Compress or not, flexible busbar or not

[Cheap 4-life]

You made this up with no basis in fact. EVE actually tested the pressures and reported them in their datasheet.

Again I’ll say they tested one cell, not many against each other

 

[Cheap 4-life]

Using foam is no different than using springs. The foam exerts 300kgf uniformly across the face of the cells just like springs outside of a movable plate do. The pressure is the same. One difference is that springs increase the force with compression distance, but the foam is engineered to keep the pressure constant over a range of compression distances. In other words, the foam holds a constant 12PSI over a portion of the expansion range, while springs only have 12PSI at one distance.

I thoroughly disagree with a lot of this comment... using foam is absolutely different than springs... if a cell pushes/bulges more in the center as they do then a softer surface like foam would allow that bulge to happen more so than a rigid end plate.. that’s common sense foam is softer than a solid end plate.. when the cells are pushing harder against that foam at high SOC there is more pressure at that time,, if there’s not then there would be expansion.. the rigid end plate and springs allows the endplate to slightly move which allows for slight expansion which lessons the force with compression distance compared to using a rigid case or foam that doesn’t even shrink at all during high SOC as you say.. but you also say the cells move in your case so I’m not sure what your saying.. I would say the idea of using springs at all is to lessen the pressure/compression as the SOC rises compared to a rigid case and lessen the pressure compared to using foam in a rigid case that has no noticeable movement as you say.. Using foam would do the same as springs but imo not as good as springs. But again foam is a viable option for our home batteries just not what I would choose. I like the idea of having the cells compressed together like the data sheet with hard end plates and each cell against another hard cell. The hard surface would better prevent bloating simply because it’s a lot harder than foam. But if the cells are compressed in a rigid case with foam to the point where they don’t move at all then bloating wouldn’t be a problem but over compressing at high SOC could be but again that amount of compression might not matter..

 

[Cheap 4-life]

You made this up with no basis in fact. EVE actually tested the pressures and reported them in their datasheet.


Using foam is no different than using springs. The foam exerts 300kgf uniformly across the face of the cells just like springs outside of a movable plate do. The pressure is the same. One difference is that springs increase the force with compression distance, but the foam is engineered to keep the pressure constant over a range of compression distances. In other words, the foam holds a constant 12PSI over a portion of the expansion range, while springs only have 12PSI at one distance.

The great advantage I see to rigid compression is that the cells cannot move in any appreciable way. This means that your busbar issues will go away since they will no longer move and cause trouble.

Cells move and should be allowed to very slightly move or they imo are being over compressed. That’s why welded busbars and flexible busbars are used in almost all EV batteries and used in the good server rack batteries.. So there isn’t cells being read incorrectly by loosening busbars..

 

[Cheap 4-life]

Just for kicks, let's look at the amount a 1/4" stainless rod will stretch when six of them are used to hold EVE 230 Ah cells. The datasheet from EVE says that the maximum pressure should be 15kN if the cells rupture. I'm doing math in public here, but that should come out to about 3,300 pounds of force. A stainless rod should have a Young's Modulus of about 140 gigaPascals, or roughly 20,300,000 PSI. If we calculate the stress on one rod, we should be able to divide later by the number of rods to see how much each rod will expand. Here we go....

Stress = Force/Area = 3,300 lbf / 0.049 in2 (for a 1/4" rod) = 67,227 PSI
Strain = Stress/Young's Modulus = 67,227 PSI / 20,300,000 PSI = 0.003 inches per inch of rod

If we assume that we have 8 EVE 230 Ah cells in a case, we'll need about 17" of rods to cover the length.

Now, 17 inches * 0.003 inches per inch = 0.056 inches.
If we use six rods, that would be 0.056/6 = 0.0094", or about 0.24mm.
The relative motion of each of the eight cells would be 0.0094" / 8 = 0.0011", or just under 29 micrometers.

That's if the cells rupture. Normal use would see much less motion.

I here all your advanced math and such.. but 1 inch of allthread in my world does not stretch at all.. and under the force that these batteries exert, longer allthread would barely stretch and cause to much pressure on the cells which is why foam or springs are used..

