Should pouch cells be in constant external pressure?

[rg12]

I have read somewhere that pouch cells maintain their impedance and capacity or something better when they have static external pressure.

My first logical thought is that if the cell wants to expand and we are holding it from doing so, wouldn't it be more prone to bursting?

I also see many manufacturers of both li ion pouch cells and lipo cells that use fiberglass tape which cannot be stretched and I'm thinking about how the cell will be held really tight only in two spots if it wants to swell.

Would love any input you guys have on the subject

 

[fechter]

That's what I've heard also. Seems to be particularly true of LiFePO4 cells. Compression seems to prevent the cells from puffing in the first place. With Li-ion or Lipo, I'm not so sure. I have seen Li-ion pouch cells that puffed dramatically and failed, but not sure if compression would have prevented it.

 

[spinningmagnets]

Everything I've read indicates that adding an even pressure to the sides of pouch cells is a benefit. I have seen tech data on using as much as 100-PSI, though others wrote that they used as low as 20-PSI.

 

[DaLanMan]

The way it was explained to me when it comes to expressing transit vs outgassing (the first is what happens when you squeeze something with the lid offf. it exits via path of least resistance) whereas Outgassing only occurs if there *is* a release path. so if you have pressure across the entire surface which is greater pressure than the pressure level at which outgassing *can* occur you won't get it.

So they only fart if there is room in the box, otherwise they just keep it to themselves.

 

[999zip999]

I believe that you should keep them from swelling but as far as putting a hundred PSI on them maybe in my race car but not in a 80 to 100 amp demand on 150 to 200 amp cell. As in ebikesca needs.
So A123 20ah cell with stiff plastic with reinforced duct tape. Held up for me for 9 years at 80amps. I did have the bolt together factory plates but they way a ton. So game away.
I know you guys used to love the lipo 4S hard packs. Where those really that strong cuz I thought if they going to Puff they're going to puff.
Wow just like me if I want to Puff I'm just going to puff.
As far as DeLanMan was saying boy if you are going to far you are going far. Just give us a heads up. Little stinker.

It depends on the quality of your cells the internal resistance and are you just flat out abuse them like you stole them.

 

[amberwolf]

If you use stiff enough end plates on the stack for the largest-area-surfaces of the cells, as all of the cell modules I've seen images of show, where these plates are then compressed together with the pack between via tension bolts or other mechanisms that allow specific tensioning amounts (and thus surface pressure of compression), you'll get sufficient compression and even pressure across the whole required surface of the cells.

If you are having outgassing/swelling problems on the cells with the correct pressures applied per the cell specification sheet, the cells are probably defective, damaged, failing, or being used outside their actual capabilities (regardless of what a seller says they can do).


If you're not using the cells near their limits of current or voltage, and they are well-made, you may never have any swelling at all even without compression. This is the case with my ancient EIG NMC C020 cells, for instance.

Cells people have posted with swelling problems usually also indicate there are cell failures, and sometimes mention cell (ab)use outside or at their limits, etc. Whether the swelling in their cases is a result of no compression or other factors, or a combination, is unknown.

There's a few threads people have shown compression methods in; like this

Compression of pouch cells, info/data

Advantages of pouch cells … Easier assembly. Disadvantage… Lower energy density than 18650 (usually). and they blow up. if that is an advantage or not depends on who you ask here, some people actually think that puffing pouches is actually a good thing.... :confused: I've heard that having...

endless-sphere.com

I've posted images of various prebuilt packs and modules (found on google) in assorted pouch- and prismatic-cell battery threads, but the forum search is terrible and can't find them. :/

Same thread has this

Compression of pouch cells, info/data

There’s lots of places that pot batteries. Luna does one https://lunacycle.com/luna-wolf-v2-52v-battery-pack/ Use a polyurethane that’s somewhat softer. I forget exactly the duro I use but confirmed to be the same resin by other places as said by bjbenterprises. This is not going to crack...

endless-sphere.com

3-8psi seems to give best cycle life and long term low impedance for many pouches made today. This is just enough to get the micro formations of gas to migrate into the upper portion of the pouch in the foil weld region. The gas doesn't conduct ions, or let the active material contacting it store energy. You need the clamping load to be uniform or its shorter life than unclamped, as the pressure hot spots are local ionic pathway minimums, so active material is used non-uniformly.

