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Compress or not, flexible busbar or not

I’ll have to go back and check up on Cinergi - he was the one who originally told me he got no compression after the first few cycles (which is what I’ve experienced).

If he has had to double his clamping force to prevent excessive expansion and stress in his rigid busbars as his cells have aged, I may need to keep an eye on my cells behavior.

On the one hand, because I’ve gone with flexible busbars, I’m certain I’ll have nothing to worry about in any case.

And on the other hand, should I ever see increased expansion or should new data emerge suggesting cycle life will greatly improve under higher clamping force of 600kgf (or whatever), it just means I’ll have to invest in new springs…

But what springs.. they would still need to only exert 300kgf at low SOC (like a rigid fixture is done) and then also apply enough pressure to all but completely stop expansion so busbar loosening isn’t an issue and so expansion doesn’t cause cell degradation or damage. Or if not then what’s the point of compressing at all..
 
I’m not onboard with this yet.. please explain how you are. Again they simply showed how they tested a cell. That doesn’t mean that optimal for prolonged health of our cells. Unless I’m missing something
I was just looking at those two tables which seem to indicate you don’t need to worry about causing ‘interval defects’ with clamping force below 5000kgf.

The original datasheet showed greatly increased cycle under 300kgf of force (always).

So unless something regarding that has changed, for optimal cycle life I still think you want a 300kgf clamping fixture (with the longest springs you can fit).

Under 300kgf, pack expansion after the first few cycles is nonexistent (though I’ll confirm that now after one year of use).

Also, eco anion is not the same thing as deformation. My pack is not exoanding, but that does not mean that cells are not rotating a bit as my ‘accordion-like’ pack charges and discharges.

My flexible busbars will easily be able to accommodate a small amount of angular displacement / stress without loosening, while Cinergi’s long, rigid, multicell busbars could easily loosen under such angular stress/rotation (I recall now that that was one of the primary concerns that caused me to go the custom flexible cable route rathe than following his design).
 
My thinking is that the expansion force does not rise by much till near EOL, not a gradual increase over its life. Or at least that’s what’s would make sense to me
I agree (but we’ve got no data).

It’s a pity they did not include a midlife expansion force measurement…
 
The original datasheet showed greatly increased cycle under 300kgf of force (always).
I don’t think a data sheet ever said always. I think the 300kgf was always to be applied at low SOC. And people just thought that it was implied as the cells should always be held at 300kgf and started using all kinds of springs etc in an attempt to keep the packs at 300kgf (or close) throughout its entire SOC..
 
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Yes common sense to me previously said why not compress with springs instead of rigid, it can’t hurt and would avoid over compression at high SOC and rigid could most likely not avoid that.. now it’s seeming like it’s best to all but completely stop expansion with a rigid structure and the cells will not be over compressed at high SOC as per the data sheet if that’s actually what it says. I’d throw in some poron foam to further prevent over compression at high SOCZ
I agree with you except that you pointed out that the new data only speaks to ‘internal danage’ and says nothing about cycle life.

Unless there is something new, 300kgf (always) maximized cycle life.

I believe when I built mine the guidance was to stay as close to 300kgf as possible at both extremes (which was complicated to achieve).

With this new data, I’d probably just get the longest springs I had space for and calibrate to 30mg for at the maximally discharges point…
 
My point is that there only is possibly not more expansion/force with more cells if expansion is all but completely stopped from even starting by the compression.
As I said if the same exact springs are used for a 16cell and 4 cell pack, then the spring on the 16 cell pack will be compressed more than the springs on the 4 cell pack.. to me that means there’s more force in the 16 cell pack because the extra cells compressed the spring more than the 4 cell pack.. that’s if the springs WERE NOT strong enough to completely stop all compression from happening/starting..
is this correct?, or are you still saying that even if the springs are not strong enough to completely stop expansion and expansion does happen in the pack, that there’s still no more force with 16 cells vs 4 cells? As I said that wouldn’t make sense for you to say because the pack with more cells compressed the spring further due to the extra expansion force...
Yes, the force changes by using different springs. You would not use the same springs on a 4-pack as a 16-pack (well, you could if you stacked four of the springs per threaded rod).