 

[noenegdod]

Yes it takes the same amount of psi to keep 16 cells compressed or 1 cell compressed if that compression could vary thru the SOC.. However when those 16 cells try to expand in a fixed case there would be a possibility for 16 times the amount of room that isn’t available due to the fixed case which would increase the pressure/compression within the fixed case more than the pressure seen in a fixed case with only one cell, or am I wrong?
I wouldn’t think the all thread stretches anywhere near enough to keep the pressure from rising. I think the pressure/compression would simply rise and maybe not enough to matter but with springs that significant rise in pressure/compression that’s not needed could be lessened. The springs allowing enough compression to prevent swelling but not to much to allow the compression to rise more than needed at high SOC

My friend, your not understanding the linear relationship between 1 cell and the length of the restraint and 19 cells and the length of the restraint for those 19 cells.
Look at the image below really carefully. The cell is clamped between 2 plates and there are 6 rods holding the plates. Those rods are the same length as the width of the cell. Either the rods are stretching or not. If they are not stretching then the answer is that it is irrelevant and you can stack as many as you want to and everything is fine. If they are in fact stretching then it is still irrelevant (as long as you use the same material for the rods) because when you add a second cell, you add length to the rod and exactly double the amount of stretch to match exactly double the cell expansion. Its all relative.

At the end of the day, put springs, dont put springs. Do what you want to but based on the information below, your extrapolation that you can do it with 1 cell but not with 2 or more is incorrect.

 

[Ellcon123]

1" of allthread does stretch. You just can't see it. Under the same load 2" will stretch twice as much. This happens in everyones world. Every bolted joint in the world relies on this stretch. It's what produces the clamp load. Why you torque a bolt.

 

[42OhmsPA]

I've read enough about springs, foam, rods, and buss bars to make my brain tingle...
Attached is what I'll be using for 2 packs.
For the next packs I'll be going with flexible buss bars (cutting drilling and sanding is a pita) ; if I do anything else I'd add sections of strut across the top, middle and bottom of the cutting boards with some slight overlap on the edges to run some all thread through and tighten down.

 

[Cheap 4-life]

My friend, your not understanding the linear relationship between 1 cell and the length of the restraint and 19 cells and the length of the restraint for those 19 cells.
Look at the image below really carefully. The cell is clamped between 2 plates and there are 6 rods holding the plates. Those rods are the same length as the width of the cell. Either the rods are stretching or not. If they are not stretching then the answer is that it is irrelevant and you can stack as many as you want to and everything is fine. If they are in fact stretching then it is still irrelevant (as long as you use the same material for the rods) because when you add a second cell, you add length to the rod and exactly double the amount of stretch to match exactly double the cell expansion. Its all relative.

At the end of the day, put springs, dont put springs. Do what you want to but based on the information below, your extrapolation that you can do it with 1 cell but not with 2 or more is incorrect.
View attachment 112721

I just don’t think these cells will expand enough or put enough pressure on the allthread to make it stretch. Instead there would simply be to much pressure/compression on the cells because the rigid/fixed case wouldn’t move. Not the same if only one cell is put under compression. If x amount of compression is put on the outside but the cells pressure themselves rises when trying to expand, then the pressure at the outside rigid end plates actually increases as the SOC rises in a fixed rigid case. That’s why foam and or springs are used, to lessen that

 

[Cheap 4-life]

1" of allthread does stretch. You just can't see it. Under the same load 2" will stretch twice as much. This happens in everyones world. Every bolted joint in the world relies on this stretch. It's what produces the clamp load. Why you torque a bolt.

Right but I don’t think allthread will stretch enough to matter or at all from just the pressure these cells exert

 

[42OhmsPA]

Threaded rod / bolts / all steel will stretch, though it may be so miniscule it can't be seen by the naked eye.
I wish my buddy still worked in the lab at work, we'd see what the instron could do ?

Different grades have different elongation rates https://www.allthreadrod.com/grades/

Bolt Stretching and Tensile Stress

Tensile stress and Hooke's Law.

www.engineeringtoolbox.com

Threaded Rods - Loads in Imperial Units

Weight rating of threaded hanger rods.

www.engineeringtoolbox.com

Some good info in here that can relate to this thread, I wouldn't use my ARP hardware I used to build my motor to clamp my cells ??

Stretched to the Limit — What All Those Bolt Alloys And Numbers Mean

Understanding the differences — both in price and performance — of ARP's different bolt materials can be challenging. Here we break it down.

www.enginelabs.com

 

[justgary]

over compressing at high SOC

Again, the new EVE datasheet implies that this is no longer a concern. You don't seem to agree, but that's fine with me.

Cells move and should be allowed to slightly move or they imo are being over compressed.

Please point to the part of the datasheet that says the cells will be "overcompressed."

1 inch of allthread in my world does not stretch at all.

But in the real world it does. I just showed you how much it will stretch.

