Compress or not, flexible busbar or not

[noenegdod]

9min into this video

He basically says more cells is more force..
For example, if I added more weight to the rig I used to do my testing of my springs, then the spring would get shorter because there was more weight on the spring. Therefore at the ends of the springs there is more force/pressure being applied.. Now if those same springs are used on my 19cell pack, then as the 19 cells expand (up to .5mm each) the spring would get shorter. Then the same as I just explained happened on my test rig would happen,, more force/pressure would be applied to the cells because at the ends of the springs there’s more force/pressure being applied do to the expansion of 19cells and how much shorter they made the spring get. Now the kicker,, IF THOSE SAME SPRINGS were used for 1 cell (not 19) the spring would NOT get nowhere near as short as it does with 19 cells. Now if it’s understood the shorter the spring gets the more pressure/force that’s applied, then there’s no way it can’t be agreed that there should more pressure/force in the 19 cell rig than the 1 cell rig..
Yes I understand to use different size springs to make sure the pressures/forces are the same in a 19 cell or 1 cell pack.. but that doesn’t change that more cells are causing more pressure/more force (due to the additional expansion length) in a fixed fixture that can’t change its springs because it doesn’t have any..

Sadly, you still dont have a clue what your talking about.

 

[Cheap 4-life]

Sadly, you still dont have a clue what your talking about.

Well feller I’m not the only one.. he says the same thing in his video and two others have said the same thing on this thread and im sure many others are thinking the same thing. If I’m so wrong then explain how more cells compressing a spring more than fewer cells isn’t because of more force

 

[noenegdod]

Well feller I’m not the only one.. he says the same thing in his video and two others have said the same thing on this thread and im sure many others are thinking the same thing. If I’m so wrong then explain how more cells compressing a spring more than fewer cells isn’t because of more force

I said it all previously. Early on in this thread and you refused to take the time to attempt to understand it. You even said that one of the examples I was trying to use to get through to you was irrelevant because it used gravity as a load. Now, you are using gravity to test your springs. You just dont get it.

 

[Cheap 4-life]

I attempted and it still doesn’t make sense to me.. More expansion distance with more cells will equal more force therefore more pressure within a rigid fixture. Cells cannot be completely stopped from expanding unless they are over compressed. One cell expanding .5mm in a rigid fixture is less force than 19 cells expanding 9.5mm in a rigid fixture..

 

[noenegdod]

Here ya go

pushing a car is a bad example for a couple different reasons so try this instead.

If two people are lifting a 400lbs object and are standing side by side they are both supporting 200 lbs. Correct?

You have to ignore the weight of the second person. If instead of standing side by side, you picked up the second person who picked up the 400 lbs object. You now have to be able to hold 400 lbs and so does the second person. Does that make sense?

Side by side you both only have to be able to pick up 200 lbs. If you are in a row where you are picking up your weightless friend and he is picking up the object you both need to pick up 400 lbs. Please think this through. This is not opinion. it is fact. Simply the way the universe works.

^^^Post 252 and then your reply:

Yes and yes.. but that still does not change the fact that if someone is pushing on my back like a cell is pushing on another, there’s is then more force.. you are adding gravity into the physics,, when it’s pushing instead of lifting then gravity is irrelevant other than the friction the weight has in the ground due to gravity but that's also irrelevant in my examples.

demonstrating a complete lack of understanding.

 

[noenegdod]

I attempted and it still doesn’t make sense to me.. More expansion distance with more cells will equal more force therefore more pressure within a rigid fixture. Cells cannot be completely stopped from expanding unless they are over compressed. One cell expanding .5mm in a rigid fixture is less force than 19 cells expanding 8mm in a rigid fixture..

Will you try to follow the logic with me and not argue why I am wrong? I will take you down the road again but you have to be willing to just listen and not close your mind and insist you are right.

 

[Cheap 4-life]

Will you try to follow the logic with me and not argue why I am wrong? I will take you down the road again but you have to be willing to just listen and not close your mind and insist you are right.

I have been trying to follow/listening to what your saying. I’m not insisting I’m right or closing my mind. I’m explaining why that doesn’t make sense to me.. I think it would be best if you can tell me how what I’m saying is wrong instead of just saying it’s wrong..

 

[noenegdod]

I think it would be best if you can tell me how what I’m saying is wrong instead of just saying it’s wrong..

Several people have tried that already. Its like herding cats. You simply insist on introducing noise.

Will you try to listen and follow without arguing and introducing noise into the discussion.?

 

[justgary]

One cell expanding .5mm in a rigid fixture is less force than 19 cells expanding 8mm in a rigid fixture..