Lots of ways to achieve this, but the challenge is in starting out at 3psi or so, and not climbing beyond say 8psi by end of life, as the Z axis of the cell stack you're compressing should permanently grow by 11-13% by EOL, so all of this expansion displacement over the cells life must be compensated for in the initial starting stackup.

It's a challenge due to the widely different compression set behaviors over time of compliance foam sheet materials. This let's the datasheet compression set values get you into the right family of materials, but then long term testing needs to be done on your individual pouches expansion behavior with accelerated aging to confirm you left the required expansion stackup height.

There's a post here

A123 pouch cells LIFePO4 balancing issue

Hi, I would like to ask you. I have A123 LiFePO4 pouch cells 20Ah on my ebike. I bought them from Cina, from Lithium Batteries,Motorcycle Batteries,Solar Batteries,KOK Power Limited. They got me very good price, two years before. When I got them, I was amazed. All of them had exactly 3,300 V. I...

endless-sphere.com

with a datasheet link to A123's specs.


This thread is from someone that worked at A123

A123 20Ah Pressure Thoughts--and bits for experiments.

I thought it might be productive to start a new thread dedicated specifically to this topic. The big(moose) A123 build thread below seems to have gone quiet with the revelation of just how important it is to put at least some nominal level of evenly applied compression force to these cells...

endless-sphere.com

 

[rg12]

Thanks for all the info guys.

I went through the threads you mentioned and after working with cylindrical cells for 10 years it seems that im getting into a ticking time bomb business and I don't like it.
All that puffing and compression changes over time...
How do they do this in cars and other applications?
It seems so unsafe compared to cylindrical.

 

[DaLanMan]

In cars, they have monsterous rigging. Have you ever looked at a tesla cel? I mean as hard core as that is, at least you will recognize it's basic form factor.. look at the Nissan Leaf (a pouch cel IIRC) they are all all kinds of hard core, I have thought about buying them in lightly used states.. It is just a bit daunting...

 

[amberwolf]

Cylindrical also has compression and pressure changes, etc., it's just all inside the cylinder, which acts as the compressive device.

 

[rg12]

Well I had a big plan on building packs from pouch cells for surron bikes but now am very skeptical about safety issue pottentials.
What do you guys think?

 

[DogDipstick]

....yes

3-5psi across the face is what aI use. This is not a trivial amount of force over such a surface area. It is calculated and torked into compression with my banding tools. I have read about certain results in papers of upwards of 15psi and even in vacuume environments... However I go nowhere near that level of compression. 3-5psi is my goal.

Banded at 50% soc.

Here are some blocks, before banding, and after.

 

[DogDipstick]

Well I had a big plan on building packs from pouch cells for surron bikes but now am very skeptical about safety issue pottentials.
What do you guys think?

They are very safe and the bet out there,. Longest lasting, best cycles, best power, best best best. they dont die. You die first violently, in a crash... or you give them to a kid when you get bored. I do not know why anyone would use anything else.

LukeW. ( LIVEFORPHYSICS ) said ten , wait.. thirteen years ago...

"Also, as far as life-cycle for LiPo goes, the LG presentation I sat through yesterday showed 50,000 cycles at 40% DOD, 40,000 cycles at 50% DOD, then a big drop to 20,000cycles for 60% DOD, and something like 7-8,000 at 70%."

Also Luke:
" Buy a volt battery (hopefully from a wrecked one at an auto wrecking yard) and put together a 44Ah 24s pack made from those delicious LG chem NMC cells.
That would be all the ebike any reasonable person would ever need IMHO. 10's of thousands of cycles, never even be able to strain the setup, it would always run cool to the touch everywhere."

Also Luke:

"Original Volt only permitted access to ~8kWh of the 16kWh pack. They do this because it's much easier to get >10k cycles if you only do 50% DOD cycles."

Also Luke: ( Speaking to Dr. Bass,)

" They have extremely good safety, 10,000's of thousands of cycles, and a usable C-rate for Ebikes."

