How tight to compress a LFP package?

[Jenta]

Originally Posted by Steve_C

Hi Jenta, where did you find cells for that price? Massively cheaper than Winstons. I am getting ready to build a new bank for my ordered Lagoon.

Thanks!

I ordered them while we were still in the Bahamas so we had time and a place to receive them for us in the US - so I ordered them from Alibaba.

This is what I got and they had studs just like the image which was very nice.
https://www.alibaba.com/product-deta...191032531.html

No busbars or nuts but I made my own copper ones and tinned them and then got aluminum washers and nuts from McMaster as they only need to go to 8N of force and that way it's all aluminum and tinned copper and nothing else.

When I ordered on March 14th the price was $74 per cell and that had gone up from $68.50 per cell a couple weeks earlier.
I do see the price is up to $98 but you can always try to negotiate if you are buying a decent quantity.

[Jenta]

Too add something more pertinent to the compression issue - not an answer for doing it or not - I have a YouTube link you can look at.

This is from a friend that is hauled out in the same yard we are in and we started talking by email earlier this year about Lithium and based partly I think on our conversations he decided to tackle the project in addition to everything else he is doing.

His boat was already 24v and he built two 8s packs and decided to "compress" them. He did a beautiful job and they fit perfectly in the space his 8Ds were before and of course weigh less and store/deliver more power.
He basically did like Jedi was talking about where he didn't compress them so much as restrict them to prevent bloating.

He's French and says he did these for some interested friends I believe which is why he is speaking in French - but if you're looking at using EVE cells you could reach out to him on YouTube if you like anything he did or have any questions. He's a super nice guy, very intelligent and can speak English very well. Not sure about German though.

https://www.youtube.com/watch?v=yro0QiO2w2o

[Dirk01]

Many thanks to all comrades. MANY hints ...
To draw a conclusion:
Compression is recommended. How much has to be clarified with the manufacturer. (In the first video the statement is: Any compression is better than no compression.) However, if used incorrectly, it can do the exact opposite and damage the cells. It seems important, however, that bloating is strictly to be avoided in any case.
For me I have recognized the following:
Using springs with a known spring characteristic (F / mm) it is possible to compress batteries in the range between 5 and 12 psi, even if the SoC changes (whereby 12psi is the sweet point). For a 48VDC battery pack there is a maximum difference of about 8mm (0.5mm / cell). Presumably without additional compression. IMO that is A LOT!
Side note: The automobile manufacturers also had to put in a lot of effort into the mechanical design to prevent the batteries from bloating (apart from safety reasons in the event of an accident).
But projecting only some springs and rods seems to be the easier part of the exercise.
It is much more complicated to obtain a surface pressure that is as uniform as possible. Because ALL constructions I've seen, provided the batteries expand evenly over their surface and ultimately exert their force on the battery edges. IMO it is not the best idea to make it bombproof. It should be „elastic“. In Germany there’s a saying - everything that doesn't give way breaks.
Fortunately, the batteries don't expand evenly. The rectangular construction does not allow that. The bloating in the area of the battery terminals will also be less due to the inner design. A typical beer belly will be formed. So a bump that is a little below the middle between the battery terminals and the bottom but pretty much in the middle between the sides.
The questions as far as I can see are now:
1. Are the batteries (more or less) even @ 0% SoC? (Hopefully not really. Would make it easier IMO)
2. What is the bloating result @ 20% SoC? (that should be the lowest discharge point for permanent use)
3. What is the bloating result @ 80% SoC? (that should be the optimal load point for permanent use)
4. What is the bloating result @ 100% SoC? (this should be the maximum load for top balancing and is the upper limit for a compression construction)
Or can we just estimate even @0%SoC and 0.5mm beer belly @100%SoC in a perfect proportional manner?
And the most important question:
5. Which bending of the end plates occurs with which load due to the compression-screw-spring-construction? And what is the effective surface pressure for the different SoCs?
I think the best way to answer that is with FEM (Fenite Element Method). Is there anyone in the forum who could support? It’s more or less a try and error combination of end plates and spring charachteristics (not to forget the lengthen of the rod, supporting the spring and the connection to the opposite side of the battery). This could be done easily (correction: much more easy) with the help of a computer.
I am convinced that in this way the overall problem of surface pressure could be solved in a sustainable way. Applied to different battery sizes, a solution would result which, at least for one manufacturer with clear information such as EVE-cells, would be a profound solution with a uniform design (but of course with different end plate thicknesses and with different spring characteristics). However – it will only be a compromise. But a compromise is better than nothing and better than a wrong decision.
Is it more complicate than just wrapping it? Is it better to have such an expensive system (and I’ll have to of them in the 15kWh range) well engineered than just screwed together? Yes, I think so. Or ist it „overengineered“? No. It’s to expensive for an Ooops.
Only my 5 cts. Hopefully I'm not getting beaten up now…

Cheers
Dirk