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EVE-280 cells should these be clamped tight or spaced for expansion?

It depends on your desired compression tolerance range from 0-100% SOC the greater the total spring compression distance, the less sensitive it is to changes in pack length due to SOC. Its fairly easy to calculate.

pack length change /Total spring compression distance. SO for a 16 cell stack that's something like 8mm max? So then a spring with 16mm of compression would result in a 50% tolerance. So if you set the spring at 50% SOC, then you would get +25% force at 100% and -25% force at 0%.
Yeah ... with a longer pack ... a longer spring might be better because of the greater potential for expansion and contraction.
 
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You can also stack two springs (or put one on either end). You need double the compression distance (same for each single spring), which results in cutting the sensitivity in half.

Really I think plus/mins 25% is likely just fine based on the info we have (secondhand) from the EVE engineering dept.
 
It depends on your desired compression tolerance range from 0-100% SOC. The greater the total spring compression distance, the less sensitive it is to changes in pack length due to SOC. Its fairly easy to calculate.

Pack length change / Total spring compression distance = % change in clamping force.

So for a 16 cell stack that's something like 8mm max? So then a spring with 16mm of compression would result in a 50% tolerance. So if you set the spring at 50% SOC, then you would get +25% force at 100% and -25% force at 0%.

If this range is too great, simply choose a spring which requires more total travel to reach the desired clamping force. lets say 32mm. so 8/32=25% or plus/minus 12.5%

Of course I am just using ballpark numbers, and the cells may not expand that much at all. Remember the springs solid height needs to be avoided to prevent the clamping force going very non linear.

I think you have clearly articulated the jumbled still-forming thoughts I had in my head. Thank you.

How about the second case, what is the effect of parallel 'rows' of cells on calculations?

This
Screenshot_2020-11-13 PowerPoint Presentation - LiFePO4 Cell Configurations pdf(1).png

vs this

Screenshot_2020-11-13 PowerPoint Presentation - LiFePO4 Cell Configurations pdf(2).png
 

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Really I think plus/mins 25% is likely just fine based on the info we have (secondhand) from the EVE engineering dept.
Don't forget A123 as well (also second hand via Dacian and Ian George), but the EVE info and the A123 info corroborate each other.
 
I think you have clearly articulated the jumbled still-forming thoughts I had in my head. Thank you.

How about the second case, what is the effect of parallel 'rows' of cells on calculations?

This
View attachment 27465

vs this

View attachment 27467
Assuming you have a single set of rods in the middle of the double row pack, then the center rods simply need to have two times the compression force of the outer rods.
 
Assuming you have a single set of rods in the middle of the double row pack, then the center rods simply need to have two times the compression force of the outer rods.
While that would result in the correct total force .... If you are going to only have 6 rods, it seems to me that it would be better to have the force spread equally over all 6.
 
If the value for a single cell is 300KGF fixture, is it correct to say that that value remains the same for any number of cells in a row, but if you introduce a second or third row, your target KGF increases from 300 to 600 or 900 (while the PSI target would stay the same since that factors in area)?
 
If the value for a single cell is 300KGF fixture, is it correct to say that that value remains the same for any number of cells in a row, but if you introduce a second or third row, your target KGF increases from 300 to 600 or 900 (while the PSI target would stay the same since that factors in area)?
That's the way I see it.
 
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If you have one end locked in place and each cell expands 2 mm. Then 16 cells in a row will expand 32 mm
The weight of the cells cause some pressure so they won't move that much. The shape of the cells will take up some space.
I know the end one won't move 32mm but it will move more than 2 mm and the pressure will be more than if you checked just one cell.
 
I am looking at these.
Not a great choice IMO it would not allow for expansion and compression of the cell. The last thing you want to do to these cells is over compress as you can see from the charts a few pages back.

Something like this with a longer working range would be much better. If you compressed this spring to around a 1/2 inch of compression @ 0SOC that should put you at around 7psi of compression and leave around 3/16 for expansion. Max load of that spring would be just over 13psi. (calculations for 4 springs)
 
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My 2 cents:

We all know the recommendation:
300kgf compression force.

It supposed to give even 1000 extra cycles, for what those numbers are worth anything.

I know from my own experience that it highly dependent on the cell.

I have 48* 152Ah, in March my Idea was bigger is better..
First indent was the 120Ah cells.
These where a fraction more expensive and same casing size......

