gobain sell to use 2mm thickness, which is good because it won't inflate the battery pack size very much. He will also want to know the sq ft amount when you talk to him. A group buy may be a good thing too.
When I know more I'll post more.

I would say 2 mm is to thin, I used 5mm... But since I still did not dismantle the pack, I can't know if it's good enough.
I used foam that are used in some shipping boxes, it's called plastazote it's a closed cell polyethylene, the one we use as a mat when hiking.
By using this foam, my idea was to resist to cell expansion but let it happened when pressure is to high, kind like a spring, but the advantage is that cells do not move relatively to each others.... well... it's hard to mesure the tension on busbars.

 

[cinergi]

WRT worrying about cooling these cells .. my 280Ah cells increased all of 10F during a 220A discharge followed by a 130A charge. I wouldn't worry about it unless you're consistently pushing 1C through the cells.

 

[Zwy]

And how are you applying the needed compression . . . back to a torque wrench?

Simple, measure length of bare pack compressed to touching, then add material between cells. Run compression to the length of total material thickness added to bare pack length. Then compress 25% of total material thickness. Let's say 4 cells of 2" with 0.125" thick material between plus each end between cell and plate. That comes to 8.625" with 5 sheets of material. Compress to total length of 8.469" as an example. That is 0.156" compression which 25% of 0.625". Metric example use 200mm x 4= 800mm plus 4 mm x 5= 20 mm. Compress 5 mm for total length of 815 mm.

 

[Zwy]

I received an email today from Richmond in which he said:

So that is the state of things on the PF100

If you want to order, I'm in as I have several batteries to build yet. I'd do 12 feet.

 

[Capt Bill]

Hi everyone, been lurking in the shadows and finally decided to put up my first thread and post. I've read thru many of compression threads and can say there is much discussion about this subject. My background is automotive, I'll just leave it at that. This is how the EV manufacturers are handling cell compression with prismatic cells. https://www.tapesolutions.saint-gob...icles/foams-are-driving-ev-battery-innovation As you can read here, compression is important and how it is done is equally important.

One more article on this subject from the same manufacturer, although it is primarily direct toward pouch type packs but I feel it equally applies to prismatic cells also. https://www.tapesolutions.saint-gob...ssembly-robust-efficient-batteries-norseal-pf

Interesting Reads ... In looking at how those researchers are talking allot about EV batteries/ I am wondering if compression of LiFePO4 are much more important for Electric Vehicles as compared to stationary home solar battery banks ... Just wondering out loud; / ... while thinking if I eventually compress my 16 280Ah LiFePO4 cells on my next battery pack update, I would also want flexible bus bars. I am still wondering about real life benefits on my stationary cells that have very slight wave on sides / showing me my LiFePO4 battery cells sides are not exactly flat planned.

from interesting reads: " ... research indicates that optimal battery lifespan occurs when a moderate amount of pressure is applied to ensure electrical and thermal connections while the battery ‘breathes’ during its discharge and recharge cycles. ... help prevent delamination and deformation in pouch-type, actively-cooled, Li-ion battery packs, thereby keeping more EVs and hybrids on the road every day."

 

[dokmartin]

i'm in for some of the norseal pf100 4mm if a group buy happens

 

[curiouscarbon]

another option available https://www.mcmaster.com/norseal/

 

[zzyzx]

i'm in for some of the norseal pf100 4mm if a group buy happens

A tad expensive :

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[curiouscarbon]

i don't have a napkin handy so the math is happening here

the frey 100Ah cells i'm working with have a ~3mm air gap created by the plastic cell holders
cells are 5.25"/133mm wide and about 10"/250mm tall air gap
3/16"/4.7mm thickness, 4"/100mm wide, 36"/900mm long, strip available is like 90 usd.
75% face coverage (it's expensive, optimizing here)
900mm long strip / 250mm tall cell = ~3.6 cells per strip.
90 / 3.6 = ~25 usd per cell (combining scraps)
25 dollars * (n-1) cells OR (n+1) to include outside faces too
8 cell pack, cost added 25*7 or 25*9 usd or 175-225 usd per pack
4.7mm foam compress to 3mm about 40% compression seems ok.
% dimension reduction = % compressive strain?