One more time... in a rigid fixture, the cells cannot expand (that would be the definition of a rigid fixture). All they can do is build pressure. If one cell builds X pressure as it charges in a rigid fixture, a whole pack of cells will build the same pressure in a rigid fixture. I demonstrated that with the image of a pack of cells with rigid plates in between.

You continue to confuse springs with a rigid fixture. They are different. The more you constrain the cells, the less they expand and the more pressure they build (presumably). Once they are fully constrained, they no longer expand. Since they don't expand, a pack of 18 of them will not expand in a rigid fixture either, which means they all build the same pressure. Keep in mind that the pressure is caused by the walls pushing back against the cells as they try to expand.

 

[Cheap 4-life]

One more time... in a rigid fixture, the cells cannot expand (that would be the definition of a rigid fixture). All they can do is build pressure. If one cell builds X pressure as it charges in a rigid fixture, a whole pack of cells will build the same pressure in a rigid fixture. I demonstrated that with the image of a pack of cells with rigid plates in between.

You continue to confuse springs with a rigid fixture. They are different. The more you constrain the cells, the less they expand and the more pressure they build (presumably). Once they are fully constrained, they no longer expand. Since they don't expand, a pack of 18 of them will not expand in a rigid fixture either, which means they all build the same pressure. Keep in mind that the pressure is caused by the walls pushing back against the cells as they try to expand.

I agree with everything you said.. except that the cells don’t expand in a rigid fixture.. my understanding is that each cell expands from roughly 71.5mm to 72mm thru SOC regardless of being in a rigid fixture unless the cells are compressed more than 300kgf/over compressed. Are you saying at 300kgf the cells are fully constrained in a rigid fixture, as in the cells can’t expand at all? I here ya basically saying the cells can’t expand but there is apparently rod stretch. As I said before way earlier in this thread, I can understand that if expansion DOESNT HAPPEN that there isn’t anymore expansion force..

Are you saying that in a fixture THAT DOES allow the cells to expand that the additional expansion from more cells creates more pressure in the fixture? That’s all I’m saying is happening as I explained by using the same exact springs for a one cell pack and a 19cell pack.. I don’t think there’s anything I’m not understanding here.. I seen @cinergi 16 cell pack expand a good bit and he had it compressed to 300kgf with springs..

 

[Cheap 4-life]

Several people have tried that already. Its like herding cats. You simply insist on introducing noise.

Will you try to listen and follow without arguing and introducing noise into the discussion.?

If you consider me explaining why I don’t agree with you as arguing and introducing noise then you and I do not need to discuss this any further.. that’s how conversations happen..

 

[justgary]

That’s all I’m saying is happening as I explained by using the same exact springs for a one cell pack and a 19cell pack.

You did mention this before, but the example you used had the springs bottoming out before the supposed 0.5mm of expansion could happen to all 19 cells. My response was that to keep the pressure curve the same for 19 cells as it is for one cell, you need 19 sets of those identical springs.

Either you want to go by the old data and keep the pressure constant, or you want to go by the new data and keep the distance constant. Which do you want to do?

 

[justgary]

Note also that the 20Ah cells mentioned in the 2014 paper are pouch cells, and are not contained in an aluminum box.

It is possible that EVE now puts a sheet of Poron foam inside of the box for each cell....

 

[fafrd]

Update on my 16-cell pack.

Using a ruler I could never measure any expansion but it occurred to me to position washers against the sides which should be pushed out by expansion and left there if/when the cells contract.

The washers were ever so slightly pushed out fro the walls of the fixture - under 1/2mm per side so under 1mm total.

This was for a very modest charge/discharge cycle of only 10-15% of rating, so there would probably be more expansion from a deep discharge to 20% to a full charge up to 90%…

 

[Cheap 4-life]

You did mention this before, but the example you used had the springs bottoming out before the supposed 0.5mm of expansion could happen to all 19 cells. My response was that to keep the pressure curve the same for 19 cells as it is for one cell, you need 19 sets of those identical springs.

Either you want to go by the old data and keep the pressure constant, or you want to go by the new data and keep the distance constant. Which do you want to do?

I don’t think I did say the springs were bottoming out..
I did say in my other example with one person pushing on the others back that they would create more pressure pushing on the car that’s against a concrete wall than one person alone would create... I still agree with that.. however I realize why using humans is kinda a bad example. If expansion of the cells is close to stopped by the rigid fixture then the cell behind a cell isn’t applying more force even though the second person pushing on the back of the first person can apply more force because nothing is stopping the second person from creating more force like the second cells expansion is stopped therefore stopping the extra expansion force..
however I’m still saying that not all expansion in a rigid fixture is stopped simply because it’s in a rigid fixture..
doesn’t every cell still expand by up to. 5mm thru SOC unless compressed to tight? Yes that’s only 8mm for a 16cell pack but that’s still more expansion force than one cells .5mm. I’d assume the rigid fixture would stretch the 8mm due to the extra force of having more than one cell in it’s fixture.. and if it can’t stretch enough for the .5mm per cell expansion, then could there possibly be to much pressure being applied to the cells..

