Compress or not, flexible busbar or not

[Cheap 4-life]

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.

Correct so wouldn’t that extra distance from 16 cells therefore cause more force in a fixed/rigid rig.

 

[fafrd]

Correct so wouldn’t that extra distance therefore cause more pressure in a fixed/rigid rig?

The expansion per busbar will be independent of pack size (assuming you’re using springs with a wide-enough flat spot in their force / deflection curves).

 

[Cheap 4-life]

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.

Man that is a hell of an implication in my mind.. just because they tested a cell like that and showed us how they tested a cell doesn't mean that’s optimal for cell longevity..

 

[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.

So your saying a different spring (possibly a different length) should be used for different amounts of cells? Basically saying that the same exact spring shouldn’t be used for a 4 cell and 16 cell pack? I agree
If that’s so then how can the same rigid/fixed type of inclosure be used for 16 cells or 4 cells?

 

[justgary]

Correct so wouldn’t that extra distance from 16 cells therefore cause more force in a fixed/rigid rig.

That statement is true. Your previous statements about the difference in force between one or sixteen cells are false.

The datasheet clearly says that a rigid fixture will result in more force on the cells. The rigid fixture essentially no expansion (less than 0.001" with six 6mm rods). Force and distance are essentially fungible for these cells. You can have everything from no force and all expansion to all force and no expansion. Pick your poison. EVE is now saying to pick force.

 

[Cheap 4-life]

That statement is true. Your previous statements about the difference in force between one or sixteen cells are false.

The datasheet clearly says that a rigid fixture will result in more force on the cells. The rigid fixture essentially no expansion (less than 0.001" with six 6mm rods). Force and distance are essentially fungible for these cells. You can have everything from no force and all expansion to all force and no expansion. Pick your poison. EVE is now saying to pick force.

Wait what’s true? I said 16 cells cause more force in a fixed/rigid rig than 4 cells.. that’s all I have ever been saying..
“EVE is now saying”. I don’t think they are saying that at all, as I said you are thinking that the6are implying that. When in fact all they are doing is showing the data from how they tested a cell, again not saying that’s how we should do it

 

[justgary]

Man that is a hell of an implication in my mind.. just because they tested a cell like that and showed us how they tested a cell doesn't mean that’s optimal for cell longevity..

We need to look at EVE's intent here. They sell a product. They want us to believe that their product is superior to their competitor's product so that we buy the one from EVE. Why would they write a datasheet that doesn't put their product in the best light possible?

They tell us that they had over 80% of the original capacity left after 4,000 0.5C cycles in that fixture. If they could have gotten 4,500 cycles some other way, they would have told us how to do that. This is their most current guidance, and I can accept that it supersedes their previous information.

Before you throw calendar aging back in my face, I'll remind you that nobody knows the true answer to that one. EVE probably knows best on that one too. Calendar aging is irrelevant here anyway since we probably can't do anything about that except try to manage the temperature of our cells.

 

[justgary]

I said 16 cells cause more force in a fixed/rigid rig than 4 cells.

No, you didn't say that before. This statement you just made is false. Still.

 

[Cheap 4-life]

“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.”
That was his comment
This was my response
Correct so wouldn’t that extra distance from 16 cells therefore cause more force in a fixed/rigid rig.
Your response was
“That statement is true. Your previous statements about the difference in force between one or sixteen cells are false.”
Notice I said extra, meaning more than the distance from 4 cells

 

[Cheap 4-life]

We need to look at EVE's intent here. They sell a product. They want us to believe that their product is superior to their competitor's product so that we buy the one from EVE. Why would they write a datasheet that doesn't put their product in the best light possible?

They tell us that they had over 80% of the original capacity left after 4,000 0.5C cycles in that fixture. If they could have gotten 4,500 cycles some other way, they would have told us how to do that. This is their most current guidance, and I can accept that it supersedes their previous information.

Before you throw calendar aging back in my face, I'll remind you that nobody knows the true answer to that one. EVE probably knows best on that one too. Calendar aging is irrelevant here anyway since we probably can't do anything about that except try to manage the temperature of our cells.

I think their intent was nothing more than to show us there results from there test which happens to be cells that we buy.. best light? They did put a cell thru the ringer and shared the results. That doesn’t mean that’s what we should do with our cells.
I wouldn’t throw calendar aging in your face. I agree compression is best

 

[justgary]

Notice I said extra, meaning more than the distance from 4 cells

If that was your meaning, the statement is false.

 

[justgary]

That doesn’t mean that’s what we should do with our cells

What, then, should we do with our cells? Please site your research so that we can know what to do.