- that matter a lot.
You can not keep on piling sheets into a casing without getting in the danger zone..

They needed to be compressed, if not they will bloat / delaminate.

My 280Ah came a few months after the 152's, and together they have made up 736Ah battery.

It never was overcharged, never undercharged, kept between 15 and 85%.

After 150 days I'm ready to make my new Powerwall installation...

150 days ago most of the 152Ah where still flat.
I ordered 3 replacement as I knew 3 where starting to bloat a little.

IMG_20201114_195348_copy_750x1000.jpg

All the 152Ah are bloated / delaminated.
And all about the same.
They have 1.25mm (in total 2.5) bloating.

The 280??
IMG_20201114_195355_copy_750x1000.jpg

0.0 mm, no bloating at all.

I'm in day 4 of building the new Powerwall.

When finished I'll post the progress.

Then this thread popped up again..
I could not let it pass to show what not compressing can have for result.
I have no idea how much capacity I lost due the delaminating.
Perhaps I'll test some day one to know for sure.
I guess 10% or less.


Should you compress???
Absolutely.
Even if your cells are not "overfilled" like mine, it still won't hurt to compress them

And allowing expansion??
That would be stupid.
The cells are make of sheets LiFePO4.
Once they get space between them, it doesn't revert.
It's not a balloon...
You can't "de-bloat" or re-compress cells.

There is air inside what we all know is really good comprassable.
If the sheets need to expand due to temperature difference, it can do within the housing.
Sure, you should not smash the cells.
Too much of everything is bad.

Allowing space between the cells for expansion...

Look at my lovely 152Ah...
That is what will happen.
Without over charging, without undercharged, without too high C rates.
On average C0.1..max C0.2
 
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I’m building a 16s2p bank devided into 4 sections of 8 cells each. Expansion would then stop at a maximum of 4mm, but that would still introduce alot of unneccessary tention to cells being connected to the same busbar? Then it would be better to use cables instead. If using busbars, wouldn’t it be better to just add the ”correct preassure” to a pack at 50% SOC?
 
When you attach the supplied busbars at their full limit, you have approx 3mm space between the cell walls on the xuba 280AH's. I packed this with a compressible insulation material and tightened the pack between clamped end caps. This keeps the pack rigidity overall, but allows for very slight expansion of the cell at full charge. This means that at the outer edges, where the aluminium casing is strongest, it wont change dimensions, so pack remains tight, however, at the larger side panels, there is a very slight 'give' in the compressible insulation. I reckoned that, as regards heat, cell against cell would trap heat more(no delta between two same temp cells) than cell against 3mm insulation.
I'm impressed with all the calculations that have gone on here, with regards the perfect compression, but i just think there are too many variables to be confident of the outcome. If i lose 500 cycles, then so be it. I don't think i will be 'over compressed', but for my RV situation, the cells need to behave as one solid unit.
 
I really think it comes down to personal choices if you’re referring to Lifepo4 batteries. Unless you seriously overcharge/discharge these types you should never have any expansion. They should never warm up either unless you under size your bank and overdrive them. I have 32 200ah going for 2 years now running 8kW inverter avg 3.6kW in a room that’s about 85 degrees year round. 8s packs with hose clamps tight 2” from the top and 2” from the bottom and kapton tape across the top.

I hope my experience will take some worry out of your experience. I stressed a lot when I first received them. I was overseas and nobody to call. Great guys on another forum help me thru it.
Cheers!
 
I really think it comes down to personal choices if you’re referring to Lifepo4 batteries. Unless you seriously overcharge/discharge these types you should never have any expansion.
I have noticed my EVE cells expanding and deflating at very low C rates and I have never overcharged them. This is perfectly normal according the EVE and referencing the EVE data sheet.
 
Mine expanded and contracted too when I charged and discharged.
I bought more threaded rod today.
I found some blue plastic tube that fits over the threaded rod. That might be fancy. I didn't buy it.
 
Mine expanded and contracted too when I charged and discharged.
I bought more threaded rod today.
I found some blue plastic tube that fits over the threaded rod. That might be fancy. I didn't buy it.
Just look in the plumbing department of your favorite home improvement store. They sell clear flexible tubing the match the ID size of the tubing to your threaded rod diameter. its pretty cheap!
 
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