 

[Cheap 4-life]

I suppose if threaded rod itself stretched enough to not over compress the cells at high SOC, then there wouldn’t be so many people using springs/foam..
Yes the test rig in the data sheet is rigid/fixed but that’s not saying that the force exerted at high SOC is good for the cells.. going by the 17psi is imo good to go by and shouldn’t just be pushed aside because a data sheet shows how they test one cell. They are not saying this I should how it should be done long term and certainly are not saying this is how it should be done with 16+ cells in a row. If that many cells in a row are supposed to be in a completely rigid/fixed structure then I’d assume most of the good quality premade battery would be doing it, but they are not. They all allow some expansion and not just from threaded rod stretch because if there’s very that much pressure in your cells there’s a problem and probably the blow off valve would pop

 

[justgary]

I've read enough about springs, foam, rods, and buss bars to make my brain tingle...
Attached is what I'll be using for 2 packs.
For the next packs I'll be going with flexible buss bars (cutting drilling and sanding is a pita) ; if I do anything else I'd add sections of strut across the top, middle and bottom of the cutting boards with some slight overlap on the edges to run some all thread through and tighten down.

I find it curious that you are worried about springs, foam, and whether steel rod will stretch under force, yet you plan to use HDPE cutting boards as end caps. The datasheet says 8mm steel or aluminum. I cheated that one by using 1/4" aluminum, but then again I built to the old datasheet that said to keep the compression constant. I also used aluminum bar for the sides rather than threaded rods.

The datasheet no longer concerns itself with maximum compression.

 

[noenegdod]

I’d have to disagree with welded busbars causing massive stress on the terminals. The terminals can flex the amount needed. Never have heard of anyone’s terminals breaking or issues because of it.. unless the cells were over charged but in that case there’s other issues. Look it up the better made newer premade packs use welded busbars. BYD SOC etc

Disagree all you want but the fact remains: If you take 2 cells and place them side by side in contact with each other, weld the busbar to the terminal, if the cells expand the busbar will pull on the terminal. Its a fact you can not get around.

I dont have to look up to see that busbars are welded to terminals on premade packs, It is the most reliable and economical way of attaching them but they have taken steps to manage expansion.

What I am curious about is exactly how you know and can state with authority that the terminals can "flex the amount needed".

 

[Cheap 4-life]

I find it curious that you are worried about springs, foam, and whether steel rod will stretch under force, yet you plan to use HDPE cutting boards as end caps. The datasheet says 8mm steel or aluminum. I cheated that one by using 1/4" aluminum, but then again I built to the old datasheet that said to keep the compression constant. I also used aluminum bar for the sides rather than threaded rods.

The datasheet no longer concerns itself with maximum compression.

I agree the cutting board is to weak, but at least he has some compression I guess if some compression was his goal

 

[noenegdod]

I just don’t think these cells will expand enough or put enough pressure on the allthread to make it stretch. Instead there would simply be to much pressure/compression on the cells because the rigid/fixed case wouldn’t move. Not the same if only one cell is put under compression. If x amount of compression is put on the outside but the cells pressure themselves rises when trying to expand, then the pressure at the outside rigid end plates actually increases as the SOC rises in a fixed rigid case. That’s why foam and or springs are used, to lessen that

If it is acceptable for 1 cell it is acceptable for 2, 3 or 20.

 

[justgary]

not saying that the force exerted at high SOC is good for the cells.

They say they still had at least 80% capacity after 4,000 cycles at 0.5C.

a data sheet shows how they test one cell.

How many cells do you suppose they tested in a row before they wrote the older datasheet?

 

[Cheap 4-life]

Disagree all you want but the fact remains: If you take 2 cells and place them side by side in contact with each other, weld the busbar to the terminal, if the cells expand the busbar will pull on the terminal. Its a fact you can not get around.

I dont have to look up to see that busbars are welded to terminals on premade packs, It is the most reliable and economical way of attaching them but they have taken steps to manage expansion.

What I am curious about is exactly how you know and can state with authority that the terminals can "flex the amount needed".

Correct if there’s expansion there will be pull on the terminal. Never disagreed with that.. the terminals will be able to flex with that expansion if some compression measures are used. At least that’s how a lot of major EVs and premade server rack packs are doing it.
I’m saying that with authority that under the proper compression there has never been to my knowledge a problem with terminals breaking. They can flex

 

[SparkyJJO]

Have any EV packs used springs?
If no, then it is a non issue

 

[justgary]

I suppose if threaded rod itself stretched enough to not over compress the cells at high SOC

Again, please post the part of the datasheet that discusses overcompression.