 

[Ellcon123]

I looked at post #237 and you stated it will take more force to stop 16 cells expanding than 1. On the face of it that is true. If 1 cell expands say 1mm then to stop 16 cells expanding past 1mm will take 16 times the force.

Issue here is pressure and it would increase. The correct analysis is 16 cells can expand a total of 16mm and the force does not change. You are applying the restriction of 1 cell to all 16 which is incorrect.

 

[fafrd]

So your saying a different spring (possibly a different length) should be used for different amounts of cells? Basically saying that the same exact spring shouldn’t be used for a 4 cell and 16 cell pack? I agree
If that’s so then how can the same rigid/fixed type of inclosure be used for 16 cells or 4 cells?

You need to use longer springs for 16 cells than for 4 cells (longer flat spot on the force / displacement curve)…

 

[justgary]

Here is a quiz for everyone. It is open book. Let's assume that the pressure on the cell in A varies from 12 PSI to 20 PSI during our usage, and that the cells have a cross-sectional area of 56 square inches. Further assume that the cells in B and C behave the same way.

At A we have one cell held in a rigid frame just like the datasheet says to do.

At B we have four of those cells, each in their own frame, but the four frames are rigidly clamped (bolted) together.

At C we have four cells, but we have removed all of the inner frame parts and have them in a rigid frame like the datasheet says to do, except we did four cells at a time.

Question 1: How much pressure do the cells in B have on them?
Question 2: How much pressure do the cells in C have on them?
Bonus 1: What is the range of force on the left wall in C?
Bonus 2: What is the range of force on the right wall in C?

 

[fafrd]

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.

Has EVE released a new datasheet for 280Ah-class cells?

The last datasheet I saw spoke about a ‘300Kgf clamping fixture’ with various indirect indications through back channels about minimum and maximum clamping force - is there now a more direct indication from the datasheet about the minimum and maximum clamping force required to achieving specified cycle-life?

 

[42OhmsPA]

Here is a quiz for everyone. It is open book. Let's assume that the pressure on the cell in A varies from 12 PSI to 20 PSI during our usage, and that the cells have a cross-sectional area of 56 square inches. Further assume that the cells in B and C behave the same way.

At A we have one cell held in a rigid frame just like the datasheet says to do.

At B we have four of those cells, each in their own frame, but the four frames are rigidly clamped (bolted) together.

At C we have four cells, but we have removed all of the inner frame parts and have them in a rigid frame like the datasheet says to do, except we did four cells at a time.

Question 1: How much pressure do the cells in B have on them?

12-20 psi

Question 2: How much pressure do the cells in C have on them?

12-20 psi

Bonus 1: What is the range of force on the left wall in C?
Bonus 2: What is the range of force on the right wall in C?

240~560 lb / sq ft.

View attachment 112893

 

[fafrd]

Here is a quiz for everyone. It is open book. Let's assume that the pressure on the cell in A varies from 12 PSI to 20 PSI during our usage, and that the cells have a cross-sectional area of 56 square inches. Further assume that the cells in B and C behave the same way.

At A we have one cell held in a rigid frame just like the datasheet says to do.

At B we have four of those cells, each in their own frame, but the four frames are rigidly clamped (bolted) together.

At C we have four cells, but we have removed all of the inner frame parts and have them in a rigid frame like the datasheet says to do, except we did four cells at a time.

Question 1: How much pressure do the cells in B have on them?
Question 2: How much pressure do the cells in C have on them?
Bonus 1: What is the range of force on the left wall in C?
Bonus 2: What is the range of force on the right wall in C?

View attachment 112893

If within a rigid fixture force never exceeds 20 PSI when maximally charged and force never drops below 12 PSI when maximally discharged, those force extremes remain the same whether you have a single cell, 4 cells, or N cells (as long as the structure is truly rigid) and whether you have individual fixtures for each cell or a single fixture for the entire pack (again, assuming that the fixture is truly rigid).

 

[fafrd]

Correct so wouldn’t that extra distance from 16 cells therefore cause more force in a fixed/rigid rig.

If the cells do not displace at all, there is no force on the busbars.

The need for flexible busbars is driven only by the assumption that cells will displace as a discharged pack expands when charged (which will be prevented by a rigid fixture).

 

[justgary]

Has EVE released a new datasheet for 280Ah-class cells?

The last datasheet I saw spoke about a ‘300Kgf clamping fixture’ with various indirect indications through back channels about minimum and maximum clamping force - is there now a more direct indication from the datasheet about the minimum and maximum clamping force required to achieving specified cycle-life?

I think the new datasheets hit the streets this spring. You should be able to find yours in the Resources section. Don't confuse a LF280 with an LF280K, though.