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

I decided on using four 160# die springs, I set them at 11.6psi at 50%soc. Based on the expected expansion they should be in the sweet spot. I was worried just clamping with no room for expansion that the pressure could build over the max recommended.

S Davis said:

View attachment 71722I decided on using four 160# die springs, I set them at 11.6psi at 50%soc. Based on the expected expansion they should be in the sweet spot. I was worried just clamping with no room for expansion that the pressure could build over the max recommended.

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Do you have a link to the springs?

I suppose the 300kg compression is the total force onto the cell surface. other units do not make lot of sense

Simi 60 said:

Anyone got any further input as to what the "actual" torque wrench setting would be to get to the 12ft/lb in the eve280 specs?

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here is the tricky park with that question when looking at the EVE fixture spec and how I am reading what they are TRYING to tell us...


From the EVE 280Ah spec version E page 4/11 we get ONLY the following:


Notice it mentions a fixed FORCE; it does not say 300kgf at 30% and <some other>kgf at 100%; it just says a 300kgf is applied.

We can derive the PSI (whichis a force over an area) by looking at the same spec and getting the area of the cells so that is how we get to PSI.



so now we have a FIXED force per square inch.

the next issue is the spec clearly states the THICKNESS of the cell WILL change (ref page 4 of spec):


so the challenge is how to maintain a FIXED force across a battery in a fixture that WILL expand.
Worst case expansion of a NORMAL battery is:
(71.5mm - 1mm) = 70.5mm at 30%SOC
(72mm+1mm) = 73mm at 100% SOC
====UPDATE===
others have pointed out below that what the spec sheet really means is that the expansion due to charging is only 0.5mm (that is 20thou of an inch); the rest of the +/-1 is manufacturing tolerances. This could be, however, measuring my cell actually expand more than 0.5mm and they are grade A eve cells so not sure what to make of it..could still be a variance which means over a series fixture of many cells it should "average out"...
============


so the battery WILL change its width by 2.5mm PER CELL and that is normal!

A 24v-8cell battery pack in a series fixture will increase its overall length by (8 * 2.5mm) = 20mm or 0.78in when charged from 30% to 100%

this is why you need to be careful on using a RIGID fixture if you are trying to follow the spec; if the cell fixture is rigid the pressure will get quite high as it is normal for the cells to expand when charged, you must actually let them expand.

basically you need to design a system that provides 300kgf at 30% capacity AND a 300kgf force at 100% capacity.
This is why engineered linear compression springs are important as they provide a true spring constant(lbs/inch of travel) so you can actually calculate what will happen to your battery pack as the series cell expands.

I use engineered foam spacers every other cell (so each cell has at least 1 foam side to push against) AND springs so that cells can still expand while the springs are continuing to provide a known force (but not to much force hehe)

another option is if your cells spend most of their time charged, well, make sure to apply the 300kgf when at 100% and for the time when cells are disharged let the pressure drop a bit ;-)

Bob B said:

Do you have a link to the springs?

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I used these. I measured their K values to be ~165#/inch. $3.29 each. Easy to get.

Diysolar123 said:

so the battery WILL change its width by 2.5mm PER CELL and that is normal!

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I've measured my Eve 230s at closer to~.5mm per cell.

100 Proof said:

I've measured my Eve 230s at closer to~.5mm per cell.

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keep in mind the spec is literaly +/- 1mm at 30%SOC and again +/-1mm at 100%SOC... so your measurement is in that range ;-)

Diysolar123 said:

keep in mind the spec is literaly +/- 1mm at 30%SOC and again +/-1mm at 100%SOC... so your measurement is in that range ;-)

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I only mention it because purchasing and/or preloading springs to handle expansion of ~20mm vs ~4mm (in the case of 8 cells lined up) could be critical.

Diysolar123 said:

keep in mind the spec is literaly +/- 1mm at 30%SOC and again +/-1mm at 100%SOC... so your measurement is in that range ;-)

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I think you misunderstood that part (but really nice post otherwise BTW ), the +/- 1 mm is just a manufacturing tolerance, the expansion will be around 0.5 mm between 30 and 100 % no matter what.

