Pack / Cell compression Optimized By Using Springs.

[100 Proof]

My mistake, sorry.

If I try to fit the springs, is an 80x20x2.5mm = 5.2kg/cm spring enough?

I calculated it myself and got something like the car's spring class, I don't think it's right ?

These springs are ~$4 each and easy to get.

Pack / Cell compression Optimized By Using Springs.

If each pack has independent plates on either end, you will get expansion/contraction on both ends even if the springs are only one end (so the center cell or two will be stationary with a small amount of expansion / contraction on either end of the pack). With a common plate on one end, both...

diysolarforum.com

 

[Capt Bill]

In wondering if I want to have space between my 280Ah LiFePO4 cells over compression in solar system set up that might get 100 amp through battery banks now and then; Question: When folks using compression with bulging cells that are not flat sided (thus seeing space between the cells along side where bulge(s) interface ; Do those bulges EVER FLATTEN OUT ??? UNDER COMPRESSION ... n running amps through those battery banks ??? Who has got some info. from real time look see mental records ? > Curious, and Learning More All the Time ;+)

 

[cinergi]

Yes my EVE 280's flattened out under compression after pulling their SoC down sufficiently low. They immediately bulge when removed from compression if the SoC is sufficiently high.

 

[Capt Bill]

Yes my EVE 280's flattened out under compression after pulling their SoC down sufficiently low. They immediately bulge when removed from compression if the SoC is sufficiently high.

Wow, That is new pertinent information in my brain. Any others with stories like that ? ... I would then think loosening the bus bars is a good idea if you were planning to remove the compression. Wonder if those solid type bus bars seem fine tight and solid through all the High to Low SOC cycles. ??? This new info. for me; "EVE 280's flattened out under compression after pulling their SoC down sufficiently low" ; does recently have me seriously considering compression.

 

[cinergi]

Wow, That is new pertinent information in my brain. Any others with stories like that ? ... I would then think loosening the bus bars is a good idea if you were planning to remove the compression. Wonder if those solid type bus bars seem fine tight and solid through all the High to Low SOC cycles. ??? This new info. for me; "EVE 280's flattened out under compression after pulling their SoC down sufficiently low" ; does recently have me seriously considering compression.

I've recently discovered that rigid bus bars are still put through stress -- and apparently to the point of causing higher resistance between the cell terminal and the bus bars as time goes by. The last maintenance I had to do required me to *pry* off the bus bar due to cell expansion (I did it at 100% SoC). I'm going to swap out my rigid bus bars with flexible as soon as they arrive.

 

[Capt Bill]

swap out my rigid bus bars with flexible as soon as they arrive.

Flexible bus bars make lots of sense to me, when going for the compression option. Wondering what you have found, price and all. I would like finding reasonable cost on flexible bus bars that could handle the larger amp flows on a 280 Ah EVE set.

 

[cinergi]

Flexible bus bars make lots of sense to me, when going for the compression option. Wondering what you have found, price and all. I would like finding reasonable cost on flexible bus bars that could handle the larger amp flows on a 280 Ah EVE set.

I got the ones I mention in my thread here: https://diysolarforum.com/threads/c...w-inverter-rv-build.13786/page-36#post-464888
I'll report back in that thread once they're installed and have been running a while etc etc

 

[Capt Bill]

I got the ones I mention in my thread here: https://diysolarforum.com/threads/c...w-inverter-rv-build.13786/page-36#post-464888
I'll report back in that thread once they're installed and have been running a while etc etc

https://www.aliexpress.com/item/1005003230279908.html?spm=a2g0o.cart.0.0.6ba53c00dcAw5S&mp=1 found at that link/ ... I would like to read more about how it is flexible? (Did not see specific info of construction. ??? Is that a waffle of thin layers of copper ??? ...vs https://www.aliexpress.com/item/1005003515631469.html?spm=a2g0o.cart.0.0.59b43c00hXZdJt&mp=1 also looks like good option (maybe better) ???.