You are still conflating expansion with pressure though. The cells expand. The expansion changes the spring length. The different spring length changes the force.

If you have no constraint on the cells, they expand but have no force applied on them.
If you have rigid constraint on the cells, they don't expand but have more force on them.
 
Also, eco anion is not the same thing as deformation. My pack is not exoanding, but that does not mean that cells are not rotating a bit as my ‘accordion-like’ pack charges and discharges.
What do you mean by this. I figure the only way the cells can move at all is from expansion/contraction thru the SOC. I’m not saying bloating
 
But what springs.. they would still need to only exert 300kgf at low SOC (like a rigid fixture is done) and then also apply enough pressure to all but completely stop expansion so busbar loosening isn’t an issue and so expansion doesn’t cause cell degradation or damage. Or if not then what’s the point of compressing at all..
I’ve been where you are. I was worried about all these same things. And I know what I’m trying to tell you is nit going to ease your concern, but I’ll repeat it.

There is no meaningful expansion under 300kgf.

After all this back-and-forth, I’m going to check my pack again over the next charge / discharge cycle, but I’m already pretty sure it’s going to show no change (after ~365 cycles since I put the battery in service exactly one year ago).

No clamping force / unconstrained expansion results in reduced cycle life. EVE is clear about that.

They characterize cycle life under a ‘perfect’ 300kgf clamping fixture, si that is what you should aim for if you want to achieve similar cycle life.

We can’t afford ‘perfect’ 300kgfclamping fixtures, so getting as close to that as possible using the longest springs you have space for (with a wide of a constant ~300kgf ‘plateau’ as possible is the best we can do.

Bus bars are a whole different issue but the only reason to go for a rigid fixture is because you have rigid busbars, not to further improve cycle life.

Flexible busbars plus a 300kgf clamping fixture is the best for cycle life…
 
I don’t think a data sheet ever said always. I think the 300kgf was always to be applied at low SOC. And people just thought that it was implied as always and started using all kinds of springs etc in an attempt to keep the packs at 300kgf (or close) throughout its entire SOC..
What are you talking about? The datasheet shows how cells tested under 300kgf delivered improved cycle life. EVE fid not build a ‘clamping fixture’ - they have a piece of professional test equipment that applies 300kgf to the cell under test. Perfectly 300kgf and for the entire duration of the test.
 
Yes, the force changes by using different springs. You would not use the same springs on a 4-pack as a 16-pack (well, you could if you stacked four of the springs per threaded rod).

You are still conflating expansion with pressure though. The cells expand. The expansion changes the spring length. The different spring length changes the force.

If you have no constraint on the cells, they expand but have no force applied on them.
If you have rigid constraint on the cells, they don't expand but have more force on them.

I’m saying if you did use the same springs on both size packs. The spring is rated for the exact same pounds at 50% of its length.. if the same pounds (tightened up spring to the same length) is applied by the same spring to both size packs then how would the spring on the 16 cell pack be compressed (shortened length) more than the 4 cell pack.. spring is supplying the same lbs to both packs.. imo that’s because the cells are allowed to expand and the expanding force/length is causing the spring in the 16 cell pack to compress more (get shorter) than the same spring on the 4 cell packs.. yes if the same spring was tightened up enough to prevent expansion at all in both packs then the spring length probably wouldn’t change hardly at all but then might as well be a rigid fixture
 
No clamping force / unconstrained expansion results in reduced cycle life. EVE is clear about that.

