mscdirect.com has an excellent selection of die spring. They give dimensions and force at a compressed dimension. I wouldn’t exceed 660lbs total or 165lbs for each of the 4 rods on a big cell. The trick is going to be coming up evenly. Triple check your numbers for your cells type. Don’t rely on my estimates. I use a single centralized die spring and did a load test and settled on 550lbs. Let us know about the laminated busses and if they keep needing re torquing every week or so.
Compress or not, flexible busbar or not
- Thread starter Cheap 4-life
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Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
I’ll check out mscdirect.
My understanding is the purpose for using the laminated flexible busbars is to prevent the re torquing. @cinergi uses them for that reason as he explained above. There has been complaints of the braided flexible busbars needing to be re torqued. And complaints the same for the solid busbars.
RCinFLA
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I wonder how many folks erroneously think that compression has any impact on the damage done by electrolyte decomposition that causes cell bloating.
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Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
Wouldn’t compression help to prevent cell bloating/delamination
RCinFLA
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Delamination yes, cell bloating no, at least within the constraints of bus bars not ripping battery terminals out because of bloating. On the flip side, bloating with compression will cause cell internal gas pressure to rise higher increasing the possibility of popping the overpressure vent port.
Cell bloating is the result of electrolyte decomposition, most often due to overcharging.
Electrolyte is called an organic compound, which is a politically and environmentally correct way of saying it is mostly hydrocarbon based, just like petroleum products.
Electrolyte decomposition breaks down to several chemical compounds. Several gases, mostly carbon dioxide and carbon monoxide, and hydro-carbon tars. It is the hydro-carbon tars that clogs up, primarily, the negative graphite electrode pores restricting lithium-ion migration through cell that does the real cell damage.
The bloating you see is just the gas pressure on the outside metal container. The actual cell lamination wrap is open at top to allow pos and neg foil terminals to escape the wrap. Gases within the wrap vent up through the open-ended wrap into the sealed metal container. The actual cell laminate wrap does not bloat.
Compression will help reduce delamination of negative electrode graphite from copper foil negative current collector and LFP positive electrode from aluminum foil. Delamination is most often causes by high cell currents with repeated heating and cooling cycles of the electrode material.
When the electrode material gets dried out due to extreme electrolyte loss it is also more prone to delamination, but the cell is pretty much toast by the time the electrolyte gets that depleted.
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Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
Ok, so are you saying that compression helps nothing when the cells are used in a low power demand/slow charging solar setup? Since the cells won’t see high cell currents or repeated extreme heating/cooling cycles? Or is the regular heating/cooling cycles of a low power demand solar setup enough to warrant compression due to it possibly causing delamination. I’ve seen several posts of cells swelling from minor usage and not being overcharged when they are not compressed
The cells do expand/contract as a result of changes in SoC regardless of the C rate. In my case, I wanted to secure the cells for a mobile environment so that no matter the SoC, 300+/- kgf was being applied to keep them securely in place and in response to the EVE recommendations at the time about the recommended fixture. With 16 cells in a row, there's no fixed fixture that would maintain the cells in a tight configuration from 0% SoC to 100% SoC without exerting a ridiculous amount of force on the cells at 100% SoC.
chrisski
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Do you see any harm in putting 8 cells between two pieces of plywood, separated by some sort of padding, and then severed between four pieces of threaded rod?
Honestly, coming from a world of technical data, procedures, and quality assurance, a lot of what we do can be worded as “best practices” by some and “cowboying” by others.
RCinFLA
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The negative electrode graphite, which is about 20% of cell, expands by 9-11% of its volume between zero and 100% state of charge.
