I may have missed something, but I was trying to follow the top balancing logic and it put a question in my head.
Everything has some resistance. In this case, it is the electro chemical resistance inside of the cells that I am thinking about. I have not worked much with LFP yet, most of my experience is RC car LiPo, Samsung 18650's and my LG Chem NMC cells. All of these do have very low internal resistance, but it is still there. If I am fast charging at a relatively high current, and then just cut it off, the cell voltage drops quite a bit to the real internal cel voltage. The absorption or saturation charge time where the voltage is held for a period of time, allows for the internal cell chemical voltage to "catch up" with the terminal voltage. I do not see any benefit in cutting off the charge and not doing at least some saturation charge at your desired voltage. So my question is... Is there a reason some here are just doing a quick shut off through a power supply or BMS and not doing an absorb phase? Will's parallel balancing is all about a long top absorb charge to get the cells internal chemical voltage to all match. From what I understand about how LFP cells react, I would think holding all of the cells at the desired top balance voltage until the current falls near zero would be the best option. Then when they are separated and allowed to rest, they will all drop back some, and should still be very close. If one fall more, it probably needs to be held at the set voltage even longer to catch up. In the end, the resting voltage should all be the same if the cells are at the same state of charge, under no load, and at the same case temperature. Am I missing anything?
When I built my 14S e-bike batteries, I had to manually balance the cells as I did not have a balancing charger for 14 cells or a BMS yet. I used my RC car charger that can do 1S to 6S and decided to split the pack into 3 sections, two 5S and one 4S. I balance charged each of the sections until they all sat in absorb for at least 15 minutes and the current dropped to just 0.02 amp. After the pack sat, I checked and all 14 cells were balanced within 3 mv on my Fluke meter. I had to do it one section at a time since the charger outputs are not isolated. Trying to charge a cell in the middle of a wired pack requires that the charging source be fully isolated, or you end up smoking things. It is bad enough working with 5 AH cells. Having a mishap with 100+ AH cells is a whole different story. I had one little slip while checking cell voltages on my 360 AH pack. My clip lead to my meter fell off, and on it's way down just taped one of the buss bars. It burned 1/4 inch off the end of the alligator clip and turned it into flying molten metal bits. Please be careful when working with all this power.