Right but the springs should be so tight to not allow barely any expansion almost like rigid or the cells are still expanding. Yeah most likely not as much because they have 300kgf applied to them but if they are still expanding unlike in a rigid fixture than only part of the benefit of compression is gained. Which is still a partial gain in cycles but if they are still expanding then it might sway a lot of people from compressing with springs if they are only getting part of the benefit or compression..
In my newest opinion the springs need to put enough compression on the cells to basically completely stop expansion and from what I have read from several users, their springs at 300kgf still allow expansion. Enough that their busbars are loosening. However even in a rigid fixed inclosure I would still use flexible busbars just in case
 
What do you mean by this. I figure the only way the cells can move at all is from expansion/contraction thru the SOC. I’m not saying bloating
Until you have charged and discharged your own cells, you can not really understand what I mean. The cells are actually a folded pouch internally. When they are at minimum SOC, the sidewalks are ‘undulated’ with one half-side sucked in slightly and the other half pretty much flat with the corner.

When you have a pack of cells connected by rigid busbars, they will ‘accordion’ when charged. By that I mean that where there’s is a rigid busbar connecting two adjacent cells, that distance will remain roughly constant while the opposite side where there is no bus bar, the space will increase (so a long thin triangle gets formed).

Because the rigid busbars are connected to alternating sides, the ‘base’ of those long thin triangles will alternate sides (hence the ‘accordion’).

Without any clamping force at all, tjus ‘accordion’ expansion is alarming (and makes you think you want a clamping fixture to minimize it).

With a 300kgf fixture and flexible busbars, the cells settle into a position where there is a long thin rectangle between each cell. So to a casual inspection, it appears that there is no expansion / contraction and no accordion-like rotation. But I‘ve not bothered to characterize / check my ling than rectangles to be certain they are identical in charged and uncharged state, so their may be a small amount of angular displacement.
 
What are you talking about? The datasheet shows how cells tested under 300kgf delivered improved cycle life. EVE fid not build a ‘clamping fixture’ - they have a piece of professional test equipment that applies 300kgf to the cell under test. Perfectly 300kgf and for the entire duration of the test.
My mistake if that’s so. I’m new to this compress or not stuff. Never even seen old data sheets.. although I did read members here saying that they thought the 300kgf was kept by EVEs testing rig throughout the entire test but I was unsure if that was confirmed
 
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Until you have charged and discharged your own cells, you can not really understand what I mean. The cells are actually a folded pouch internally. When they are at minimum SOC, the sidewalks are ‘undulated’ with one half-side sucked in slightly and the other half pretty much flat with the corner

I don’t need to have my own cells to understand that. I would say that accordion you are referring to is from expansion (not bloating) which wouldn’t happen in a completely rigid fixture. Or at the very least much less
 
As I said @cinergi and other say have shown that there is.. also just because your cells might not have been moving/expanding doesn’t mean other cells might not?
As I stated, I’ll confirm (more carefully) no change after one year of service. But yes, between eyeballing my cells in the morning when they are empty and eyeballing them in the afternoon when they are full, they look the same.

I have enough respect for Cinergi that if he’s now saying that with a 300kgf clamping fixture he’s getting much more movement / expansion that when he first characterized his cells, I’ll check whether I see any difference as welll.

But if all he’s said is that his rigid busbars are loosening up and so he’s gone back to flexible (without any statement regarding increased expansion) then I’ve already provided an explanation of how a celloack might be deforming enough to loosen up rigid busbars without increased expansion (from angular rotation).
 
Again, knowing what the expansion force at HOL (half of life) sure would be helpful, but my general sense is that it’s going to be practically impossible to design a rigid fixture that remains rigid under 5000kgf(!).

I doubt any EV battery out there will not deform under 5000 kg of force. So first, batteries are unlikely to be used for anywhere close to the service life needed to generate 5000kgf of expansion force, and second, even if they did, the metal box is unlikely not to deform a bit and hence apply less than 5000kgf.

I think you underestimate how rigid metals are. Say I wrap my battery in a 10mm steel box, similar to what EVE seem to be doing when welding. Having 250mm height and 130mm width I get 2 x 250 mm x 10 mm + 2 x130mm x 10 mm = 5000mm² + 2600mm² = 7600 mm² cross-section area.