.... he said all that ^^^ on this forum years ago. he was right. I stillhave not had a pack die, get out of balance, get returned, not output to the MAX of the controllers I can afford, or give any problems.. even for eyars in all seasons with no bms and just blunt bulk charging every day, balance 4x a year... balanced to 0.0005v~ +/-.

 

[DogDipstick]

Everything I've read indicates that adding an even pressure to the sides of pouch cells is a benefit. I have seen tech data on using as much as 100-PSI, though others wrote that they used as low as 20-PSI.

NASA published some riff raff once with explicit testing over the range of 2-15psi in varied environments such as low gravity or vacumme.

100psi is ridiculous.... lets look at that.

lets say the cell is 6x10... ( like a chvvolt)..60 sq inches.. to ave "100psi" on that face.. you would need a force ( weight ) of 6000lbs on the 6x10 face. IDk. im saying shannaigans of it didnt happen. cell would crush. Mr. Ron Magnets, our friend.

The nasa papers and shtuff say 2-15psi tested and graphed extensively, OCV over time vs atmospheric.

In my case, 3psi goal, 6x10 cell... this is 180lbs force on the face (still alot, not trivial as i said above.. but aint no 6000lbs lol) divided by the two faces and the four straps in tension, I can easy meter the face plate force applied to the cell.

I read here from experts to restrain @ 50% SOC.

 

[DogDipstick]

If you use stiff enough end plates on the sta..

If you are having outgassing/swelling proble.


If you're not using the cells near their limits of current or voltage, and they are well-made, ...

Cells people have posted with swelli..

There's a few threads people hav.]

.....the one sure fire way to make a ell ' swell" or ' puff" is to let it hit LVC repeatedly and or hard... ie... getting that cell near 0v. Guarantied puff. Hitting low voltage on the cell is abusive.

NEVER do this... ( stay using only 50%-70% SOC ) and you wont have a puffed cell.

Ok Im done. lol.

 

[rg12]

....yes

3-5psi across the face is what aI use. This is not a trivial amount of force over such a surface area. It is calculated and torked into compression with my banding tools. I have read about certain results in papers of upwards of 15psi and even in vacuume environments... However I go nowhere near that level of compression. 3-5psi is my goal.

Banded at 50% soc.

Here are some blocks, before banding, and after.

I see that you use a heatshrink for pressure?
How much of a pressure do they add?

I have seen a certain manufacturer using a schrader valve on the alloy cover of the battery pack sealed metal case and they recommended to put in 2-3 PSI for helping with water proofing which in itself doesn't make sense to me other than to test of leaks rather than helping prevent leaks.
Anyway, would it help adding pressure to the case?

 

[DogDipstick]

I see that you use a heatshrink for pressure?
How much of a pressure do they add?

I consider the heatshrink rather negligible in the force it applies. I dont try to attain any real value ( in psi, kPa, etc) with the heatshrink.

....the real forces come form the " Bandit" straps. Uline banding with a crank tool. banding on the faces of plates inside ( under ) the heatshrink. Poly banding..... cranked to a certain tension with a screw and handle duck bill tool that grabs a strap, and tensions the other against it with accuracy.

Here is pic of me tightening up bands . You can clearly see the heatshringk does nothing (bows outward when the plates do their force application). the hea t shring just a protective cover, no mechanical force is attained by it.

 

[DogDipstick]

Using the above tool, I can dial in the application of force to the plate(s) that does the pushing, restraining. The torque on the crank tool directly relates to a tension value that directly relates to a application of force value divided against the four straps.

You know. Numbers.

... not just will y nilly heat shrinking... but I try to get an avtual value to the psi attained to the face.

Ive tested the force with a few differnt ways of measuring pressure across a surface ( like a scale)(.... yes, I banded a plate to a scale to see how much force was over that area of the plate).

 

[DogDipstick]

I have seen a certain manufacturer using a schrader valve on the alloy c
Anyway, would it help adding pressure to the case?

Yes, that would work, for pressure is pressure.

Right now, you are in a vessel and it has 14.7 lbs of air over your head and all around you from this pressurized vessel. ( Earth). Gauge pressure doesnot show this but Absolute pressure would.