That amounts to less than 2 mm thickness change on a 280 AH cell. Barely noticeable.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
To answer your question, not really. However as @cinergi and some others are saying, compression without flexible busbars can cause issues like busbar nuts/screws loosening or busbars sliding and or cell resistance/voltages being read incorrectly by the bms due to the loosening. Without flexible busbars it seems to me (from everything I’ve watched and read) that compression shouldn’t be used. Instead the snake pattern @Will Prowse is using or the space between cells that offgrid garage implemented should be done when solid busbars are used
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
You seem very knowledgeable regarding this issue. More knowledgeable than I am. but from numerous sources, I have seen how much cells expand. That’s from normal/low c-rates and not over charging. Sometimes it’s a lot more than 2mm or at least it looks like more
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That's like backwards of what I read/understood a couple months ago. What I understood was a fixture should be used to prevent cell movement when using rigid bus bars (and the cells do better with some compression). So I designed mine using rigid bars, with two rows of 8 cells, each cell separated by a plastic sheet for an insulator, and 3/4" ply end plates, using 6 threaded rods (2 on one side, 2 down the middle, 2 on the other side), and the nuts just snugged down at middle of SoC but not going too crazy.
Too much conflicting info out there on this
Screw it, I'm sticking with this config I guess ?
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
I here ya, my heads spinning trying to figure out what’s best. Preventing all movement can’t happen. It’s how much they move and if that’s enough to cause the busbars to loosen. To me it seems that If the busbars are welded (instead of held by screws or nuts) then compression is fine and should be used. Then the terminals will simply withstand the movement and flex. The busbars couldn’t loosen/slide etc due to being welded. I’ve seen some of the better made (more expensive) batteries using the welded busbars and compression of some sort.That's like backwards of what I read/understood a couple months ago. What I understood was a fixture should be used to prevent cell movement when using rigid bus bars (and the cells do better with some compression). So I designed mine using rigid bars, with two rows of 8 cells, each cell separated by a plastic sheet for an insulator, and 3/4" ply end plates, using 6 threaded rods (2 on one side, 2 down the middle, 2 on the other side), and the nuts just snugged down at middle of SoC but not going too crazy.
Too much conflicting info out there on this
Screw it, I'm sticking with this config I guess ?
Maybe with only 8 cells in a row the cells using mechanically held solid busbars wouldn’t move enough under compression to make re torquing of the busbars needed.
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Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
Post in thread 'Eve lfp280 w/ overkill + victron settings'
https://diysolarforum.com/threads/eve-lfp280-w-overkill-victron-settings.11346/post-198413
Post in thread 'Cinergi's 28 kWh / 4 kW Solar / 10 kW inverter RV build'
https://diysolarforum.com/threads/c...lar-10-kw-inverter-rv-build.13786/post-457384
Both posts where solid busbars with compression caused the need for re torquing due to loosening.
There is a post where re torquing of flexible busbars was needed but they were the braided type not the laminated type.
https://diysolarforum.com/threads/eve-lfp280-w-overkill-victron-settings.11346/post-198413
Post in thread 'Cinergi's 28 kWh / 4 kW Solar / 10 kW inverter RV build'
https://diysolarforum.com/threads/c...lar-10-kw-inverter-rv-build.13786/post-457384
Both posts where solid busbars with compression caused the need for re torquing due to loosening.
There is a post where re torquing of flexible busbars was needed but they were the braided type not the laminated type.
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RCinFLA
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I would not use wood, but it is a good idea to have some amount of thermal barrier and if you are compressing cells have some spacing compliance 'flex' between cells. That way if one cell shorts out it does not create a cascading meltdown to adjacent cells, popping off the whole string.
Main reasons I am not an advocate for compression is:
1) You should not be using thick electrode cells regularly above 0.5C(A) current due to onset of electrode ion migration starvation, therefore little internal electrode heating, therefore little benefit of compression.
2) If you don't have some compliance in compression pressure between cells you can create a high-pressure point on cell surface causing a separator punch through causing cell to short out. Cell wraps do not have a perfectly uniform thickness.
Nobodybusiness
Solar Sponge.
Yes..
But amounts to 1.25 inches for a pack of 16. That's a LOT of pressure in a rigid fixture.
EDIT: If the fixture was snugly assembled at a low SoC
RCinFLA
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Absolutely agree.
Also goes to point that compression can get you in a lot of trouble if you don't know all the in's and out's of what you are doing.
Cheap 4-life
My body is 2.63 trillion volts, .07v per cell
....
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RCinFLA
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Not a good idea to have hard fixed constraint.
