Compress or not, flexible busbar or not

[rhino]

Given that we both agree that a rigidly constructed fixture is not a good idea, how would you recommend accomplishing mounting eve 280 cells in a mobile application without compression?

Mine is not in mobile environment but the one battery I built is in a rigid fixture but uses Poron foam between cells which still allows expansion and contraction and keeps the cells "rigidly" together so won't jostle around in a mobile situation.. Poron foam has advantage that it is good at expanding readily back to it's original uncompressed form so the cells will still be kept in place. Example of opposite of this is EPS foam which once you compress it it stays in that compressed form.

 

[Ampster]

They also do not tell you the fixture provides cell cooling.

In my configuration cooling is not an issue. The only applications where cooling is obviously a factor is in EVs, especially with other more volatile chemistries and high DC fast charging rates.

 

[noenegdod]

Mine is not in mobile environment but the one battery I built is in a rigid fixture but uses Poron foam between cells which still allows expansion and contraction and keeps the cells "rigidly" together so won't jostle around in a mobile situation.. Poron foam has advantage that it is good at expanding readily back to it's original uncompressed form so the cells will still be kept in place. Example of opposite of this is EPS foam which once you compress it it stays in that compressed form.

The frame of your fixture is ridged however the foam is part of your fixture therefor your fixture is not ridged.

 

[Cheap 4-life]

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.

What about all the premade packs out there that do not use springs.

 

[Cheap 4-life]

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.

It’s being noticed that not compressing the cells and using solid busbars causes the busbars to loosen therefore needing to be constantly checked re torqued. Being loose can damage cells overtime by not allowing the bms to properly read the cells. And by some cells supplying less of more to the load.

 

[Cheap 4-life]

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.

But yet many high quality pre made packs do not use springs so basically rigid compression and use welded busbars.

 

[Charleswgibbs]

But yes many high quality pre made packs do not use springs so basically rigid compression and use welded busbars.

Is there any premade packs that have 16+ 280 cells in one long line with solid compression?

 

[Cheap 4-life]

Is there any premade packs that have 16+ 280 cells in one long line with solid compression?

Most I seen is 8. Even with only 8 there can be over 3/16” of movement. I will have 19 in a row so I am definitely using flexible busbars and spring compression.

 

[RCinFLA]

Most of the metal rack mount cases I have seen, strong enough to maintain structure have at least compressible end pads to pack cells together. Many good ones have some gap between cells to reduce possibility of cell cascading thermal runaway.

 

[Cheap 4-life]

Mine is not in mobile environment but the one battery I built is in a rigid fixture but uses Poron foam between cells which still allows expansion and contraction and keeps the cells "rigidly" together so won't jostle around in a mobile situation.. Poron foam has advantage that it is good at expanding readily back to it's original uncompressed form so the cells will still be kept in place. Example of opposite of this is EPS foam which once you compress it it stays in that compressed form.

Do you have a link for the poron foam you used. All I can find is small pieces for keyboards. Amy said she is also sending the separators so I’ll very see what she sends

 

[Cheap 4-life]

The spec sheet for the CATL cells I’m getting says 300kgf of compression using 15mm steel plates. If come up with 21.7ftlbs of torque to achieve 300kgf. Is that correct? Does that mean I apply 21.7ftlbs of torque to each nut? I will be using springs

 

[S Davis]

The spec sheet for the CATL cells I’m getting says 300kgf of compression using 15mm steel plates. If come up with 21.7ftlbs of torque to achieve 300kgf. Is that correct? Does that mean I apply 21.7ftlbs of torque to each nut? I will be using springs

No, you don’t torq the nuts. With die springs the springs will have a certain pressure at a given compression.

The springs I used are rated at 160 lbs compressed to .68” so just below the 12 psi recommended. This is for a stack of four.

If I was doing more cells I would look for a spring with more travel so as they expand the pressure doesn’t rise above the 17 psi upper limit.

 

[Cheap 4-life]

No, you don’t torq the nuts. With die springs the springs will have a certain pressure at a given compression.

The springs I used are rated at 160 lbs compressed to .68” so just below the 12 psi recommended. This is for a stack of four.

If I was doing more cells I would look for a spring with more travel so as they expand the pressure doesn’t rise above the 17 psi upper limit.

Shit now I’m lost (which don’t take much to achieve) I’m actually using a “stack” of 19. So is that 12psi when theyare at a low SOC?

 

[Cheap 4-life]

I would use something like these. Four threaded rods and four of these valve springs are in the right ballpark for the EVE 280 cells, so if yours have similar requirements, the same should work well. The pressures within the pack should be similar.

https://www.oreillyauto.com/detail/c/melling/melling-valve-spring/mel0/vs360?q=Melling+VS-360&pos=0

How do you know these springs are in the ballpark?

 

[rhino]

Do you have a link for the poron foam you used. All I can find is small pieces for keyboards. Amy said she is also sending the separators so I’ll very see what she sends

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

But I'm not planning on compressing my next battery at all.

 

[Cheap 4-life]

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

But I'm not planning on compressing my next battery at all.

Worried to not compress at all. I seen a lot of pics showing cells swelling pretty barely from low charge/discharge and not from overcharging. I guess that wouldn’t happen to all cells but could happen to some of them from what I’ve seen. And even more so because I had to order CATL cells which supposedly are more prone to swelling if not compressed

 

[Dynoman]

The spec sheet for the CATL cells I’m getting says 300kgf of compression using 15mm steel plates. If come up with 21.7ftlbs of torque to achieve 300kgf. Is that correct? Does that mean I apply 21.7ftlbs of torque to each nut? I will be using springs

Do not torque the nuts to 21.7 ft.-lbs.!

Don't have the post with the math handy, but it was something like 5 to 10 in-lbs. of torque on each nut with no springs. My 8s battery banks are compressed, without springs, to about 5 in-lbs. with a torque wrench at greater than 80% charge. If I remember right, it seemed just a little more than finger tight.

 

[MattiFin]

Do not torque the nuts to 21.7 ft.-lbs.!

Don't have the post with the math handy, but it was something like 5 to 10 in-lbs. of torque on each nut with no springs. My 8s battery banks are compressed, without springs, to about 5 in-lbs. with a torque wrench at greater than 80% charge. If I remember right, it seemed just a little more than finger tight.

This might be usefull: https://sabreindustrial.com/content/Tech PDFS/003 Bolt Clamp Load Chart.pdf

 

[Martijn]

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.

What happens if you compress too tight?

I've got threaded rods, with nuts. The sides of the top of the cell don't touch each other, but the walls do (with non-compressable isolation between cells). So when the SOC is near 100%, will there be a lot of force on the cell walls?

 

[Cheap 4-life]

What happens if you compress too tight?

I've got threaded rods, with nuts. The sides of the top of the cell don't touch each other, but the walls do (with non-compressable isolation between cells). So when the SOC is near 100%, will there be a lot of force on the cell walls?

Over 17psi is supposedly worse than not having compression at all