Pouch Compression wiki review

svetz

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I wasn't aware you could compress jelly-roll (cylindrical) construction. Well, something to fix in the wiki assuming it can be done!
 
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svetz

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Jelly-Roll Construction
1618002679099.png

I don't know that you can compress a cylindrical jelly-roll cell, at least I can't think of a way to do it (pressurized oil bath?).
Update: There are a number of studies around compression of jelly-roll cylindrical cells, mainly because as they "swell", the pressure on the center increases and limits life. Probably not desirable to externally compress such a cell.

There are definitely pouch cells (see below) and they can be compressed. The wiki entry doesn't mention prismatic or jelly-roll, just "pouch" so hopefully no confusion there.

Pouch Cell construction
a-Core-electrode-separator-stack-of-a-prismatic-lithium-ion-battery-with-planar.png

,,,it was like unrolling toilet paper. It didn't look like pouches to me.,,,
It'll take someone more knowledgeable than me to explain why a prismatic would unroll like TP... I thought they were all pouch cells too.
 
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GXMnow

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My Chevy Bolt cells are pouches. They are all stacked in pairs with a thin aluminum cooling plate between each pair. The ends when in the car have a heavy stamped steel plate and long studs holding it all together. There was a plastic spacer in the middle separating it into 2 sections. I am only using one half, and only had one of the plastic spacer plates, so the inside end of my cell stacks was just the thin aluminum plate. I used a 1/8 inch thick aluminum plate on the end of that, and had to use a washer on the 4 corner studs to get it to lay flat. After 4 months of daily cycles, I noticed the 1/8 inch aluminum plate was bowing out nearly 1/8 in in the middle. I added a 1 inch square tube bar diagonally across the front of the plate with a small block right in the middle of the cells. I tightened down the bolts holding the bar until the 1/8 inch plate was just flat again. It actually did not take much pressure to pull it in to flat. But I now understand why the GM factory plates are not only fairly thick steel, they are also stamped to give it a vertical cross section like a series of beams across the cells.
IMG_8931.JPG IMG_8937 (1).JPG IMG_2408.JPG
Here is the bar across the front of the cells now. You can also just see them through the vented door. The acorn nuts are the corners of the cells. And the third pic is the factory Chevy mounting plates. Pouch cell certainly do need to be carefully mounted.

The prismatic cells are much easier to deal with, but they should still be constrained from bulging. When the cells are new and flat, you don't need much pressure, but the plates on the ends need to be stiff to not bend if the cells try to grow.
 

svetz

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Just to keep the thread on topic... it's about reviewing the wiki page on pouch compression. ;)
 

Paraterra

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IMHO following is not very clear:
"Below 72°F the effect is linear. Over 72°F the effect becomes exponentially negative."

Does the "exponentially negative effect" refer to the capacity fade that will become bigger over 72°F ?
Or to the effect of compressing ?
 

svetz

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IMHO following is not very clear:
"Below 72°F the effect is linear. Over 72°F the effect becomes exponentially negative."

Does the "exponentially negative effect" refer to the capacity fade that will become bigger over 72°F ?
Or to the effect of compressing ?
Thanks for the feedback!
I see what you mean... I went back to the source where @ghostwriter66 wrote:
For temp - and I know this was not one of the questions -- BUT 72 is perfect -- and actually its better to be at 52 than 82 .. although 72 is perfect - when temp starts going above 72, the UNHAPPY FACE scale starts going up exponentially as the temp climbs and when the temp lowers its more linear ... (if that makes sense). That also will affect the PSI.

But that didn't quite make it clear and quite possibly I paraphrased it incorrectly.

So I did some digging and found this paper (Lithium, but not LFP) and while the temperatures were extreme it had the table off to the right which seems to indicate compression isn't important at low temperatures in regards to fade, so I've updated the wiki entry ... hopefully clearer/accurate...
1618064173928.png
 
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Gazoo

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Just to keep the thread on topic... it's about reviewing the wiki page on pouch compression. ;)
I don't see how the jelly roll can be compressed. Compression helps prevent delamination of the jelly roll because it expands and contracts when the cell is being charged or discharged. That's my understanding.
 

Maast

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I don't see how the jelly roll can be compressed.
Because it's not cylindrical, its a flattened jelly roll. There's a video around here somewhere that shows a guy taking one apart and its a big flattened jelly roll with tabs spaced along it for the neg and pos terminals.

The upshot of all this is that yes, both pouch and flattened jelly rolls can be compressed. Whether they need to or not is up to the manufacturer to specify.
 

Gazoo

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Because it's not cylindrical, its a flattened jelly roll. There's a video around here somewhere that shows a guy taking one apart and its a big flattened jelly roll with tabs spaced along it for the neg and pos terminals.

The upshot of all this is that yes, both pouch and flattened jelly rolls can be compressed. Whether they need to or not is up to the manufacturer to specify.
I saw that video. There are also x-rays of jelly rolls. Still...I do not understand how it's possible to compress the jelly roll without crushing the aluminum case it's housed in, and I will add the cell is at a 0 SOC. I guess it would be possible if the cell has expanded and then some compression is applied to it?
 

Substrate

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Um, jelly-rolls are already compressed, like tightly rolling up newspaper. (remember those?). Heck people used to use them in a pinch as faux-fire logs which had to be tightly compressed otherwise you just set your fireplace - um, on fire. :)

Ridiculous example, but you can duplicate how tight things get when you roll up a magazine.

So compression is not really an issue with cylindrical cell batteries.

Pouch type prismatics need to be compressed for two reasons:

1) In case of a catastrophic overcharge event, you want them to vent, rather than expand like an accordion.

2) You also want compression to help prevent the individual plates of the pouches from becoming mis-aligned internally. High quality batteries have plates of slightly different physical sizes (get your micrometers out!) to help prevent dendrite formation from happening near the edges of the plates - and when plates that are expanding and contracting lose this alignment, there is typically an easy-to-cross temptation for dendrites to cross.

Other than plates getting out of alignment from skewing due to lack of compression, by not being tightly held against each other over the entire surface of the plates, there is a simulated loss of capacity, and also putting the material that IS properly compressed under more strain trying to pick up the slack. Known as "hot spots".

Thermal runaway is now possible. Hot spots get hotter, leading to more expansion .. rinse and repeat.

With sub-c loads, you may not see this happening. Subsequent hammering at high recharge rates exposes some of these mis-alignment and even dendrite issues (in extreme cases).
 

svetz

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Append the entry with information from the link in Frick's post:

Periodic Adjustment​

This reference indicates cell thickness has irreversible changes over time, so any compression should be periodically checked:​
The second change is the cell thickness increase from BoL (beginning of life) to EoL (end of life). This change is typically not reversible. This irreversible thickness increase of the cell is gradual and happens over repetitive charge and discharge cycles... If the cells are restricted more than optimal, the capacity fade occurs faster and at the same time, it can be detrimental​
 
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