Before, during and after top balancing...

[howardq99]

I am in the process of doing the first top balance of my new LF280K cells from Amy Wan. I sorted the cells by capacity according to the test results provided by Amy and I've got the 16 highest capacity cells in parallel right now on their way to 3.65V (currently at 3.326V from a starting voltage of 3.282V) using this power supply:
Kungber DC Power Supply Variable, 60V 5A Adjustable Switching Regulated DC Bench Linear Power Supply with 4-Digits. I'm doing it in steps and so I'm on the first charge up to 3.40V on my way to 3.65V.

I've got the cells in a compression fixture with threaded rods and ZUSANSR-315 foam that specs out to give a firmness @ 25% deflection of 13 PSI (Foam Roll, Water-Resistant Closed Cell, 36 in W, 10 ft L, 3/16 in Thick, Black, I found it on Zoro for $132). I fixed the cells with 9 layers of the foam at 25% compression at the cell voltage of 3.282V which is the voltage that I received them at. I tried to get more information about the foam and a deflection curve of what the PSI would be at different deflection percentages but the manufacturer has not responded to my inquiries.

So after I charge the cells up to 3.65V is it okay to decompress the cells and remove them from the fixture so I can reassemble them in a 24V configuration (my inverter is currently 24V with a 48V unit on order), and then put them back into compression with a foam deflection of 25% so I have the 13 PSI at a 100% SOC? Would the cells expand/bloat when decompressed at 100% SOC or does the expansion stop once the charging stops and they are safe to remove from the fixture?

Also, it seems to me that accurate compression of 13 PSI would be more important at the higher SOC as this is when the cells would potentially be at maximum expansion. None of the cells are shipped with compression so it seem they are not worried at all about expansion/bloating/swelling at the voltage they ship the cells at. My cells arrived after almost exactly 3 months from the time that Amy did the testing, gave me the results and shipped the cells. They were at 3.297V when they shipped and they arrived at a voltage range of 3.281V - 3.282V.

I believe I've read that once a cell bloats out there is no returning it back to it's original form and the internal damage has been done. After spending almost $5K on these LiFePO4 cells I really want to make sure I don't do anything that has a permanent negative impact to my packs.

Finally, I did purchase this if l need to draw down the cells after they are top balanced so I can loosen the fixture and reconfigure the cells: 150W 4-Wire Electronic Load Tester 2.4 inch DC Tester APP Electronic Adjustment Constant Load Lithium Battery Capacity Monitor Discharge Charge Power Meter Supply Checker
I'm thinking I can discharge the cells to the voltage that they shipped the cells at, 3.297V and be safe that they don't expand when I remove them from the fixture.

Thank you for any and all advice!

 

[sunshine_eggo]

So after I charge the cells up to 3.65V is it okay to decompress the cells and remove them from the fixture so I can reassemble them in a 24V configuration (my inverter is currently 24V with a 48V unit on order), and then put them back into compression with a foam deflection of 25% so I have the 13 PSI at a 100% SOC?

Yes.

Would the cells expand/bloat when decompressed at 100% SOC or does the expansion stop once the charging stops and they are safe to remove from the fixture?

Only if they are defective. Bloat does not occur even in uncompressed charging.

Also, it seems to me that accurate compression of 13 PSI would be more important at the higher SOC as this is when the cells would potentially be at maximum expansion.

Yep.

I believe I've read that once a cell bloats out there is no returning it back to it's original form and the internal damage has been done. After spending almost $5K on these LiFePO4 cells I really want to make sure I don't do anything that has a permanent negative impact to my packs.

Cells bloat because they are damaged/defective, not because they are charged uncompressed.

Finally, I did purchase this if l need to draw down the cells after they are top balanced so I can loosen the fixture and reconfigure the cells: 150W 4-Wire Electronic Load Tester 2.4 inch DC Tester APP Electronic Adjustment Constant Load Lithium Battery Capacity Monitor Discharge Charge Power Meter Supply Checker
I'm thinking I can discharge the cells to the voltage that they shipped the cells at, 3.297V and be safe that they don't expand when I remove them from the fixture.

Completely unnecessary.

 

[howardq99]

Thank you Sunshine_Eggo for your reply. Also a big thanks to Zwy here on this forum, he's been very helpful in also sharing his knowledge and answering many questions!

 

[howardq99]

So after charging them up to 3.65V and letting them sit in parallel for almost a week I need to assemble them into two 24V 2P8S configurations until I get my 48V inverter as I currently only have 2 JK BMS units.

What is the best practice for combining the cells in each 2P group? Should the pairs be arraigned by capacity with the top two highest capacity cells combined in in the first 2P group, then the next two highest capacity cells combined in the second 2P group and so on? Or is it best to have the highest capacity cell combined with the lowest capacity cell for the first 2P group and so on?

 

[toms]

In theory match the highest and lowest capacity in a parallel pair to optimise pack capacity.

In practice you will find significantly different “capacities” depending on how you measure, and keep in mind you have different current rates when the batteries are in use.

Ideally in theory you would find out the average charge profile of your intended system - then run a capacity test using this profile to match your highest and lowest cells.

In practice you won’t see any difference between that method and randomly matching the cells.

 

[howardq99]

Thanks for your insight, I appreciate the input!

 

[Nobodybusiness]

Yes.



Only if they are defective. Bloat does not occur even in uncompressed charging.



Yep.



Cells bloat because they are damaged/defective, not because they are charged uncompressed.



Completely unnecessary.

You’re absolutely sure that cells do not need to be compressed while top balancing at max voltage?

What is the point of compression then?

 

[A.Justice]

What is the point of compression then?

This is an often talked about issue on this forum. I have my cells bound with wood on each end and hand tightened with pair of hose clamps. There are members here who would look at that setup in horror. It works fine for me.

As long as the cells are immobile (relative to each other), have a rigid surface on each end, and that rigid surface is tightly held at the end, you should be fine in all but the most demanding application.

I've discharged my packs at 5c, and charge at 1c, never had any type of issues, expansion or otherwise. My first pack was made from CALB cells, and sat on the floor, uncompressed and functional, for 6+ months running all of my electronics and battery charging stuff. They never swelled or expanded. Swelling shouldn't happen unless the cell is defective, or, in some cases, you discharge / charge at a VERY high C rate.

I think compression is an important issue, but not quite as important as people make it seem. With the amount of effort you took to compress properly, I think you'll be just fine.

 

[sunshine_eggo]

You’re absolutely sure that cells do not need to be compressed while top balancing at max voltage?

With every fiber of my being.

What is the point of compression then?

Because EVE cell datasheets list two different cycle lives- One for compressed. One for uncompressed. If you compress them with the prescribed pressure in the datasheet, you can get a noticeable bump in cycle life.

Eve also lists a fully charged width vs. discharged width.

 

[Nobodybusiness]

d