The box would nee a good seal though.

it helps with water ingress to0. cause a sealed vessel doesn't let water in.. esp if the interior of that vessel is pressurized above atmos .

But then the problem becomes the pressure escaping.

this is what I use to band the packs. Uline poly banding.

Uline Poly Strapping - 1/2

16 x 6" CORE Strong polypropylene strapping is 1/2 the weight of steel strapping. Easier, safer and less expensive to use. Embossed surface. ULINE offers over 41,000 boxes, plastic poly bags, mailing tubes, warehouse supplies and bubble wrap for your storage, packaging, or shipping supplies.

www.uline.com

Here; read this.
"Tolerance of Li-ion Pouch Cells to Varied
Space Environment Pressures"

"Under restraints, the performance of the cells at reduced pressure
And vacuum remains similar. The performance for both without cell restraints
is very poor"

https://ntrs.nasa.gov/api/citations/20130000451/downloads/20130000451.pdf

 

[rg12]

Very informative, thanks for all the references!
I'm working with a tech company to develop cases, they can do anything as they work with rockets and crazy stuff so as long as it is not too high cost I would maybe go with the air pressure.
What PSI do you think would be helpful for pressurizing the case with the cells?

 

[DaLanMan]

I would go for strapping and the uniform mesh aluminum alloy, it is in essence 2 thin sheets of aluminum with a gridwork of triangles hooking the 2 sides together, stuff is ridiculously strong. I got the supply I have from an old contract, they were done with a bunch and were gonna toss it. I rescued it and gave it a good home.

If ya need me to, I can probably track down the info on where to get it besides the back end of a warehouse.

 

[rg12]

I would go for strapping and the uniform mesh aluminum alloy, it is in essence 2 thin sheets of aluminum with a gridwork of triangles hooking the 2 sides together, stuff is ridiculously strong. I got the supply I have from an old contract, they were done with a bunch and were gonna toss it. I rescued it and gave it a good home.

If ya need me to, I can probably track down the info on where to get it besides the back end of a warehouse.

Unfortunately im very tight in measurements already so I cannot add thickness.
Maybe got 1mm free on each side.

 

[j bjork]

Using the above tool, I can dial in the application of force to the plate(s) that does the pushing, restraining. The torque on the crank tool directly relates to a tension value that directly relates to a application of force value divided against the four straps.

You know. Numbers.

... not just will y nilly heat shrinking... but I try to get an avtual value to the psi attained to the face.

Ive tested the force with a few differnt ways of measuring pressure across a surface ( like a scale)(.... yes, I banded a plate to a scale to see how much force was over that area of the plate).

What do you use on the cells to get uniform pressure on the surface, it looks like just relatively thin plastic sheets?

 

[DogDipstick]

What do you use on the cells to get uniform pressure on the surface, it looks like just relatively thin plastic sheets?

There is a 0.20 inch, or 0.250 inch, thick poly carbonate plate: on either face of the cell(s), that the straps apply force to. 6 x 10 x 0.250. I build the core, place a plate top and bottom.. and wrap the heatshrink. THEN the straps apply force on the plate. The plate distributes the force to the cell. The plate(s) live(s) under the heat shrink covering.

Increase to battery dimension (width) by 040 or 0.500", but I find it good protection and gives a good stiff surface to apply the restraint, constraint.

Here in this picture you can see the two ( black, ) plates ( these are 0.20 inch thick) under the covering. I try to buy clear or transparent heat shrink too: One item of note. So I can inspect over time. These packs then get the bands for further restraint. These packs are currently on my bike.

 

[rg12]

I am making a pressured box for li ion pouch cells and would like to know the recommended PSI to push into the boxin order to have enough pressure on the cells.

I have calculated (could be way off) that 1PSI will put about 40kg of pressure spread across the whole area of a cell based on a 36x10x1cm cell.

But what if I have 20 cells in there? how is it calculated if I want to know how much pressure it will provide from all directions on 20 cells?
Pressured air will press from all directions, not just the flat parts but also from the sides.

Would be happy for any input on this.

Roy

 

[TorgueRPM]

I don't think you need to put any pressure on the sides of a stack of pouch cells, just 2 endplates