What is the easiest way to compress prismatic Cells?

[time2roll]

Duct tape would provide zero pressure on that face.

Could use a plate or support for the ends to give some moderate pressure.

 

[MiPaul]

This is absolutely not the same product as Rogers Corp EVExtend Poron foam. Review the specs, they are not the same thing at all.

I didn't say it was the same product.

It's spring rate is very close to the proper value to hit the 300kg pressure target. What's still unknown is the amount of expected growth

 

[MiPaul]

Could use a plate or support for the ends to give some moderate pressure.

You could, but spec is 300kg or 661 lbs. I'm not sure duct tape is that strong.

 

[Rednecktek]

5/16" plate steel and 1/2" threaded rod? Those I have in the garage.

 

[time2roll]

600 pounds would take six layers of duct tape. That is the calibration

 

[MiPaul]

5/16" plate steel and 1/2" threaded rod? Those I have in the garage.

I haven't looked at plate design yet, but that has a gut feel of sufficient. Assuming a 1/2"-13 thread, it has a minor diameter of 0.404". Using the tensile yield strength of common A36 steel, that rod would yield at around 4,600 lbs. (0.128 sq in of solid area * 36,000 psi).

Using 4 rods, each should only see 165 lbs at ~12psi. The problem with threaded rod is there is no compliance, no stretch, at these low loads. The expanding cells would be pushing against the solid wall of the end plates. Who knows how high the pressure might be?

 

[Zwy]

I haven't looked at plate design yet, but that has a gut feel of sufficient. Assuming a 1/2"-13 thread, it has a minor diameter of 0.404". Using the tensile yield strength of common A36 steel, that rod would yield at around 4,600 lbs. (0.128 sq in of solid area * 36,000 psi).

Using 4 rods, each should only see 165 lbs at ~12psi. The problem with threaded rod is there is no compliance, no stretch, at these low loads. The expanding cells would be pushing against the solid wall of the end plates. Who knows how high the pressure might be?

You must think these cells are like hydraulic pressure. They aren't.

First problem with springs on the end is the ability of the cells to actually move inside the fixture. You might not think it's important. however for an application such as my truck camper, the springs would be worthless. Have you seen any EV manufacturer use springs for a compression fixture? I haven't. There is a reason why.

BTW, my end plates have foam on them.............

 

[MiPaul]

This is absolutely not the same product as Rogers Corp EVExtend Poron foam. Review the specs, they are not the same thing at all.

The SDS of the McMaster materials says it's Rogers Corp Poron. The 4701-40 is rated (soft), and the 15 pcf from the description has a spring rate that's in the 11 psi range at 25% compression.

The yellow curve in the last graph on the first page is 4701-40 at 15pcf. The thickness is 0.125", not 0.250 as is the McMaster material.

https://www.stockwell.com/data-sheets/poron-cfd-curves.pdf

You must think these cells are like hydraulic pressure. They aren't.

First problem with springs on the end is the ability of the cells to actually move inside the fixture. You might not think it's important. however for an application such as my truck camper, the springs would be worthless. Have you seen any EV manufacturer use springs for a compression fixture? I haven't. There is a reason why.

BTW, my end plates have foam on them.............

Well the manufacture of pretty much every large prismatic cell call out a fixture pressure during cycling, so yes I do believe they require a force applied to the wide side to control expansion. They've even went to lengths of testing cells to show the lifetime benefits of compression at a specific pressure. I suspect they know something.

A stack of cells under 300kg / 660lbs of pressure shouldn't go very far. Unless you plan to hold the end cells and suspend the middle ones, I'm not sure what you're getting at?

No, EV pack manufactures use a specifically engineered foam to control expansion pressures. I'm sure they're privy to a lot more cell & foam data that we DIY'ers have access to. Springs are very know in the engineering world, are readily available, easy to work with, and can be verified for proper force during operation with layman's tools.

They're your cells, you are free to do as you wish. BTW, what's the pressure your end plates are applying to the stack at 40% & 80% SOC?

 

[Rossman]

The SDS of the McMaster materials says it's Rogers Corp Poron. The 4701-40 is rated (soft), and the 15 pcf from the description has a spring rate that's in the 11 psi range at 25% compression.

Rogers manufactures many types of Poron foam. Just was making it clear that the linked foam is *not* the Rogers Corp Poron foam designed for battery applications - specifically called EVExtend - and has entirely different properties & thicknesses than many of the poron foam products that people have been linking from mcmaster/grainger.

Edit: I actually spoke with people at Rogers and the only way to get the Poron foam EVExtend product (which is their foam specifically designed for this purpose) you need to go through one of their preferred converters.
Here's a link to the LEGIT Rogers foam product that is engineered for this purpose: https://rogerscorp.com/elastomeric-material-solutions/poron-industrial-polyurethanes/poron-evextend

 

[S Davis]

You must think these cells are like hydraulic pressure. They aren't.

First problem with springs on the end is the ability of the cells to actually move inside the fixture. You might not think it's important. however for an application such as my truck camper, the springs would be worthless. Have you seen any EV manufacturer use springs for a compression fixture? I haven't. There is a reason why.

BTW, my end plates have foam on them.............

My cells have not moved at all, they are clamped at 640lbs. But they are mounted on vibration dampers. They do expand length wise I can see the spring length change from 10% to 90% soc, so I would be concerned about too much pressure building in a ridged fixture. My cells are in a work rig that is driven on very rough roads.