The typical cell will be 71.5 mm @ 30 % and 72 mm @ 100 % but for example you can have a cell that is 70.8 mm @ 30 % then it'll be 71.3 mm @ 100 %.

I guess the datasheet should have been clearer and said 71.5 mm +/- 1 mm @ 30 % and +0.5 mm @ 100%. But I think we're quite lucky with what we have, I've seen datasheets much worse than that...

Damn interwebs and it's rabbit holes
Woke up this morning, read comments, searched some more, got even more confused
Went down and backed off nuts, did up until finger tight and gave them a couple of turns more.
Not enough for my torque wrench to register.

Bob B said:

Do you have a link to the springs?

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Yes I got them at Penn tool.

Penn tool

I finger tightened my cells when I did the top balance for the first time. Perhaps 30%-40% SOC after removing the cells from the shipping container when I hand tightened them. I needed a wrench to get the bolts off when the top balance was done.

I’m not positive a torque is as important as sandwiching the battery between two pieces of something that won’t bow out. 1/4” plywood will work on my Tiny 25 ah cells, but not for the bigger 280 ah cells.

I think the nuts need to be tight enough not to fall off when the cells get to a low SOC.

I think a XXX ft LBS would not be correct.

BiduleOhm said:

I think you misunderstood that part (but really nice post otherwise BTW ), the +/- 1 mm is just a manufacturing tolerance, the expansion will be around 0.5 mm between 30 and 100 % no matter what.

The typical cell will be 71.5 mm @ 30 % and 72 mm @ 100 % but for example you can have a cell that is 70.8 mm @ 30 % then it'll be 71.3 mm @ 100 %.

I guess the datasheet should have been clearer and said 71.5 mm +/- 1 mm @ 30 % and +0.5 mm @ 100%. But I think we're quite lucky with what we have, I've seen datasheets much worse than that...

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I have updated my posting above as your interpretation of the spec including manufacturing tolerances and SOC tolerances is certainly different than what I was thinking they were trying to say.
I have several cells I guess I could take them out of the fixture and run them thru a discharge cycle while watching with my mitutoyo dial indicator. From the initial cell I did a test cycle on I have more than 20thou of expansion when charged to 100% because of how it felt (not a huge bulge but it definitely did grow so I could easily feel the difference).
I could put the battery on a surface plate to keep things stable; I hate lugging that thing around, even the small one weighs a lot hehe...
Not sure how to handle the fact that the growth is not uniform, maybe if I secured one side of the battery, then use my surface plate height guage with a large plate attached to it and put the dial indicator on the back of that...hmmm, that should work, maybe???

BiduleOhm said:

I think you misunderstood that part (but really nice post otherwise BTW ), the +/- 1 mm is just a manufacturing tolerance, the expansion will be around 0.5 mm between 30 and 100 % no matter what.

The typical cell will be 71.5 mm @ 30 % and 72 mm @ 100 % but for example you can have a cell that is 70.8 mm @ 30 % then it'll be 71.3 mm @ 100 %.

I guess the datasheet should have been clearer and said 71.5 mm +/- 1 mm @ 30 % and +0.5 mm @ 100%. But I think we're quite lucky with what we have, I've seen datasheets much worse than that...

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well I did a test and carefully measure the expension of an EVE brand new cell from 50% to 100% SOC...this was NOT in a fixture, just free standing.

The answer is that I measured an expansion of .762mm so that is much less than what I thought it would be however it is certainly more than 0.5mm... so an 8cell fixture would need at least 0.24" of room.
I wonder if the 0.5 expansion is when the cell is compressed with the 300kgf???

Interesting. I'm not sure about if the spec is with or without the 300 kgf tho.

No, my expansion under compression is less then 0.5mm per cell on average.

cinergi said:

No, my expansion under compression is less then 0.5mm per cell on average.

