Compression of pouch cells, info/data

[Hummina Shadeeba]

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 or fall apart.
I’ll post up specific resin I use later. A vacuum isn’t needed.

Think it’s wc-766 or maybe some other clear. I’ve done many in different resins. Even a very soft 80a duro is good if you’re just looking to hold cells together instead of building a structure.

 

[john61ct]

Note that "prismatic" often means "in a hard case" as opposed to raw pouches.

Traditionally from top LFP vendors, a hard dielectric plastic case, but cheaper recent products increasingly in lighter weight lower volume aluminum, which requires the pack builder to put rigid dielectric dividers between the cells.

Neither provide protection against swelling, both require strong compression.

As with pouches, I do not think huge strength is required while there is no swelling.

The key is stiffness, rigidity, **resistance** to the huge outward forces produced by cells' attempted swelling, containing those forces to prevent significant movement, say 3-5% at most.

 

[spinningmagnets]

https://iopscience.iop.org/article/10.1149/2.1121915jes#f1

Here's a PDF on pouch-cell compression. The cells were tested at "Mega Pascals", which is a pressure comparison to atmospheric pressure. If you add 0.10 MPa to anything that is at sea level, you have doubled the pressure it is under. Denver is the "Mile high City", and pressure experiments performed there must take into consideration that the item is naturally under 0.08 MPa (12 pounds per square inch) at that altitude.

0.10 MPa = 14.5-PSI (atmospheric pressure at sea-level is approx 14.5-PSI)
0.42 MPa = 61-PSI
0.84 MPa = 121-PSI

"...Due to their extreme volume expansion, Si/C-composites suffer from fracture or delamination and consequent capacity fading during the Li-ion cell operation. One approach to reduce the electrical contact loss and improve the performance is the application of mechanical pressure on the cell..."

Compress the flat sides of pouch cells to prevent de-lamination of the layers. Delamination initiation is most prevalent near the positive cathode, resulting in the cell current having less layer surface area to transmit the current through, which causes the remaining layer connections to run hotter, which cause a downward spiral of more expansion/more delamination/more heat concentration in the remaining contacts.

"...a comprehensive aging study of Si/C|NMC811 pouch cells is conducted with cells in different compression configurations as uncompressed and under flexible and fixed compression at pressure levels in the range of 0.08 MPa, 0.42 MPa, and 0.84 MPa..."

 

[spinningmagnets]

Side note: Here is a graphic that shows my understanding of what a "prismatic" cell is (I referred earlier to prismatic as a "folded pouch cell"). Its relevant to this thread because I don't like that the folded edges would likely have uneven compression, compared to flat pouch cells:

Here is a graphic from a patent detailing a prismatic cell, displaying the two most popular configurations:

 

[flippy]

the cheap cells are "folded" as its cheaper and faster to produce, the more expensive cells are layerd.

there is a guy that tore open a winston: https://www.wanhunglo.com/2019/citysax-160ah-lifeypo4-battery-discharge-and-teardown-whl-51

 

[liveforphysics]

Are there any published or derived specs on the compression pressure - too much is probably as bad as none

I communicated to a battery engineer from Zero motorcycles and and 100-psi is a helpful goal. They used flat pouch Farasis cells, and then potted them after testing. He mentioned anything is better than nothing, and not much of a benefit above that.

It's my understanding that prismatic means a pouch cell that is folded, but I don't have a reliable reference for that from the web. I've looked for a good pic to show. 404 not found.

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.

 

[spinningmagnets]

Awesome info. I stand corrected.

serious_sam, you were correct.

 

[nicobie]

Good to see Luke back. He's been missed. 8)

I wonder how the Li battery recycling gig is going? How do I invest in it?

 

[spinningmagnets]

I too, would not mind investing.

https://www.electricbike.com/start-up-redivivus-recycles-lithium-batteries-with-new-process/

 

[Hummina Shadeeba]

I heard how they freeze and then recycle the cells by grinding them up on the spot and seems the obstacle is how every car and accident is different and they have to get the cells out somehow. They need the jaws of life to get to the battery and seems fraught with obstacles and dangers doing that.

