Compression vs restriction

[Zwy]

Emailed my steel shop about lead time of 8x 3/8 6061 aluminum plate 8”x16”. Hopefully they respond soon.

I'm working on the shop system now and those are double wide. I use 17" wide. You want room for a center rod and the outside.

I think a 10 footer in 8" wide bar stock and 3/8" thick is around $300 at my local welding shop I buy from unless I can find some drop. I have a big metal cutting bandsaw to cut it.

https://www.midweststeelsupply.com/store/castaluminumplateatp5

$35 each for 8

That is cast plate, I prefer 6061 extruded bar stock. That shows $67.61 each for 3/8" x 8" x 17".

 

[Brucey]

These may be an option, $79 for single wide and $100 for two row, think you have to supply your own rods:

Compression Plates for 280Ah 304Ah 314Ah LiFePO4 Cells -EVE CATL REPT

This is a Compression Kit for use on any LiFePO4 cell that is 204mm x 173mm x 72mm in size from many manufacturers like EVE, CATL, REPT, Trina. Cells in this size come in capacities of 280ah, 304ah, 314ah among others.

jag35.com

 

[HRTKD]

These may be an option, $79 for single wide and $100 for two row, think you have to supply your own rods:

Compression Plates for 280Ah 304Ah 314Ah LiFePO4 Cells -EVE CATL REPT

This is a Compression Kit for use on any LiFePO4 cell that is 204mm x 173mm x 72mm in size from many manufacturers like EVE, CATL, REPT, Trina. Cells in this size come in capacities of 280ah, 304ah, 314ah among others.

jag35.com

Capitalism at work!

 

[fatjay]

From what I've seen, all the bulging originates in the middle. So why wouldn't you want to have the rods focused in the middle more, instead of at the top?

Also, how are people moving these lf280k packs? They're close to 200lb built. I thought people build them in place, but apparently that's not always the case.

 

[HRTKD]

Also, how are people moving these lf280k packs? They're close to 200lb built. I thought people build them in place, but apparently that's not always the case.

My battery box is two columns of 8 cells. I assembled each column on the bench and then placed them in the box. In retrospect, building them in the box would have been easier, but I had warming pads to adhere to the sides of each column and that could not be done once the cells were in the battery box. There wasn't enough room on the sides to put the warming pads on after the cells were in the box.

 

[sunshine_eggo]

I promise I'm trying not to be thick. If it was spread out among 4 bolts, would it be divided? Like 4x2lb springs in parallel would be 8lb

You're not being thick. When determining to total clamping force between two components from multiple fasteners, it's pretty complicated due to the limited area of affect. However, when using them to externally clamp a plate or bar and transfer that pressure into an end plate, I'm pretty sure you split it between the fasteners since it becomes a simple force diagram at that point.

7075 is much stiffer but more expensive.

7075 is 4% stiffer than 6061. 7075 can be notably stronger than 6061 depending on its temper, but it also has issues with fracture toughness and stress corrosion cracking thus requiring a T7351 temper. It's still strong, but its cuts the strength over T6 substantially.

Stiffness is determined by the Young's Modulus for the material. Higher modulus means higher stiffness. They will have nearly identical stress-strain diagrams until you hit the yield strength. With lower tensile strength comes lower yield strength. If any of your samples had permanent deflection, then you got into the yield part of the curve, and it's not a representation of stiffness. It's a representation of yield strength.

In other words, if you took two identical samples, one of 7075 and one of 6061, fixtured them and applied a load, they would have the same deflection PROVIDED you did not exceed the yield strength of the materials.

I would not use any 7075 alloy unless it's cheaper and more readily available than 6061-T6 OR you need the higher strength of 7075.

 

[hardtop]

You'll want enough compression so the cells stay together as a unit. That is my approach, and also you'll want the ability to mount things to the battery. I spent $20 on plastic breadboard, threadall, threadall cover, screws. I bought stainless screws but probably could have bought zinc.

Here is my pack that is more restricted than compressed because the plastic bends more than plywood does. Since then I added another lead from the battery terminated with an Anderson connector to have my charger and inverter quick-connected at the same time. The cells aren't separated with plastic shims or whatever. I need to do that and also you'll want to be able to mount the battery if you ever transport it. I need that part too but mounting properly is difficult.

 

[S Davis]

You'll want enough compression so the cells stay together as a unit. That is my approach, and also you'll want the ability to mount things to the battery. I spent $20 on plastic breadboard, threadall, threadall cover, screws. I bought stainless screws but probably could have bought zinc.

Here is my pack that is more restricted than compressed because the plastic bends more than plywood does. Since then I added another lead from the battery terminated with an Anderson connector to have my charger and inverter quick-connected at the same time. The cells aren't separated with plastic shims or whatever. I need to do that and also you'll want to be able to mount the battery if you ever transport it. I need that part too but mounting properly is difficult.