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That's consistent with my results too, but I budgeted for .5mm in my spring selection and fixture design.

New member here. I’ve been reading a lot about cell compression using springs. All the packs I’ve seen are using the springs on the outside pushing the end supports toward the cells using four springs. Am I right at assuming these springs should compress at 165 lbs to achieve the 12 psi goal.

Could I use four springs to pull the end supports together using threaded rod with eyelets on the end? I recently salvaged the seat springs out of my old Toyota. I found they started to stretch at about 160 lbs. I’m not an engineer, what do you think? 4S 280 ah cells

Vonglick said:

New member here. I’ve been reading a lot about cell compression using springs. All the packs I’ve seen are using the springs on the outside pushing the end supports toward the cells using four springs. Am I right at assuming these springs should compress at 165 lbs to achieve the 12 psi goal.

Could I use four springs to pull the end supports together using threaded rod with eyelets on the end? I recently salvaged the seat springs out of my old Toyota. I found they started to stretch at about 160 lbs. I’m not an engineer, what do you think? 4S 280 ah cells

View attachment 71866

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An arrangement like that could definitely work. I considered doing something like that, but abandoned the idea as I felt springs in compression with rods was just going to be simpler for my use.

100 Proof said:

An arrangement like that could definitely work. I considered doing something like that, but abandoned the idea as I felt springs in compression with rods was just going to be simpler for my use.

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Thanks 100 Proof, I’ve got eight springs out of the two seats. I thought I’d give it a try since the springs were free. I’ll use 1/4” eyelets with 3/4” plywood ends. I’ll post pictures after the build.

Vonglick said:

Thanks 100 Proof, I’ve got eight springs out of the two seats. I thought I’d give it a try since the springs were free. I’ll use 1/4” eyelets with 3/4” plywood ends. I’ll post pictures after the build.

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If you can, try to hang 165 pounds off of one of your springs and then measure its extension. Use that same extension measurement to dial in your force.

100 Proof said:

If you can, try to hang 165 pounds off of one of your springs and then measure its extension. Use that same extension measurement to dial in your force.

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That’s exactly what I did. I put a hook into a ceiling stud, hooked the spring to it, made a loop with a rope and stepped into the loop. I weigh 170 lbs and had all my weight on the spring. Reached up with some calipers and measured the length of the spring. I figured, close enough!

Did anybody do compression on a larger bank of e.g. 8S4P cells? I mostly saw builds of only low-cell units but may have missed a pic or two. If somebody did it, do you compress all cells with one and the same mechanism or is there an advantage to compressing each (4P or even 2x4=8P) parallel stack by itself? I imagine so but it might not be very practical or nice looking when doing it with springs. So that's why I'm also interested in knowing if there is a final verdict on the spring vs foam issue ...?

Zjohn said:

Did anybody do compression on a larger bank of e.g. 8S4P cells? I mostly saw builds of only low-cell units but may have missed a pic or two. If somebody did it, do you compress all cells with one and the same mechanism or is there an advantage to compressing each (4P or even 2x4=8P) parallel stack by itself? I imagine so but it might not be very practical or nice looking when doing it with springs. So that's why I'm also interested in knowing if there is a final verdict on the spring vs foam issue ...?

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Mine is two compression structures of 16 cells each. Just have to account for X cells times 0.5mm of variance when sizing your spring.

cinergi said:

Mine is two compression structures of 16 cells each. Just have to account for X cells times 0.5mm of variance when sizing your spring.

Cinergi's 28 kWh / 4 kW Solar / 10 kW inverter RV build

I've completed the last revision the battery before I head to Kanas next week (in theory) to pick up the new RV and install this (at their facility with their assistance WRT any welding or physical work beyond my capabilities). The previous revision had a few spots that were +40F under 120 amps...

diysolarforum.com

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Definitely a very nice setup cinergy! Unfortunately I do not have space for a 2x16 setup like that and will need to do a 4x8, complicating things a little... Also, I am a little surprised to notice the springs compressed to just about 100%, am I seeing that right?