 

[liveforphysics]

You guys are too kind.

Our business grew so fast, we have the most beautiful two floor RnD facility now, and the equipment of my dreams to help make a beautiful future for our shared spaceship.

We are developing batteries for aircraft, satellites, long haul trucks, yikes, bikes, and grid storage for global clients on top of our recycling company.

Everyday I wake up and thank the Universe for the incredible opportunities to shine every day from morning till late night. 12hr days feel like a day off compared to our normal work hours.

Unlimited Love,
-Luke

 

[nicobie]

Someday, if you get a chance, We would love to see pics of your workshop.

I for one, love looking at other people's shops.

LoL (lots of love)

Nick

 

[liveforphysics]

Someday, if you get a chance, We would love to see pics of your workshop.

I for one, love looking at other people's shops.

LoL (lots of love)

Nick

I know you do Nick! You will love our shop, bring something you want to put up to 5,000Amps of constant current across, or a cell or pack you want to see discharged at up to 2,000Amps. We have vacuum calorimeters for precision thermal measurements of large modules and packs, and dataloggers that capture 240 thermocouples simultaneously. We have a destructive cell test chamber that's taller and wider than me made out of 3/4inch wall 316 stainless, and it lets us precisely quantify the gas volumes and pressures and gas species released. We have a LIBS that in 10seconds identifies every element on the periodic table from blasting a pin-prick sized area on the surface with a Q switched laser, and reading the spectra in the plasma light wavelengths emmited (Sci-Apps Z-300).

Im interested right now in finding someone who is serious about wanting to win unlimited class hill climb events in under 1min duration range. I've played enough with my 4500psi compressor and air tanks and solenoids jetting around the RnD lab in my office chair to see that cold gas thrusters work to give you downforce when it's needed to keep each tire in contact with the ground delivering thrust. The nozzles are on the suspension out towards the wheel hub so it doesn't pancake your suspension when it activates. I would be honored for the opportunity to make a powertrain that wins the Dubai sand dune hillclimb Unlimited category if a team is interested in sweeping every event they enter.

Unlimited Love,
-Luke

 

[nicobie]

Hot Damn!

I knew it would be something special but I sure didn't expect this.

You should be proud!

That's one hell of a PIF you are offering. It somewhat reaffirms my faith in mankind . However I should have known that you would pay it forward. If only I was a few years younger...

Luke for the win

LoL

 

[liveforphysics]

Hot Damn!

I knew it would be something special but I sure didn't expect this.

You should be proud!

That's one hell of a PIF you are offering. It somewhat reaffirms my faith in mankind . However I should have known that you would pay it forward.

*.* for the win

Im grateful for you driving me to all those distant ebike races when I had been up working during the day at work and at night preparing various deathbike iterations. You driving me was like a guardian angel for me those times I was exhausted delirious and would have still tried to drive myself. I am in your debt my friend, you get a special tour where you can bring something you want to see tested to its limits, and fuse, connector, contactor or cell I will setup on the appropriate fixture and test chamber and run it for you.

Unlimited Love,
-Luke

 

[eMark]

• INTERESTING THREAD

3-8psi seems to give best cycle life and long term low impedance for many pouches made today.
...
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.

Just to clarify (IMO) this thread has more to do with the need for "raw performance" when it comes to the degree of pouch cell psi compression. Some might categorize this degree of psi compression necessary due to what some may see as a form of pack "abuse" from beginning to EOL.
(see TrotterBob's reply p.1)

Depending on degree of "abuse" it's possible the pouch psi compression could grow to more than 8psi by EOL with cheaper cells and/or optimum "raw performance".

What would be helpful is a device that would measure the growth in psi compression at say: 10 cycles, 50 cycles, 100 cycles, 200 cycles (until EOL). In other words 3psi might not be reached until 5-10 cycles (IF some puffing is a good thing) and 8psi not until 100-200 cycles.