Sum of each cell's small SOC width expansion from all the parallel stacked together cells can drive compression pressure up exponentially. Can result in crushed cell internal perforated plastic polyethene electrodes separator wrap, shorting out cell. Issue gets worse the more cells you parallel together.
You can also crush electrode material loose, increasing resistance to rest of electrode.
LFP cell construction is like two blocks of Rice Krispy treats (block represents positive LFP and negative graphite electrodes) with thin piece of plastic, with a lot of small holes punched in it, separating the two blocks to allow marshmallow to ooze through the small holes in plastic sheet (marshmallow represents electrolyte). If the block on one side of plastic makes contact to block on other side the cell is shorted.
Charging causes the granules of graphite in negative electrode to expand by as much as 11% in size due to the fully charged stuffing of lithium ions within the graphite lattice structure.
The structure of the positive LFP electrode is such that it does not shrink in size as lithium ions leave it during charging. The 'iron' in the positive LFP electrode provides a support pillar to maintain the LFP positive electrode lattice size after lithium ions have left for charging transfer to graphite negative electrode side. The iron in positive electrode lattice support makes LFP cells' cathode much more rugged than other lithium-ion chemistries. That is the primary reason LFP cells have much greater discharge-recharge cycles lifetime.
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Horsefly
Solar Wizard
I also am not subscribed to the whole compression fixture thing. I've built four batteries, and all four have had something close to finger-tight threaded rods / bolts just to keep them from moving around and putting pressure on the bus bar connections. I think calendar time will kill my cells before the compression fixture would matter.
I hadn't noticed this thread earlier, but hopefully everyone here knows there are several fairly long and detailed threads here (circa 2020 and 2021) about building compression fixtures. Many folks have posted pics of what they have done and given links to what they bought to build them. Many others (like me) think that all the effort to compression the cells is more likely to damage the cells than extend the life of the cells. But that's just me.
Carry on.
I hadn't noticed this thread earlier, but hopefully everyone here knows there are several fairly long and detailed threads here (circa 2020 and 2021) about building compression fixtures. Many folks have posted pics of what they have done and given links to what they bought to build them. Many others (like me) think that all the effort to compression the cells is more likely to damage the cells than extend the life of the cells. But that's just me.
Carry on.
RCinFLA
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Unless you run your battery hard with high discharge and charge current (>0.5 C(A) current) there is little benefit to compression.
Compression helps reduce electrode material delamination from metal foil which is dependent on electrode heating/cooling cycles.
Cell internal heating gets worse the more cell current demand and the more aged the cells (2x-3x heating with age at given current).
Operating at cold temps (<5 degs C) increases internal impedance of cell which increases internal heating for given current and results in more temp cycle shifts.
Delamination shows up as impedance increase on a 1kHz battery impedance meter which primarily measures conduction resistance of cell.
Compression helps reduce electrode material delamination from metal foil which is dependent on electrode heating/cooling cycles.
Cell internal heating gets worse the more cell current demand and the more aged the cells (2x-3x heating with age at given current).
Operating at cold temps (<5 degs C) increases internal impedance of cell which increases internal heating for given current and results in more temp cycle shifts.
Delamination shows up as impedance increase on a 1kHz battery impedance meter which primarily measures conduction resistance of cell.
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Given the cells significantly expand and contract, and EVE tests cell failure with a rigid fixture, it seems crazy to me that folks are using completely rigid fixtures. To each their own I guess.
EVE recommends compression for maximum longevity, and they basically give us the pressure guidelines. If the user chooses to use compression, it's illogical to use anything other than some form of fairly constant rate of compression (within their guidelines) across the SoC range (basically using springs, foam, or some other media which achieves this). Seems pretty simple, yet there's still ongoing discussion about using rigid compression... I'm not sure why this is even remotely considered a good, or even safe, practice.
EVE recommends compression for maximum longevity, and they basically give us the pressure guidelines. If the user chooses to use compression, it's illogical to use anything other than some form of fairly constant rate of compression (within their guidelines) across the SoC range (basically using springs, foam, or some other media which achieves this). Seems pretty simple, yet there's still ongoing discussion about using rigid compression... I'm not sure why this is even remotely considered a good, or even safe, practice.
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