frey pack 8 cells already cost up to ~1000 usd so +200usd

it's pricy but still really appealing to me. the plastic cell holders do not provide lateral friction to fix the cells in place, and the foam would provide an extremely big surface area to keep each cell laterally aligned

if i want to get really crazy high C rate later then can put down spaced strips and use the air channels between norseal strips to circulate air

this foam option is appealing, but still thinking about the implementation that's best for me

edit: the frey 100Ah cells do not require compression to prevent breathing of big cell face as far as i know, so the PSI is less important to me. benefit of increased structural integrity and busbar strain relief appeal to me. so that sets my design goals a bit separate from large alu case cells (200,280Ah)

edit 2: seems like the thicker mcmaster carr norseal is PF40 not PF100 because thickness

 

[Zwy]

another option available https://www.mcmaster.com/norseal/

I think I'll stick with what I used, https://www.grainger.com/product/USA-SEALING-Water-Resistant-Closed-Cell-497F84 $90 per sq foot is just a little out there. For that price one could just buy some new cells. If you look at the specs, ASTM D1056/2C3 for USA Sealing compared to ASTM D1056 2D3 for the Norseal on this page https://www.allsealsinc.com/ASTM-D1056-Quick-Reference-Guide.html the only difference would be one has oil resistance with medium mass change and the other is temp resistance.

 

[curiouscarbon]

10 usd https://www.amazon.com/Sponge-Neoprene-Thick-54-Wide/dp/B001FVG3CM i’ve used this as padding and insulation in past projects

enough for 10 separators, one dollar per separator!! lol

 

[Zwy]

10 usd https://www.amazon.com/Sponge-Neoprene-Thick-54-Wide/dp/B001FVG3CM i’ve used this as padding and insulation in past projects

enough for 10 separators, one dollar per separator!! lol

While it might work for padding, the item I used has a spec of 13 psi force exerted at 25% compression. The sweet spot for cell compression to extend cell life is right at the 9 to 13 psi range. Over that and cell life will be compromised, under it and cell life is compromised but is better than over compression and no compression. I spent some time researching various materials that would meet this spec, not just any neoprene foam meets the spec.

 

[Honuz]

i don't have a napkin handy so the math is happening here

the frey 100Ah cells i'm working with have a ~3mm air gap created by the plastic cell holders
cells are 5.25"/133mm wide and about 10"/250mm tall air gap
3/16"/4.7mm thickness, 4"/100mm wide, 36"/900mm long, strip available is like 90 usd.
75% face coverage (it's expensive, optimizing here)
900mm long strip / 250mm tall cell = ~3.6 cells per strip.
90 / 3.6 = ~25 usd per cell (combining scraps)
25 dollars * (n-1) cells OR (n+1) to include outside faces too
8 cell pack, cost added 25*7 or 25*9 usd or 175-225 usd per pack
4.7mm foam compress to 3mm about 40% compression seems ok.
% dimension reduction = % compressive strain?
View attachment 38321

frey pack 8 cells already cost up to ~1000 usd so +200usd

it's pricy but still really appealing to me. the plastic cell holders do not provide lateral friction to fix the cells in place, and the foam would provide an extremely big surface area to keep each cell laterally aligned

if i want to get really crazy high C rate later then can put down spaced strips and use the air channels between norseal strips to circulate air

this foam option is appealing, but still thinking about the implementation that's best for me

edit: the frey 100Ah cells do not require compression to prevent breathing of big cell face as far as i know, so the PSI is less important to me. benefit of increased structural integrity and busbar strain relief appeal to me. so that sets my design goals a bit separate from large alu case cells (200,280Ah)

edit 2: seems like the thicker mcmaster carr norseal is PF40 not PF100 because thickness

View attachment 38324

Far too expensive, I used foam from the bin....and seems pretty good.

 

[Zwy]

Far too expensive, I used foam from the bin....and seems pretty good.

I paid $10 per sq foot, needed around 5 sq feet for the 8S pack I built. I bought what I needed off ebay. I wanted something that had an actual spec for compression similar to Norseal, not padding.

 

[Honuz]

I paid $10 per sq foot, needed around 5 sq feet for the 8S pack I built. I bought what I needed off ebay. I wanted something that had an actual spec for compression similar to Norseal, not padding.

Yes I understand, I tested compression on mine and it felt pretty good, I have 4 different foam, I used the best. The one that do not compress at 150kg but does a little at 300kg.

 

[Tkhawk01]

Has anyone used foam and then polyimide tape. I'm thinking about using the foam that @Zwy used, clamping just past 25% compression with wood working clamps and then wrapping the tape around it.