 

[cinergi]

https://www.aliexpress.com/item/1005003230279908.html?spm=a2g0o.cart.0.0.6ba53c00dcAw5S&mp=1 found at that link/ ... I would like to read more about how it is flexible? (Did not see specific info of construction. ??? Is that a waffle of thin layers of copper ??? ...vs https://www.aliexpress.com/item/1005003515631469.html?spm=a2g0o.cart.0.0.59b43c00hXZdJt&mp=1 also looks like good option (maybe better) ???.

I guess I'll find out! haha. that's how they're usually made but the pictures look like a solid piece.
Braided bars may be OK. I only saw 2-hole options and I really need 4 (so I don't need 52 pieces for my 2p16s pack).

 

[100 Proof]

Flexible bus bars make lots of sense to me, when going for the compression option. Wondering what you have found, price and all. I would like finding reasonable cost on flexible bus bars that could handle the larger amp flows on a 280 Ah EVE set.

I made mine out of tinned copper braid and 1" copper pipe. I have about 4 hours into them, not including the time it took to plate the copper pipe sections. About $50 in them. Of course, that was with copper prices from a year ago.

 

[CB-OTB]

Nevermind, found it.

 

[Capt Bill]

Yes my EVE 280's flattened out under compression after pulling their SoC down sufficiently low. They immediately bulge when removed from compression if the SoC is sufficiently high.

This post was the Best Reference Info. I have read in the DIYSolarForum ... for me to now seriously consider compression of my 8 x 2 24v battery banks of 280Ah Eve cells. Personally: ... I would only consider compression (of my Grade B (?) cells that do have some bulges on the side.) with flexible bus bars. ... Thanks cinergi for the info. reference. I Request More References: Who else is getting those results? "EVE 280's flattened out under compression (and maybe this is neccesary?) ... after pulling their SoC down sufficiently low" ??? ???

 

[Zwy]

This post was the Best Reference Info. I have read in the DIYSolarForum ... for me to now seriously consider compression of my 8 x 2 24v battery banks of 280Ah Eve cells. Personally: ... I would only consider compression (of my Grade B (?) cells that do have some bulges on the side.) with flexible bus bars. ... Thanks cinergi for the info. reference. I Request More References: Who else is getting those results? "EVE 280's flattened out under compression (and maybe this is neccesary?) ... after pulling their SoC down sufficiently low" ??? ???

I have fully charged cells under compression and removed them from compression and never had any bulging.

But I do things differently than using springs and complicated contraptions........

 

[Capt Bill]

I have fully charged cells under compression and removed them from compression and never had any bulging.

But I do things differently than using springs and complicated contraptions........

Are yours 280 Ah? EVE? ... and you never had any bulging? ... from the git go since receiving them.??? ... Wow, Who did you purchase them from? ... I did not know my 1st 8 EVE 280ah cells were bulging some here and there when I received them because on my first DIY assembly, I put sticky tape between them thinking a lttlle air space would be better for keeping em cooler when running amps through em. I was surprised when I re-assembled them to test out the compression idea, and saw the Grade B factor in cells I hoped were Grade A. ... I am not absolutely sure, but I think Grade A cells have flat sides. I wonder what other Opinions are on that. ... I have seen some Grade A LiFePO4 cells with flat sides, and still figuring out that part of the LiFePO4 picture, for my future purchase options.

 

[justgary]

The cause of the bulging is the fact that the electrolyte changes density with state of charge. Since it physically gets larger with more charge, the case can swell if unconstrained. Extra motion of the rolled foil in the cell is what reduces lifetime.

Constraining the cell keeps everything inside from moving around as much, helping to reduce dendrite formation or shorting inside the pack.

 

[CB-OTB]

Constraining the cell keeps everything inside from moving around as much, helping to reduce dendrite formation or shorting inside the pack.

This is how I understand it, which is why I don't think that springs are necessary. You want to stop movement, and springs aren't needed. Springs are only needed if you needed to have a constant pressure, but that's not where the benefit comes from, unless I'm mistaken.

