Confused about LiFePo4 cell group assembly

[Al Slitter]

I have watched many video on assembly of cell packs and some stress using some form of compression to help the batteries
to maintain there wall rigidity. Other video's show leaving and air gaps between the cells to help with heat dissipation.
My thinking is that the compression might be used with Poly type of cells and that the gap assembly will be used in aluminum walled Prismatic cells?

What are your thoughts?

 

[Colonel.lp]

Unless you are drawing very high continuous currents I doubt temperature would be a problem. It's clearly specified by the manufacturers that compression of lifepo4 prismatic cells will extend its cycle life. The industrial made battery packs for electric vehicles have a special foam between cells and the whole pack is compressed into an external case.
Your mileage will vary of course, and many don't compress without problems. It depends on your application.

 

[HRTKD]

The only change I would make to my compression frame is to put thin double sided tape between the cells, but only to eliminate any potential for movement. I don't think mine have moved, but it's a bit of added insurance. Otherwise, I'm quite happy with my threaded rod and plywood solution.

 

[Johncfii]

“My thinking is that the compression might be used with Poly type of cells and that the gap assembly will be used in aluminum walled Prismatic“

I have read hundreds of posts regarding pros and cons of cell compression, but I have never seen that suggestion. Lishen recommends that their prismatic cells be held under compression of between 9 and 17 psi, with 12 psi being the target. You can find all the details in another long thread dedicated to cell compression.

My large LiFePO4 bank is in a fixed location, and not mobile. Because I’m trying to get every possible advantage with respect to longevity, after very much consideration of all ideas and suggestions, I chose 3/4” plywood at the ends of the pack, compressed with “die” springs on the ends of 5/16” threaded rod through 3/4 square steel tubing. It added a couple hundred dollars to the total cost of my 816 amp-hour battery bank, and I’m happy with my choice.

I have never detected any warming or heating of the cells, though I have never exceeded charge or discharge rates greater than a C5 to C10 rate.

 

[Gazoo]

Lishen recommends that their prismatic cells be held under compression of between 9 and 17 psi, with 12 psi being the target. You can find all the details in another long thread dedicated to cell compression.

I believe the thread you are referring to applies to EVE cells. I don't know what Lishen recommends.

 

[Johncfii]

I believe the thread you are referring to applies to EVE cells. I don't know what Lishen recommends.

Ahhh. ... yes, I am corrected.
Thank you.

 

[Al Slitter]

OK thanks all!
From what I can make of this compression is more suitable for mobile applications. For solar having the cells together held by double sided tape and bus bars should be fine.

The other issue is an enclosure for the cells/battery; should there be ventalation holes in the storeage box for air movement?

 

[HRTKD]

OK thanks all!
From what I can make of this compression is more suitable for mobile applications. For solar having the cells together held by double sided tape and bus bars should be fine.

The other issue is an enclosure for the cells/battery; should there be ventalation holes in the storeage box for air movement?

No. All LiFePO4 batteries should benefit from a compression frame. The compression frame greatly reduces (almost eliminates) any strain on the cell terminals that could come from the cells expanding.

Given that we're seeing very little heat increase with moderate charge/discharge rates, powered ventilation isn't normally needed. Some passive ventilation is good, but not necessary. It all depends on how much heat is being generated inside the box.

 

[Al Slitter]

Thank you again for your input!

 

[timmeh]

I guess this thread is as good as any to ask being new to the forum. do you apply compression to the cells when fully charged or discharged? i'm guessing when charged as it would mean they have a max compression of what you apply and cant expand further then they are meant to when full.

 

[Steve_S]

Compression should be applied when all the cells at at Nominal Voltage of 3.200 each and at "room temp" of 25C/77F.
The cells will ALWAYS expand & Contract through their cycles. The higher the Discharge/Charge Rate the more temperature increases internally leading to expansion or contraction (if discharging).

Binding/Compressions serves TWO purposes really.
1) It prevents stresses on the terminals. Flexing causes cumulative damage which over time can result in failures.
External / Ambient Temps, as well as internal temps, will result in expansion & contraction to a point.
2) To maintain a solid "block" which resists vibrations and other forces working on the cells. The cell casings are actually quite thin and again if ANY rubbing or chaffing occurs, over time it can potentially wear through the casing and result in a failure.

It is generally advised that when Top Charging Balancing the cells in Parallel, that they also be "clamped or bound" while the process is being done to prevent any potential deformations.

For detailed assembly information and much more, download the Lyuan Tech Doc from my Signature as well as the LFP Voltage Chart.
Hope it helps, Good Luck.
Steve