 

[Rossman]

My cells have not moved at all, they are clamped at 640lbs. But they are mounted on vibration dampers. They do expand length wise I can see the spring length change from 10% to 90% soc, so I would be concerned about too much pressure building in a ridged fixture. My cells are in a work rig that is driven on very rough roads.

Every manufacturer of off-the-shelf batteries uses a rigid enclosure though, and the higher end ones include foam on one side to, presumably, take up some of that pressure from expansion under full charge.

 

[MiPaul]

Rogers manufactures many types of Poron foam. Just was making it clear that the linked foam is *not* the Rogers Corp Poron foam designed for battery applications - specifically called EVExtend - and has entirely different properties & thicknesses than many of the poron foam products that people have been linking from mcmaster/grainger.

Edit: I actually spoke with people at Rogers and the only way to get the Poron foam EVExtend product (which is their foam specifically designed for this purpose) you need to go through one of their preferred converters.
Here's a link to the LEGIT Rogers foam product that is engineered for this purpose: https://rogerscorp.com/elastomeric-material-solutions/poron-industrial-polyurethanes/poron-evextend

No, I get they're not exactly the same, the 4701-40 at 15pcf is the closest material to the EVExend that is available to consumers. Too many unknowns for me, that's why I decided to design using springs.

Every manufacturer of off-the-shelf batteries uses a rigid enclosure though, and the higher end ones include foam on one side to, presumably, take up some of that pressure from expansion under full charge.

From what I've been reading, no pressure is preferable to over pressure. Are these name brand batteries and are they using prismatic cells?

My cells have not moved at all, they are clamped at 640lbs. But they are mounted on vibration dampers. They do expand length wise I can see the spring length change from 10% to 90% soc, so I would be concerned about too much pressure building in a ridged fixture. My cells are in a work rig that is driven on very rough roads.

Excellent news. I was thinking of using isolation dampers too.

 

[Rossman]

Are these name brand batteries and are they using prismatic cells?

Absolutely, everyones favourite unit (gyll lifepower4) is built like this, and if you watch LithiumSolars YT he tears down a bunch of different batteries and shows what all the manufacturers are doing.

 

[100 Proof]

What's still unknown is the amount of expected growth

.5mm per cell.

 

[noenegdod]

I haven't looked at plate design yet, but that has a gut feel of sufficient. Assuming a 1/2"-13 thread, it has a minor diameter of 0.404". Using the tensile yield strength of common A36 steel, that rod would yield at around 4,600 lbs. (0.128 sq in of solid area * 36,000 psi).

Using 4 rods, each should only see 165 lbs at ~12psi. The problem with threaded rod is there is no compliance, no stretch, at these low loads. The expanding cells would be pushing against the solid wall of the end plates. Who knows how high the pressure might be?

You are right, that is the problem. No one knows what happens if you do not allow the ~0.5mm of expansion to happen or how much pressure is built up within the cell. Does the short side expand instead? top and bottom? What pressure do they actually see? You are right to not absolutely confine them.

 

[MiPaul]

.5mm per cell.

Thanks, is that 0.5mm documented someplace or is that your experience? And is that under a known load, or free to roam?

 

[Rossman]

Thanks, is that 0.5mm documented someplace or is that your experience? And is that under a known load, or free to roam?

There was a spec sheet posted somewhere for the EVE cells that had a width at 30% state of charge and another width listed at 100% state of charge. Unfortunately I no longer remember where I saw that particular document, it might have been in one of LithiumSolar's videos on the subject.

EDIT: I'm also not the person you replied to, so I can't attest to 0.5mm being the right figure, FWIW. I just know there was a spec sheet somewhere with this called out.

 

[Rossman]

Ok so it is 0.5mm according to the specs shown in this video at 2m 51secs

 

[Zwy]

I just tore down 16 cells that were in parallel for top balancing in a compression fixture, Poron between the cells. Poron is open cell and there is information on why Rogers manufactures Poron for EV batteries and why open cell is used. Not hard to find.

The foam had a slight edge on the outer showing where it had been compressed upon immediate disassembly. That edge is no longer there. This shows why open cell just plain works, where closed cell over time will lose the air in between the foam cells.

As for springs and all this other discussion, there are threads here in the forum, some are over 30 pages long discussing various methods for compression fixtures, springs, actual computations per sq mm, etc. If you want to rehash all this again, I suggest going there first and read up. It's pretty dry though I will warn you. And yes, you will find the amount of expansion per cell if you really want to look for it.

For me, it's a simple compression aluminum plate fixture using 5/16" threaded rod with compression limiters, with foam in between cells and the end plate. I'm pretty certain calendar aging will cause more cell capacity loss than attempting to perform hocus pocus in an attempt to gain an extra 5 or 10 cycles. The main reasons for cell compression are #1 to keep busbars from damaging cell terminals first and foremost. This can be caused by cell bulging which stresses the terminals or expansion/contraction of cells leading to loose busbars. #2 is to maintain some life span for cell capacity long term. #3 is to make a fixture to hold it all together depending on conditions or if portability is desired. An example would be having a DIY battery in an RV subject to various vibrations and movement.

This subject has been beat to death. Many think they will reinvent the wheel, but a wheel is still round.

 

[100 Proof]

Thanks, is that 0.5mm documented someplace or is that your experience? And is that under a known load, or free to roam?

As others have said, it is documented. In my own personal experience, it's been just under .5mm measured in a compression fixture. I used .5mm in all my design decisions though.