What is acceptable Cell distortion for LF280K?

[BipedalPrimate]

Have just received a shipment of 8 LF280K cell from Amy at Luyuan.

Unpacked first box of 2 cells and noticed they show some convexity on long side and concavity on short side.

Interestingly, the convexity on one side not directly opposite convexity on other side.

First picture shows top of cell on rest of photos & how distortion was measured.



Next pictures are taken from bottom left corner of first picture, moving anti-clockwise to each corner in turn.


Last picture is short side of positive terminal trying to show concavity.


Objectively, I don't think this an issue but thought I would check with the team that is more experienced than I.

So, two questions:-

1. What is an acceptable level of distortion of newly shipped B Grade cells?
2. What method of compression. if any, should be used when combining these cells?

Further cell information is:

1. Manually entering QR code into decoder reveals that they are EVE Power cells, Model Code LF280K (76), Date of Manufacture 22 May 2023.
2. Cell Voltage is 3.2888V
3. Cell IR is 0.22m Ohms

 

[sunshine_eggo]

All 9 of my cells were like that. The case is so thin, it's not going to be flat on any side.

If you can manually squeeze the cells together by hand and eliminate almost all of the gap, nothing in your pictures gives me concern.

 

[Brett V]

I have new cells that look similar, but not to that extent. I'm calling it room to grow...

Lots of differing opinions on here about cell compression. Mine sit on a shelf and have a relatively low C rate, I've just bound them snugly using fiberglass filament tape with good results.

 

[BipedalPrimate]

I've just bound them snugly using fiberglass filament tape

Is there any reason you haven't bound them in the middle as well as top and bottom?

My understanding is that cells will expand mostly in the middle.

 

[Warpspeed]

Cells have VERY thin walls for cost/size/weight reasons.
Cells are totally sealed, and can generate fairly high internal gas pressure under certain extreme operating conditions.

Only purpose of the aluminium housing is to contain the electrolyte.
If the cells swell hugely, the internal volume obviously increases, and the electrolyte level will fall, exposing part of the internal structure.
That is why we clamp our cells to prevent swelling.

People become paranoid about swelling, OMG my cells have swolen .001 inch, are they now useless ?
The walls are so thin, some distortion is perfectly normal to expect even brand new, and it has no effect unless the swelling is so gross the internal fluid level becomes dangerously lower.

Its also perfectly acceptable to clamp swollen cells under pressure to return them to their original shape.
This is successful only if this "correction" is carried out very quickly after the swelling accident.
Its absolutely no good pressing old swollen cells back into shape, the internal damage will already be done, and be permanent.
I have actually done this.
I had some Winston cells swell to the point that they were up to about an inch and a half fatter across the flat sides after a severe under voltage accident. The swelling was just eye popping when I saw it.
That was a very stressful event for me !!

But I figured, what the hell, nothing to lose by trying.
So I clamped the bloated cells between 6mm steel plates, and threaded rods, and FORCED the cells back into shape.
This was all done within hours of the swelling event.
Now three years down the track, and those cells are still working fine without any noticeable loss of capacity.
They have fully returned to their normal natural shape as well.

So the moral of the story, just arrange some totally rigid clamping, and just nip up the bolts to resist any abnormal rise of internal gas pressure.
No need to apply any static clamping force, and fitting springs would definitely be a step in the wrong direction.
The whole idea is to keep the electrolyte level exactly where its supposed to be.
Its as simple as that.

The super thin aluminium cell housing the cells come with is simply not up to the job.
It was never designed to be.
YOU have to surround the whole battery assembly with something totally rigid and unyielding that IS up to the job.

 

[Brett V]

Is there any reason you haven't bound them in the middle as well as top and bottom?

My understanding is that cells will expand mostly in the middle.

I’m just binding them together, not supplying a rigid support to the end of the cell. The first battery I built only had 1” filament tape and after two years, the cells are still the same as when new.

 

[Warpspeed]

If the cells are never over charged, or over discharged, and the charge and discharge rates are relatively low, the internal gas pressure remains very low too.

Its only under some conditions of extreme use that the internal gas pressure goes ballistic.
Not sure what those conditions are, but I know for sure, one over discharge event will definitely do it.

 

[googdot]

I have new cells that look similar, but not to that extent. I'm calling it room to grow...

Lots of differing opinions on here about cell compression. Mine sit on a shelf and have a relatively low C rate, I've just bound them snugly using fiberglass filament tape with good results.

I'm planning on doing the same...did you use any epoxy sheets or foam tape or anything in between the cells?

 

[Brett V]

I use a fiberglass silicone grill mat. Cut squares to fit between the cells.

https://www.amazon.com/gp/product/B09TZZ2KX6/ref=ppx_od_dt_b_asin_title_s01?ie=UTF8&psc=1

 

[googdot]

I use a fiberglass silicone grill mat. Cut squares to fit between the cells.

https://www.amazon.com/gp/product/B09TZZ2KX6/ref=ppx_od_dt_b_asin_title_s01?ie=UTF8&psc=1

Thanks for the reply and info. I was planning on just using foam tape (which I saw in the breakdown of the SOK batteries).

But it seems like most builds I read about are using epoxy sheets or the fiberglass mats you mentioned.