EVE-280 cells should these be clamped tight or spaced for expansion?

[S Davis]

Pressure is distributed evenly among all cells in a row.

Seems to me it would be higher pressure on the negative side of the cell if that is where the expansion occurs, not talking end to end.

 

[RCinFLA]

So does that mean one side of the cell pack is expanding more, uneven pressure from one side to the other?

Yes, there is some variation across surface of cell. Thing to watch out for is high pressure points that could crush separator.

 

[S Davis]

Yes, there is some variation across surface of cell. Thing to watch out for is high pressure points that could crush separator.

Ok i alternated my cells, positive, negative, positive, negative and used springs @640lbs with acrylic separator in between the cells so they have a flat surface.

 

[off.the.grid]

My 2 cents and just an observation, but no expert here.
My view is that there should be as little movement of the inside layers as possible, this is the reason why cylindrical cells are so good is that there is no room for the inside to move, its all pressing hard against the outer casing and has no where to go, whereas a prismatic cell there is quite a flat surface area of pressure from the expansion, along with a relatively thin outer case.
Having them clamped tight to prevent expansion would give a similar effect to the internal pressure against the outside wall of a cylindrical cell, the key is not to exert too much pressure.
From what im reading in this thread, it appears that its safe enough to do this on a discharged cell and even with the increase of pressure from charging, will be safe.
Personally I dont see any need for springs and would still mean the cells are allowed to expand and contract, my view is that this is still not ideal for the cells and is best to restrict the movement/expansion as much as possible.
As others have said here, EVE never specified the use of springs, it seems to be one interpretation of implementing applied force and perhaps is a foolproof way to do this, but it still means the cells can expand.
I would like to see how these cells are installed in EV's

 

[justgary]

My 2 cents and just an observation, but no expert here.
My view is that there should be as little movement of the inside layers as possible, this is the reason why cylindrical cells are so good is that there is no room for the inside to move, its all pressing hard against the outer casing and has no where to go, whereas a prismatic cell there is quite a flat surface area of pressure from the expansion, along with a relatively thin outer case.
Having them clamped tight to prevent expansion would give a similar effect to the internal pressure against the outside wall of a cylindrical cell, the key is not to exert too much pressure.
From what im reading in this thread, it appears that its safe enough to do this on a discharged cell and even with the increase of pressure from charging, will be safe.
Personally I dont see any need for springs and would still mean the cells are allowed to expand and contract, my view is that this is still not ideal for the cells and is best to restrict the movement/expansion as much as possible.
As others have said here, EVE never specified the use of springs, it seems to be one interpretation of implementing applied force and perhaps is a foolproof way to do this, but it still means the cells can expand.
I would like to see how these cells are installed in EV's

Bingo! That is my interpretation as well. EVE now seems to specify a static force at ~30% SOC and don't worry about the increased force as SOC increases. I built my box with Poron foam before that came to light. I would just make a rigid case if I were doing it again today.

One way to do this might be: Stack the cells as you would have them in your case (separators, etc.) in a vertical stack, then stack 660 pounds minus half the mean weight of the stack on top (about 100 pounds per 16 cells). Next, carefully measure the height of the stack at each corner. Use the mean value of the distances to build a box with that internal size. Now you either clamp the whole thing together to bolt it, or you use studs on your frame and pull the end plate together with nuts (using a tightening pattern similar to car head bolts).

The great part about this is that only one person needs to carefully measure a stack and share the measurements. If you have the same cells, your answer should be the same.

The bottom line is exactly like @off.the.grid said: Nobody worries about pressure in their cylindrical cells at all, yet they come to fisticuffs if you suggest that their prismatic cells should be treated differently.

 

[off.the.grid]

Bingo! That is my interpretation as well. EVE now seems to specify a static force at ~30% SOC and don't worry about the increased force as SOC increases. I built my box with Poron foam before that came to light. I would just make a rigid case if I were doing it again today.

One way to do this might be: Stack the cells as you would have them in your case (separators, etc.) in a vertical stack, then stack 660 pounds minus half the mean weight of the stack on top (about 100 pounds per 16 cells). Next, carefully measure the height of the stack at each corner. Use the mean value of the distances to build a box with that internal size. Now you either clamp the whole thing together to bolt it, or you use studs on your frame and pull the end plate together with nuts (using a tightening pattern similar to car head bolts).

The great part about this is that only one person needs to carefully measure a stack and share the measurements. If you have the same cells, your answer should be the same.

The bottom line is exactly like @off.the.grid said: Nobody worries about pressure in their cylindrical cells at all, yet they come to fisticuffs if you suggest that their prismatic cells should be treated differently.

I feel the 30% SOC figure is a safe bet, its good that EVE now specify this, thats near enough to the SOC i had when I clamped mine and I would not feel comfortable clamping the pack at near zero SOC.

It makes me wonder if the commercial enclosures were designed with this method in mind, but I see that the separators vary in thickness, so would need to specify what thickness to go with etc with that method.

There are a few things I would do differently next time, firstly I wish I clamped all my cells prior to top balancing, but I also would have gone with a more rigid material for the clamps.
I was originally going to go with this stuff here, its pretty damn strong and rigid.

Phenolic Cotton Cloth Laminated Sheets

Phenolic Cotton Cloth Laminated Sheets Electrotechnical sheet textolite or Phenolic Cotton Cloth Laminated Sheets is a laminated material made by hot pressing of cotton cloths impregnated with a thermosetting phenol-formaldehyde-based binder.Due t

www.gteek.com

I ended up using a thick plastic material that has shown some deformation after charging, I dont know if its a big deal in the scheme of things, but its still keeping deformation to a minimum and doesnt appear to be getting any worse with each charge cycle.
My only real concern is the possibility that bolt tension is not evenly distributed, largely because the slight bulging after the top balance affected the spacing and couldnt get all the bolts down evenly without the risk of overdoing it.
I might go around the bolts again at 30% SOC and see if any feel too tight or loose, etc.

My advice to any beginners starting out here is to use the most rigid material they can find, install the insulating fibreglass separators and carefully go round tightening each bolt just enough to keep the cells fom dropping out while lifting the pack by the clamping fixture.
Forget messaing around with foam and springs etc, we want to prevent anything that lets the cells expand to begin with.

This is what I ended up doing sans clamping prior to the top balance.
It starts to get too technical when you go down that rabbit hole of calculating force, bolt torque, etc and many torque wrenches dont go low enough to do what we are trying to achieve.
This might not be the most optimum for longevity, but its sure going to be better than not providing any fixture at all and is much safer than clamping cells too tight.
Restricting cell movement is much easier than correctly compressing cells and will give acceptable results.

 

[Sonne]

What is the updated static force spec Eve are specifying at 30% SOC? Is it still 300kgf?

 

[Daddy Tanuki]

What is the updated static force spec Eve are specifying at 30% SOC? Is it still 300kgf?

who knows? whatever they think you will swallow perhaps?

 

[upnorthandpersonal]

What is the updated static force spec Eve are specifying at 30% SOC? Is it still 300kgf?

Still the same: https://www.gobelpower.com/download/EVE-314Ah-CB31-LiFePO4-Cell-Datasheet.pdf

See page 20 and onwards.