Compress or not, flexible busbar or not

[Ellcon123]

"If all of this isn’t true then why do you use springs and why does anyone use springs." I don't know and I don't use springs. I clamp up tight and operate within parameters so there's no issue.

If you want to use springs then go ahead. I don't see the harm but it makes for a complex setup without benefit.

 

[Cheap 4-life]

You're falling into the trap of combining distance and force. They're unrelated. A sufficiently long spring would exert the same force for a 4-cell pack as a 16-cell pack regardless of their SoC. It's a cheap way to apply a relatively fixed amount of force against an object that changes dimensions.

“Sufficiently long spring” what do you mean by this. If a spring exerts let’s say 12 psi when the spring is compressed to 50% of its length. Then couldn’t that same spring be used for a 4 cell and 16 cell pack? If it’s exerting the same psi to both packs when it’s at 50% of its length then regardless of how many cells are in that pack, the length of the spring should not matter,.
BUT i think the reason your saying “sufficiently long spring” is because the 16cell pack needs more spring/room to expand therefore compressing that spring further than 50% or further than a 4 cell pack, which is caused by none other than more force against the spring in a 16 cell pack.. yes it’s more distance the 16 cell pack needs to expand but if the spring is applying the same amount of 12psi pressure to a 16cell and 4 cell pack, and IF there’s supposedly no more force with more cells, then how can the 16cells squeeze the spring more than a 4 cell pack?

 

[Cheap 4-life]

However, EVE is now stating this is not necessary. Just set the kgf to 300 at 30% SoC in a rigid fixture. When at 100% SoC, the kgf will be a lot higher, but the # of cells does not affect that upper number.

I still don’t see how they are stating this (springs) are not necessary. They are showing the way they tested a cell and the results. That doesn’t mean that’s the best way to make our DIY packs for cell longevity. The kgf might be to high in a completely rigid/fixed rig at high SOC if we are considering the best life for the cell..

 

[Cheap 4-life]

"If all of this isn’t true then why do you use springs and why does anyone use springs." I don't know and I don't use springs. I clamp up tight and operate within parameters so there's no issue.

If you want to use springs then go ahead. I don't see the harm but it makes for a complex setup without benefit.

Well the harm would be that springs do allow expansion and IF it’s not necessary, and the psi does not get to high for the best life for the cells at high SOC in a rigid/fixed rig, then expansion is most likely best to stop with a rigid/fixed structure.
It’s a big IF though, I’m still not on board. I do feel like a completely fixed/rigid inclosure will have the cells experiencing to high of a psi at high SOC

 

[Cheap 4-life]

the amount of expansion (NOT FORCE!) depends on the # of cells.

How can a 16 cell pack expand more than a 4 cell pack if both packs are compressed to the same psi with the same exact springs. If the 16 cell pack expands more therefore moving the spring more then it has to be putting more force on that spring from the force of the expanding cells..

 

[justgary]

As SOC rises, the group of 4 cells would depress their springs more than the group of 2 cells.

You are still wrong about this. Please quit trying to convince us. You will still be wrong.

That fixture drawing is nothing more than how they tested one cell.

It is quite possible that the engineers at EVE had to take physics classes, so they know that testing the pressure in one is the same as testing the pressure in many.

If all of this isn’t true then why do you use springs and why does anyone use springs.

People use springs because that was the interpretation of the old datasheet. Heck, people still leave expansion gaps because they can't believe that it is good to not let the cells "breathe."

How anyone’s thinks that 4 cells only exert as much linear expansion force as 16+ cells.

Actually, that's everyone. Everyone but you thinks that four cells exert the same pressure as one or sixteen cells.

When someone is thinking about compress or not,, are they really supposed to think they only need to account for the expansion force of one cell even if they have say 100cells in a row?

Yes, exactly. That is what we have been telling you several hundred times. Literally, we have told you exactly that, and it is true!

 

[justgary]

If a spring exerts let’s say 12 psi when the spring is deflected to 50% of its length. Then couldn’t that same spring be used for a 4 cell and 16 cell pack?

A spring is a linear device. An ideal spring has a spring constant K with units of force per displacement. If your spring above was one inch long and exerted 110 pounds when compressed 50% (0.5 inches), the constant K=220 pounds/inch. If you have another spring that is four inches long and it exerts 110 pounds when compressed 50% (2.0 inches), the constant K=55 pounds/inch.

A deflection of 1/4 inch on the first spring would change the force 55 pounds. A deflection on the second spring would change the force 13.75 pounds. The length and K of the spring have to be chosen for the task at hand.

 

[justgary]

Well the harm would be that springs do allow expansion and IF it’s not necessary, and the psi does not get to high for the best life for the cells at high SOC in a rigid/fixed rig, then expansion is most likely best to stop with a rigid/fixed structure.
It’s a big IF though, I’m still not on board. I do feel like a completely fixed/rigid inclosure will have the cells experiencing to high of a psi at high SOC

I can agree with this, except that EVE seems to be saying, "look, we told you not to overcompress, but we did more testing and compression doesn't hurt nearly as much as expansion does." Everything now points to ignoring maximum compression and not allowing expansion at all.