Or, let's use aluminium instead of steel, since it's not even half as rigid at 69 GPa.

If we apply 50kN over 7600mm², that gives 6.579 MPa. That creates a stress of 6.679 MPa / 69 GPa = 0.0000967. Over the length of 8 of my 36.35mm cells, a total of 290.8 mm, that gives 0.0000967 * 290.8 mm = 0.02mm of elongation if I'm not messing up my calculations. Or, 0.0035mm per cell.

Using 6 M16 rods of 200GPa steel we get 6 * 125mm²= 750mm². Applying 50 kN we get 66.67 MPa. Stress = 66.67MPa / 200 GPa = .00033. Over 290.8 mm we get 0.097mm elongation. Or 0.012 mm per cell. We can continue calculating how much this would decrease the pressure, but I don't think it would be that much..
So I take this new datasheet to mean that enclosing these cells in a rigid metal box such as that used for an EV battery will be fine.

If I had seen this datasheet before building my 300kgf clamping fixture, I would not have bothered.
For the LF280K it doesn't seem to be necessary, no. I'm not sure whether you have LF280K or LF280 though.
On the other hand, if you are not using a ready-made box for the purpose and are already going the route of rigid end plates secured by threaded rod, the additional cost and effort to add calibrated springs to apply 300kgf is modest.
Yeah, It's partly an exercise in welding too. Might be useful to learn.
If I’d seen this data and wanted to go to the trouble of building a clamping fixture, I would have had no concerns using a shorter calibrated spring (though the only advantage would have been slightly shorter overall dimension, since spring cost would have been about the same…).
Indeed. In my case I have the width of a 19" rack to work with, and I can't fit 16 cells from side to side, so it becomes 2 rows of 8 and thus have plenty of room for springs and bolts. I considered using LF280K and placing them from front to back instead, but 16 cells in a rack would become impossible to handle.
[regarding monitoring cell expansion]
Been there, thought about doing that, decided it wasn’t worth the trouble and wouldn’t even consider it after one year of experience with my battery now.
I might get sidetracked with other projects before adding this, but it seems like fairly easy data points to add to indicate a possibly severe issue with the battery. The cost of adding it is nothing compared to the cost of the possible consequences of an issue.

Setting SCC and parameters set up to correctly charge and discharge cells within target voltage ange is a much bigger concern to me than anything to do with expansion or clamping force at this stage.
In my case I'm trying to add this to a lead UPS where much of the battery handling is a black box I have to work around.
The BNS is there as a fall-back safety device but when the BMS cut-off kicks in, it causes a fault to the overall system requiring a manual reset.
In my case I don't trust the BMS fully, especially since it can't communicate with the UPS.
‘Leakage’ does not occur until over 100,000 kgf (before which I suspect the battery box would burst).
100 000 N, or 10 000 kgf. Looking at a data table this is within what two M12 (½") rods can hold before starting to deform permanently.
 
I have enough respect for Cinergi that if he’s now saying that with a 300kgf clamping fixture he’s getting much more movement / expansion that when he first characterized his cells, I’ll check whether I see any difference as welll.
I don’t think he’s seeing more now, but rather the same amount he’s always seen and has mitigated issues with flexible busbars
 
Right but the springs should be so tight to not allow barely any expansion almost like rigid or the cells are still expanding. Yeah most likely not as much because they have 300kgf applied to them but if they are still expanding unlike in a rigid fixture than only part of the benefit of compression is gained. Which is still a partial gain in cycles but if they are still expanding then it might sway a lot of people from compressing with springs if they are only getting part of the benefit or compression..
In my newest opinion the springs need to put enough compression on the cells to basically completely stop expansion and from what I have read from several users, their springs at 300kgf still allow expansion. Enough that their busbars are loosening. However even in a rigid fixed inclosure I would still use flexible busbars just in case
If you think you know better than EVE how to get even greater cycle life out of their 280Ah LiFePO4 cells than they do, go for it.
 
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