Just make your end plates taller and mount some angle and use vibration dampers.

 

[green mountain boys]

There has been a lot of discussion about aluminum plate used for compression in this thread. Why aluminum and not steel? Steel is cheaper. Is there some reason for using aluminum other than a slight advantage in weight reduction?

 

[HRTKD]

There has been a lot of discussion about aluminum plate used for compression in this thread. Why aluminum and not steel? Steel is cheaper. Is there some reason for using aluminum other than a slight advantage in weight reduction?

For me, I would choose aluminum over steel because:
1. weight
2. I can weld aluminum
3. I have access to an aluminum salvage yard. I bet they would beat the pants off of the price of steel plate.


That said, my first set of 4s batteries used plywood for the end pieces and I didn't detect any issues after four years.

 

[sunshine_eggo]

For me, I would choose aluminum over steel because:
1. weight

It's true that Aluminum is about 1/3 the weight of steel, but I don't know how applicable that is when you're talking about adding a few additional pounds to several dozen. It's certainly worth considering, but it may not be significant in the assembly.

2. I can weld aluminum

We're not worthy. You are a god. Seriously.

3. I have access to an aluminum salvage yard. I bet they would beat the pants off of the price of steel plate.

Since I blathered on about stiffness, steel is nearly 3X stiffer than Aluminum.

 

[fatjay]

I just ordered 12x 8x17x3/8" plates. I was going to order 8 but the price dropped by $12/plate at Qty 11+. So 8 was $300 and 12 was $312.

 

[HRTKD]

We're not worthy. You are a god. Seriously.

Well, not really. I can weld aluminum, but not anything else. Fortunately, nobody inspects my welds and I'm pretty good at cleaning them up with a flap disk.

I have found that I can weld like a mofo with Helium. But that stuff is expensive!

 

[DPC]

We're not worthy. You are a god. Seriously.

Since I blathered.... nearly 3X stiffer....

Someone was going to edit this and snicker....

 

[Aridom82]

Ignorant know it all. Also that dumb OMG youtube face.

kek

 

[Aridom82]

Let's go to the manufacturer's datasheet for the cell (not sure if you have V3):

Resource icon

EVE LF280K V3 PBRI-LF280K-D06-01 datasheet

Oct 17, 2023

EVE LF280K V3 PBRI-LF280K-D06-01 datasheet

• Sleeper85
• Battery Cell Data Sheets

View attachment 255283

This is explicitly telling you to compress the cells. The expectation is that if you don't, you will not attain the claimed cycle life.

Holy s**t 7000N? That is 713Kg. Jesus

 

[AntronX]

7000N? That is 713Kg.

That's too much. Around 11 psi is ideal. Don't over compress.

 

[sunshine_eggo]

That's too much. Around 11 psi is ideal. Don't over compress.

Curious. Why do you know better than the manufacturer?

11psi is about 606#.

 

[AntronX]

Curious. Why do you know better than the manufacturer?

I follow A123 spec in their battery pack design guide posted here a while back.

 

[sunshine_eggo]

I follow A123 spec in their battery pack design guide posted here a while back.

So, you just like one manufacturer more than another...

My experience with A123 packs is exclusively with their failed Gen2 Prius plug-in packs. Horrible implementation. Horrible reliability. Horrible support. Horrible in every way imaginable. Not at all ready for primetime when they hit the market.

 

[AntronX]

So, you just like one manufacturer more than another...

They're all pretty much the same batteries even if A123 or CATL or EVE makes them. I suspect EVE took that 11 - 12 psi sweet spot from A123 chart and calculated 300 kgf for their 280Ah cells.

 

[AntronX]

Check out my badass compression jig.

 

[sunshine_eggo]

Effing love those top crossbars!!!!!

 

[HRTKD]

Effing love those top crossbars!!!!!

A good use for Harbor Freight wrenches.

 

[Zwy]

There has been a lot of discussion about aluminum plate used for compression in this thread. Why aluminum and not steel? Steel is cheaper. Is there some reason for using aluminum other than a slight advantage in weight reduction?

I don't have to paint it and it looks so cool. No corrosion, stronger per pound.

Aluminum can be easier to work with than steel. My first cell fixture was for my truck camper, 1/2" thick aluminum and I tapped threads in it to hold the BMS.



I work with just about any material and for some things aluminum makes the most sense. It costs more but the labor involved with painting it and the paint costs money too. I want to assemble it and not have to repaint it down the road. I can weld aluminum, stainless, steel and braze/silver braze. Stainless is hard to work with but for some applications like my shower stall pan in my truck camper it is totally functional and looks great.



Yes, I fabricated the 2 piece shower pan and even coined the drain hole in the shop press. I purchased the power brush for the brushed look and passivated after welding using electrical current and phosphoric acid.

Back to your original question, it is a matter of choice but I liked using aluminum and the 8" wide flat bar is perfect for battery cells. Steel will work.