 

[noenegdod]

If you consider me explaining why I don’t agree with you as arguing and introducing noise then you and I do not need to discuss this any further.. that’s how conversations happen..

A conversation is not what is happening when two people are trying to convince each other they are right. That goes nowhere and is exactly why you have not learned this very basic concept.

A conversation is where someone says something and you actively try to understand what the person has said, restate what the person has said in your own words and they actually agree with it.

To be honest, I have not done that with your argument because:

1. I have formal education on the subject
2. I have practical experience with the subject
3. I am far from alone on the subject
4. You have acknowledged you have no education on the subject and are drawing on only "common sense" which functions as a "rule of thumb" but almost always fails at some level of analysis.

When you do not understand something you have to be willing to abandon all your preconceptions and explore the other ideas. It does not mean you can not return but if you insist on dragging them into the conversation you will stay stuck where you are.

 

[Cheap 4-life]

Either you want to go by the old data and keep the pressure constant, or you want to go by the new data and keep the distance constant. Which do you want to do?

I think the old data still stands and the new data is simply how they tested a cell and it can be done that way in a rigid fixture but not that it’s optimal for the cells.
I will be using springs and laminated flexible busbars..
springs mostly because if there ever is a problem with one cell expanding or as the cells age causing more pressure within the fixture, the springs can accommodate for that.. and springs also to stay away from the higher pressures that I believe isn't the best for these cells..
Laminated flexible busbars to keep the busbars from sliding or loosening (from any reason) to always insure the best connection to the bms and from one cell to another..
I will also be tapping my busbars with m4 threads for balance wires so that doesn’t interfere with the torque on the nuts holding the busbars to the cells

 

[Cheap 4-life]

A conversation is not what is happening when two people are trying to convince each other they are right. That goes nowhere and is exactly why you have not learned this very basic concept.

A conversation is where someone says something and you actively try to understand what the person has said, restate what the person has said in your own words and they actually agree with it.

To be honest, I have not done that with your argument because:

1. I have formal education on the subject
2. I have practical experience with the subject
3. I am far from alone on the subject
4. You have acknowledged you have no education on the subject and are drawing on only "common sense" which functions as a "rule of thumb" but almost always fails at some level of analysis.

When you do not understand something you have to be willing to abandon all your preconceptions and explore the other ideas. It does not mean you can not return but if you insist on dragging them into the conversation you will stay stuck where you are.

I think I’m understanding just fine without your special way of having a conversation that appeases you.. nothing I have said is false. Might not be said in a way a formally educated in the subject (as you describe yourself) person can understand..
More cells does cause more force than one cell if the cells are allowed to expand. It’s called additional expansion length force.. and that’s basically all I was ever saying but kept getting shot down with “more cells doesn’t cause more force”.. I explained via the exact springs used on a one cell vs 16 cell pack that the springs on the 16cell pack compress more therefore the 16cells caused more force on the spring as one cell did on the same spring..
I tried my best to get my point across. Hate that it didn’t line up with exactly the way you would say it, but people talk different ways... I build houses and I’m as educated as I need to be to do that..

 

[Skypower]

Good read that one, i know it's not Eve, so some might say irrelevant.

I'm in same ballpark as you at around 10psi / 500kgf/ 82% of Eve 300kgf per cell to start with, the springs I'm using for compression of double row of LF280K then allow 6mm of linear movement which takes kgf up to 660 / 13psi / 110% of 300kgf per cell. Before anyone says - figures rounded up / down, so not spot on, but near enough for me.

In the paper - page65 specifications psi 8 - 14psi, which equates to 200 - 350 kgf per cell. Which is all but bang on what I've designed from Eves specifications.

Interesting to read on thermal dissipation with thermally conductive cell separators, don't feel this is required for my set up as not planning on high C charge /discharge rates, but if required ideas already in my head.

Just a small breeze running across the terminal/buss/jumpers pulls an amazing amount of warmth out of the core.

 

[Ampster]

Just a small breeze running across the terminal/buss/jumpers pulls an amazing amount of warmth out of the core.

I had considered using heavy duty aluminum bus bars to dissipate heat passively. I did the math and there is not that much heat from charging my 42kWh pack at 80 Amps for a few hours in the morning while my garage is still cool.