 

[Zwy]

Are yours 280 Ah? EVE? ... and you never had any bulging? ... from the git go since receiving them.??? ... Wow, Who did you purchase them from? ... I did not know my 1st 8 EVE 280ah cells were bulging some here and there when I received them because on my first DIY assembly, I put sticky tape between them thinking a lttlle air space would be better for keeping em cooler when running amps through em. I was surprised when I re-assembled them to test out the compression idea, and saw the Grade B factor in cells I hoped were Grade A. ... I am not absolutely sure, but I think Grade A cells have flat sides. I wonder what other Opinions are on that. ... I have seen some Grade A LiFePO4 cells with flat sides, and still figuring out that part of the LiFePO4 picture, for my future purchase options.

If you search around, you will find I use foam. Poron is what I use now. I buy from Amy.

 

[Gazoo]

The compression PSI recommended by EVE is to be maintained across all states of charge. That's the reason springs seem to be the best choice, but I think foam would be a good second choice. EVE uses a hydraulic jig to compress the cell when they do their analysis to ensure perfect compression across the entire SOC.

Personally, as I have said before, I don't care about using springs, but I do admire those taking this approach. I think cell usage should come into consideration before deciding to use springs. In most cases, I am sure the cells will start aging and be the biggest cause of cell degradation if NOT using springs.

Generally, these cells are rated for 2000 cycles if not using compression. If one cycles their cells fully every day, then the cells are expected to last 5.5 years under ideal conditions with no cell compression.

Regarding bulging, some have said compression works to flatten the cells and some have said compression doesn't work. Based on what I have read if the cell has built up gas internally due to overcharging, then compression will not help to flatten the cell.

For those not using springs and or foam, be careful because too much compression is worse than no compression according to EVE.

 

[justgary]

This is how I understand it, which is why I don't think that springs are necessary. You want to stop movement, and springs aren't needed. Springs are only needed if you needed to have a constant pressure, but that's not where the benefit comes from, unless I'm mistaken.

Constant pressure is exactly what you want according to the data sheet. Not enough pressure allows the electrolyte to move too, and too much pressure gives the electrolyte nowhere to go when it changes size. The electrolyte *will* change size with state of charge in LFP chemistry.

 

[rickst29]

Constant pressure is exactly what you want according to the data sheet. Not enough pressure allows the electrolyte to move too, and too much pressure gives the electrolyte nowhere to go when it changes size. The electrolyte *will* change size with state of charge in LFP chemistry.

Exactly. Within a very small range of movement, springs are roughly constant (doing slightly more pressure when cells have expanded, and slightly less when they "shrink" in comparison to the original compression values and cell sizes). ANY kind of un-sprung box (including a "pretty" aluminum one) will apply NO FORCE AT ALL, when cells have shrunk away from the fixed-position sides, and very likely FAR TOO MUCH when expansion begins to press the cells into the non-yielding sides.

With springs, you have an area of "acceptable variation" while the moving sides vary the amount of spring compression. This variation in pack width, causing face pressure between 12-17 PSI, is the "working length" of your springs in the construction of a properly built LFP compression pack. Short springs, with high "ratings" (additional pounds per inch) will have very short working lengths. Longer springs will have longer working lengths, with less change in pressure per milimeter of spring length change.

But a box without springs provides no working length at all. When the cells are too "shrunk", the pressure is very low (and the cells may even flop around, rubbing against each other in an uncontrolled way). Then when the cells begin to expand, there will be just one TINY INSTANT while the expansion amount has matched the desired range. But any further expansion causes the cells to experience excessive pressure

It's just like using compression springs above their maximum rating, when they have completely bottomed out to "solid" height: The amount of additional pressure which is being applied, by the "solid spring" pushed past its maximum rating, reaches whatever force is being applied - with zero compensation.

Near constant pressure is the goal. Properly chosen and installed springs can come pretty close to providing that, and a solid box absolutely cannot.