Compression pressure by itself should not affect anything inside the cell if it is built to withstand it. Expansion of the pouch inside the cell case is the enemy since that represents motion of the plates and electrolyte. Motion causes bad things to happen.

 

[Cheap 4-life]

You are still wrong about this. Please quit trying to convince us. You will still be wrong.

I’m not trying to convince anyone. Just stating my current opinion. If you want to attempt to change my opinion then fine, I will be appreciative if I figure out that I am wrong

 

[noenegdod]

Everything now points to ignoring maximum compression and not allowing expansion at all.

Compression pressure by itself should not affect anything inside the cell if it is built to withstand it. Expansion of the pouch inside the cell case is the enemy since that represents motion of the plates and electrolyte. Motion causes bad things to happen.

This is essentially where I have landed. tubular cells do not expand (much).

 

[Cheap 4-life]

Actually, that's everyone. Everyone but you thinks that four cells exert the same pressure as one or sixteen cells.

Look bud, I’m far from the only one...

 

[Cheap 4-life]

A spring is a linear device. An ideal spring has a spring constant K with units of force per displacement. If your spring above was one inch long and exerted 110 pounds when compressed 50% (0.5 inches), the constant K=220 pounds/inch. If you have another spring that is four inches long and it exerts 110 pounds when compressed 50% (2.0 inches), the constant K=55 pounds/inch.

A deflection of 1/4 inch on the first spring would change the force 55 pounds. A deflection on the second spring would change the force 13.75 pounds. The length and K of the spring have to be chosen for the task at hand.

How can a 16 cell pack expand more than a 4 cell pack if both packs are compressed to the same psi with the same springs. If the 16 cell pack expands more therefore moving the spring more then it has to be putting more force on that spring from the force of the extra expanding cells..

 

[noenegdod]

Look bud, I’m far from the only one...

No, its pretty much just you.

I’m not trying to convince anyone. Just stating my current opinion. If you want to attempt to change my opinion then fine, I will be appreciative if I figure out that I am wrong

Broken clocks are right twice a day, just not on this. This isnt an opinion type of thing. It isnt subjective. it is a fact. I get it that you think you have "common sense" but common sense is basically "rule to thumb" type of logic and while it helps people with little or no understanding to grasp a concept, it often breaks down and leaves them stuck not understanding why they cant make something work or grasp an idea.

 

[justgary]

Look bud

No need to call me Bud. Bud was my dad. I miss him.

 

[fafrd]

Well no that’s not my only concern as I explained other concerns in the original post. Thanks to Rays youtube channel I know that

If using wire then I would need 1/0 to have close to the same resistance as a tin plated copper busbar that comes with these cells. With 1/0 it is no longer flexible..

I have a compression fixture on my 16-cell 2x280Ah 24V LiFePO4 pack and was concerned enough about mechanical stress to want flexible busbars. I went to the trouble of making short flexible cables using 2/0 welders wire. Plenty flexible enough to accommodate the small amount of compression / expansion of the cells and the only ‘trick’ was that I had to make the 2/0 cables 2-cells wide (so my 16-cell pack has interleaved connections, only one connection being between adjacent terminals.

It was somewhat laborious to make the 2/0 cables, but not expensive.

The braided from what I am reading/seeing on YouTube) do not flex barely at all in the direction they need to flex and have a much greater resistance than the copper busbars..
Multiple 10 gauge wires crimped into lugs seem like the best flexible busbar option to actually be flexible but idk, just seems wrong using 4+ 10 gauge wires crimped into into lugs

I’m pretty certain 2/0 or even possibly 1/0 welder’s wire is going to be much easier and more effective than attempting to crimp multiple 10-guage wires into a single lug…

 

[justgary]

How can a 16 cell pack expand more than a 4 cell pack if both packs are compressed to the same psi with the same springs. If the 16 cell pack expands more therefore moving the 12psi spring more then it has to be putting more force on that spring from the force of the expanding cells..

It is the spring that is applying more force. The cells just sit there and take it.

 

[Ellcon123]

You are mixing units. A 16 cell has to expand 4 times more than a 4 cell under the same pressure. Math. The spring is not rated in psi. It is rated in force/distance. Psi is force/area.

 

[justgary]

By the way, the datasheet also now says that the maximum allowable lateral force on the terminals is 700N = 157.36 pounds. They say "in the longitudinal direction," so I take that to mean toward the center of the cell.

 

[Cheap 4-life]

It is the spring that is applying more force. The cells just sit there and take it.

Right, why is the spring applying more force? Is it because there’s more cells in the 16 cell pack vs the 4 cell pack? Yes the spring is applying more force because the cells are expanding more in the 16 cell pack vs the 4 cell. You could measure the springs and see it’s total length is smaller when the 16ell pack is at 100% SOC VS the 4 cell pack at 100% SOC.

 

[Cheap 4-life]

By the way, the datasheet also now says that the maximum allowable lateral force on the terminals is 700N = 157.36 pounds. They say "in the longitudinal direction," so I take that to mean toward the center of the cell.

Correct but isn’t that irrelevant. We would never want anywhere near that much force because the solid busbars would loosen causing all kinds of issues. With flexible busbars that lateral force is also irrelevant