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

SherylinRM

GO GO JUICE BOOMER ZOOMER MAKER
Do any of them tell you what state of charge the cells should be in when you apply the 12psi?
I tightened mine until my lock washers were flat. That is exactly 12psi
All the of the people I trust say that the batteries should be at a low state of charge. Like just before you would recharge them normally anyhow.
I hope this helps. :)
 

HRTKD

Boondocker
As a follow up to the foam idea, here is some EPDM foam sheets. Looks like medium firmness is 13 psi for 25% compression. A 3/8" sheet compressed 1/8" should provide ~12PSI. The main issue is compression set. I don't know how much the foam will relax over time. EPDM is pretty good in this aspect, but I don't know the precise range. It may require a follow up after a couple month to retighten. You could always overshoot to 50% compression at the start.


Those sheets are rated at lbs / cu ft. Aren't we looking for something in PSI, as in pounds per square inch?
 

Colonel.lp

Solar Enthusiast
As a follow up to the foam idea, here is some EPDM foam sheets. Looks like medium firmness is 13 psi for 25% compression. A 3/8" sheet compressed 1/8" should provide ~12PSI. The main issue is compression set. I don't know how much the foam will relax over time. EPDM is pretty good in this aspect, but I don't know the precise range. It may require a follow up after a couple month to retighten. You could always overshoot to 50% compression at the start.

I have no expertise in this area, but have used EPDM for various seals over the years and under any pressure they have collapsed quite quickly and only provide enough pressure to fill a gap. Perhaps it's quality related.

I like Dacians spring idea but am finding it difficult to find springs with useful kg/mm ratings at the right size.
 

Bob B

Photon Sorcerer
So ..... my cells are pretty close to 7" x 8". Compressing them to 12 PSI would be 672 lbs of force. That would be about 305 KG .... which is pretty close to the 280 that Dacian has on his ..... but, I am probably only going to have 4 springs which would require 76.25 KG (168 Lbs) per spring.

It looks like the 20 MM OD spring on page 15 is what I would need.
 
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Bob B

Photon Sorcerer
Oh ..... @Dzl .... excellent work for the data mining you have been doing for this and other threads.

.... and @ghostwriter66 .... Excellent work for actually getting EVE engineers to talk to you.
 
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Bob B

Photon Sorcerer
@ghostwriter66 One question I would have is how many charge cycles with no compression would be required to reduce the cycle life of the cells significantly .... does even a few cycles with no compression reduce the cell life or would it be spread over the life of the cells.

I guess what I am wondering is how much damage can be done when initially capacity testing individual cells, or top balancing a pack with no compression fixture.
 

Luthj

Photon Sorcerer
I guess what I am wondering is how much damage can be done when initially capacity testing individual cells, or top balancing a pack with no compression fixture.

Minimal, especially if we are talking low C rate. This type of thing is cumulative. The miniscule movement between charge and discharge causes the capacity loss over time. Its likely exacerbated by high C rates or vibration. I am almost certain that EVE doesn't use such a fixture for initial charging/testing on each cell.
 

Bob B

Photon Sorcerer
Here's another option on springs from my post back in May...
https://www.leespring.com/compression-springs-hefty?search=LHL625D

I did buy some but never did install. Maybe I'll try them now.
Which one of those did you get? Did you have any problems buying from them direct? How much did they cost?

I was looking for an online seller. There is a vendor on Ebay that is selling the Danly springs (toolandequipsales) and I was considering sending them a message to see if they could get the ones I was looking for.
 

Airtime

Solar Addict
Which one of those did you get? Did you have any problems buying from them direct? How much did they cost?

I was looking for an online seller. There is a vendor on Ebay that is selling the Danly springs (toolandequipsales) and I was considering sending them a message to see if they could get the ones I was looking for.
I ordered online from Lee Springs. After looking at spring rates, I ordered two different ones to try:
LHL 625D 03, $3.97 each
LHL 500D 03, $3.40 each

You have to balance having enough pressure to get desired compression against having enough travel to accomodate any expansion/contraction. These two seemed in the right range to me.
 

Bob B

Photon Sorcerer
@Airtime .... AHHH .... I see now that if you click on it there is an option to add it to the cart. Please let us know how it works out .... Pics even better.
 

ghostwriter66

"Here - Hold my Beer"
OK ... talked to the senior design architect for EVE. He was very specific and short response. So these are NOT all his answers since I don't want to ramble BUT these are the ones that I think are important to us. Let me preface that he started out that although there are straightforward chemistry (liFePO4) involved -- there is still allot of mystery when it comes to LFP and why it does certain things a certain way ...

SOOOO - he said - if he was a DIY'r and doing this for an off-grid cabin or RV he would do the battery virtually the same way that the junior engineers said last night. (see my previous posts earlier) (post #215)

Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI.

Anywhere between 6-17 are good numbers - but those numbers will change on SOC and temp.

SOOO In a perfect world - when your SOC is at 50% - your PSI would be 12 and your temp is 72. He would do the adjustments at 50% SOC and then assume that 100% SOC will be a few PSI higher and 0% a few pounds lower. As long as your high and low is between 17-6 you are fine but a constant 12 is what you are looking for.

For temp - and I know this was not one of the questions -- BUT 72 is perfect -- and actually its better to be at 52 than 82 .. although 72 is perfect - when temp starts going above 72, the UNHAPPY FACE scale starts going up exponentially as the temp climbs and when the temp lowers its more linear ... (if that makes sense). That also will affect the PSI.

In fact nearly everything affects the PSI -- and over the life of a LFP you will get a couple more years from a battery at 12PSI CONSTANTLY then one going between 6-17 -- BUT that's NOT the real world unless you have a spring or mechanical controlled clamping system.

BUT technically 12 is the magic number and if you CAN KEEP it at 12 then you will probably get ALMOST 1.6X the cycles out of these batteries as you would if you do nothing.

One of the guys had a question -- ans.. Keeping it snug in a metal box at 0% PSI and then as the expeand the PSI gets higher is still better than nothing - but only marginally.

If I missed something let me know and its probably in my notes ... he really was excited that someone from the states was actually interested in this ...
 
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Luthj

Photon Sorcerer
The springs discussed should work good. There are several small (less than 1" square) coil springs available Choose a reasonable rate, say 100-400lb per inch. Then just compress the springs as needed with threaded rod. If you have 4 rods and 200lb/in springs, compress each spring by 0.825". Obviously choose a spring that wont go solid before you reach the desired clamping force.
 
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JoeHam

Photon Sorcerer
SOOOOO - i guess the next thread is - WHO HAS A GREAT IDEA HOW TO BUILD A SPRING LOADED 12PSI storage capability ???

So when I hear PSI I think tire pressure.

How bout a battery box with an air bladder in between the cells and the rigid box. Inflate to 12 PSI easy peasy.

The right sized innertube might even work?

I have no idea if that is thinking inside or outside of the box 😎
 

Luthj

Photon Sorcerer
An air bladder would be too sensitive to changes in ambient temperature I think.

For sourcing springs, mcmaster carr has a decent selection, though it can be hard to navigate filter at times.
 

Luthj

Photon Sorcerer
Well a simple tire gauge can solve that. Probably can find a Bluetooth one.

The bladder would need adjusted fairly often if the temperatures swing much at all. Plus you would need to check regularly for leaks anyways.

The coil spring method is one and done, and probably wont cost any more.
 
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