Compress or not, flexible busbar or not

fafrd

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In the resource section.
Ahhh, something new to discover (never used the Resource Section before).

Seems you meant the Resource Secrioon of the Forum, right (not this thread).

The only data sheet I did there is Rev B dated March 21, 2021 - is that the latest datasheet you are all referring to?
 

Cheap 4-life

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I’m only charging my 560Ah pack of 16 cells with about 150-200Ah these days, so if I’m getting less expansion than you are experiencing, my small change cycle might be a big part of why…
I thought you said on another thread you are using 24v for the GTIL2s. Or is that a different battery for them?
 

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As I just stated, my current daily charge generation is modest (27% to 36% of 100% of pack rating) so I can can do a check / measurement at that level quickly but getting up close an 80% charge / discharge cycle will be a hassle and will take a ~week…

And that might have a lot to do with why you are not seeing much or any expansion. That’s relatively low/slow charging. And others might also be seeing more expansion than you because they might not actually be applying 300kgf
 

fafrd

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And that might have a lot to do with why you are not seeing much or any expansion.
Which is precisely why I mentioned it.
That’s relatively low/slow charging.
I charge with SC-coupled solar and discharge overnight. By now (late September) I only get ~175Ah on a good day.

In late June I get more than twice that much but that’s a long time from now…

In the interest of science and our DIY community, I’m willing to abort discharge over a few consecutive days so I can measure the expansion closer to maximum SOC levels but this will be slightly painful and I don’t want to do it until next month when our $0.50 peak period from 4-9 pm goes away (until next June)…
 

justgary

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I’m just hoping it’s not to much at high SOC but I’m thinking poron foam as @justgary uses would stop to much pressure from happening at high SOC..
Sorry I missed a lot today; I actually had to do something else. Congratulations to everyone on 20 pages, and we are now discussing the datasheets and what they mean!

Think of Poron foam exactly like a spring. It is not really any different. I assumed no more than 0.5mm expansion per cell. The testing I did told me to use one 1/4" sheet per four cells, so my four cell pack has one sheet and the eight cell packs have two sheets each.
 

justgary

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I think you underestimate how rigid metals are. Say I wrap my battery in a 10mm steel box, similar to what EVE seem to be doing when welding. Having 250mm height and 130mm width I get 2 x 250 mm x 10 mm + 2 x130mm x 10 mm = 5000mm² + 2600mm² = 7600 mm² cross-section area.

Or, let's use aluminium instead of steel, since it's not even half as rigid at 69 GPa.

If we apply 50kN over 7600mm², that gives 6.579 MPa. That creates a stress of 6.679 MPa / 69 GPa = 0.0000967. Over the length of 8 of my 36.35mm cells, a total of 290.8 mm, that gives 0.0000967 * 290.8 mm = 0.02mm of elongation if I'm not messing up my calculations. Or, 0.0035mm per cell.

Using 6 M16 rods of 200GPa steel we get 6 * 125mm²= 750mm². Applying 50 kN we get 66.67 MPa. Stress = 66.67MPa / 200 GPa = .00033. Over 290.8 mm we get 0.097mm elongation. Or 0.012 mm per cell. We can continue calculating how much this would decrease the pressure, but I don't think it would be that much..
Please check your math on this. You only want to use the cross-sectional area of the pieces under expansion force, so don't add the end caps. 2 X 250 X 10 mm = 5000mm^2 is probably what you want if I read your description correctly.

For the rods, the cross section is PI X r^2, or 3.141 * 3^2 = 28.27 mm^2.
 

Cheap 4-life

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Sorry I missed a lot today; I actually had to do something else. Congratulations to everyone on 20 pages, and we are now discussing the datasheets and what they mean!

Think of Poron foam exactly like a spring. It is not really any different. I assumed no more than 0.5mm expansion per cell. The testing I did told me to use one 1/4" sheet per four cells, so my four cell pack has one sheet and the eight cell packs have two sheets each.
4 cell pack has only one sheet on one side?
 

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I’m willing to abort discharge over a few consecutive days so I can measure the expansion closer to maximum SOC

Much appreciated by everyone I’m sure, but your low charge rate might prevent or mitigate expansion
 
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Cheap 4-life

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Think of Poron foam exactly like a spring. It is not really any different. I assumed no more than 0.5mm expansion per cell. The testing I did told me to use one 1/4" sheet per four cells, so my four cell pack has one sheet and the eight cell packs have two sheets each.

Yeah I had to work today.. I was pondering this all day while I was doing a metal roof. This stuffs a fun hobby to me and would rather be thinking about this than working.. but gotta work sometimes I guess..

So how would I then incorporate 4 sheets of foam into a 16cell pack? Or what would you recommend now with the possible implications of the new data sheet. If your saying the foam is just like a spring then I already have springs coming so might as well use them?
I’m thinking atm I want to use 3/8” threaded rod and foam with aluminum end plates, I’ll have 19 cells though not 16.
Really hard for me to know if I got the right springs.. 20mmx10mmx80mm at 50% deflection is 160lbs.. so 640lbs total, but I’m starting to think that’s not enough. I got the springs a little long because I have so many cells in a row but I’m starting to think that I don’t want expansion to occur anyways or barely any..
 
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justgary

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Would it be better to use 1/8” foam on each side (for 4 cell) or doesn’t it work that way
The thickness is not arbitrary, just like spring length. Assuming up to 0.5mm expansion per cell, the force curve of a 1/4" sheet will keep you in the magical 12 to 18 PSI range with four cells.

If you look at the testing I did you might be able to use fewer sheets at higher max pressure. My scale only went to 20 pounds, so I don't have data beyond that.
 

justgary

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PXL_20220922_034118802.MP.jpg

You can see the foam at each end of each box. This was at 100% SOC. Not much expansion from 30%.

I do agree that higher charge rates probably do lead to more expansion.
 

Cheap 4-life

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It really doesn't matter, but I would probably either put two at each end or one every four cells.

Maybe one at each end and two in the middle.
2 at each end seems best to me maybe because I’m lacking the physics knowledge but putting the foam between some cells and not others makes me think some cells are being treated differently than others..
 

justgary

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PXL_20220922_034025528.jpg

By the way, I thought I might show the printed covers also. I thought they came out ok. They snap into the center bars. Maybe they will keep me out of trouble when I open my box lid. I have not put them into the big box yet.

The BMSs will attach to a panel above the batteries.
 

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The thickness is not arbitrary, just like spring length. Assuming up to 0.5mm expansion per cell, the force curve of a 1/4" sheet will keep you in the magical 12 to 18 PSI range with four cells.

This still boggles my brain.. why would 4 cells be any different than 16 cells if more cells to do not apply anymore force to the 1/4 single piece of foam. Are you saying the psi would go above 18 with 16 cells if using one piece of 1/4” foam? if so I would think that could only happen if the extra 12 cells was causing there to be more force applied to the foam.......
 
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Cheap 4-life

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How are the bms leads attached. Hard to see. Are all the bms leads the same length?
 
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