Is there a digital readout device to track psi compression through the cycle life? Such a device would allow an astute operator to track and even tweak the psi compression depending on the degree of "raw performance" at say 10 discharge cycles. And again tweak at 50 or 100 discharge cycles depending on the operator's knowledge and need for optimum raw performance and cycle life longevity.

BOTTOMLINE: IF there is no way for an ebiker to measure the psi (discharge) compression during life of the pouch pack then 3psi to 8psi is meaningless. Also, IF some puffing is good then wouldn't there be closer to 1psi at the beginning with 3psi only after say 5-10 cycles.

Lots of ways to achieve this, but the challenge is in starting out at 3psi

What is one KIS approach to achieving this and the challenge of tracking the growth in compression (to 8psi) during discharge (raw performance) until EOL ?

 

[TrotterBob]

My comment was aimed more towards lipo cells which under normal use should not puff at all.

I dont have enough experience with lifepo4 cells to say the same. But I suspect that keeping them fully charged all the time is in fact abusing them.

Just wanted to clear that up since I've been called out for my previous comment.

 

[TrotterBob]

Found a nice video on the topic. Not sure if this is accurate but it seems to make sense. Skip to the 15:25 mark for the spoiler.

[youtube]inyEPezasE4[/youtube]

 

[spinningmagnets]

I followed youtube suggestions, and found a video of this model of cell being disassembled. The jelly roll is arranged like a roll of toilet paper that is being flattened on two sides (*Fig3, in the previous posted pic). The manufacturer calls these "prismatic".

This is why the squarish aluminum can is slightly pushed-out near the corners, and slightly depressed in the center of the broad flat faces of the sides.

I can see why a large capacity cell would be slightly cheaper to manufacture this way.

I don't know if a low C-rate LiFePO4 chemistry is less prone to layer delamination, compared to Li-NCA/Li-NCM, but I think there are too many differences to be helpful with the much smaller pouches that we use to build a pack for this video to be useful.

 

[flippy]

Found a nice video on the topic. Not sure if this is accurate but it seems to make sense. Skip to the 15:25 mark for the spoiler.

[youtube]inyEPezasE4[/youtube]

what an utter dumbass.

"you dont need compression but you do need to prevent the cells from expanding".

no shit sherlock, that is what compression does.....

TL;DR: you need compression.

 

[john61ct]

PSI is not worth that level of attention in real life.

No puffing is never "a good thing". The only point of compression is restraint, preventing movement beyond a few mm.

Like preventing a short across terminals, basic necessary part of building your pack, that is all

Nothing to do with "delivering top performance"

3-8psi seems to give best cycle life and long term low impedance for many pouches made today

 

[john61ct]

"you dont need compression but you do need to prevent the cells from expanding".

… that is what compression does

TL;DR: you need compression.

Yes, this, that is all.

 

[john61ct]

I suspect that keeping them fully charged all the time is in fact abusing them.

True for every lithium chemistry.

But when cycle life is decades, and/or the other longevity factors are "more abusive"

then that one factor may not be so influential, if the use case requires it, so be it.

Like high C-rates, sometimes there is no choice, just accepted.

 

[eMark]

My error in using "puffing" ... as puffing is a NO NO. What i should have said is "expansion" as it relates to this post which could use some further explanation (underlined portion) ...

liveforphysics wrote: ↑May 16 2021 4:44pm
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.

That's indeed a mouthful to shallow/digest and apparently only of concern for certain DIY pouch (?folded?) builds such as this DIY diagram and possibly the following photo ...

 

[flippy]

I suspect that keeping them fully charged all the time is in fact abusing them.

True for every lithium chemistry.
But when cycle life is decades, and/or the other longevity factors are "more abusive"
then that one factor may not be so influential, if the use case requires it, so be it.
Like high C-rates, sometimes there is no choice, just accepted.

this is why cylindrical is natually more robust and durable, every cells has the perfect compression casing built in.