Several on this thread have been asking for more information about how over voltage can damage cells. Here are two web site links that talk about cell over voltage hurting the capacity and cycle life of the batteries, but they don't go into a lot of detail, other than to say, don't do it.
LiFePO4 Care Guide: Looking after your lithium batteries - Technical Information LiFePO4 Care Guide: Looking after your lithium batteries
www.evworks.com.au
Lithium iron phosphate batteries are made up of more than just individual cells connected together. They also include a battery management system (BMS). A BMS makes sure each cell in the battery remains within safe limits. A well-designed battery management system can help maximize lifetime, and...
relionbattery.com
I also attached 2 PDF papers that go into great detail about the chemistry of batteries, and more on the science of what is actually happening in the cell. I am still trying to read it all, as it can be quite tedious. On the second paper "TVJOB..." around page 45 they start talking about the aging process and cell damage in detail. This is still in Chapter one, the introduction. The whole document is 272 pages. It will take me a few days to truly understand all the data in this paper. If you are a data geek like me, it is a good read, but wow, just wow! I never knew how much I didn't know about lithium batteries.
In short, the higher the cell voltage, the faster things degrade. Even stored at no current, having a high voltage on the cell is causing the electrolyte to degrade. Lithium metal will start plating onto the negative electrode (cathode). In extreme cases, this can lead to a short of metallic lithium across the anode and cathode. Even in small cases, the plated out lithium is lithium that can never actually hold charge again. This obviously reduces the cell capacity. Even if the cell recovers and keeps working, the loss of capacity, and reduction of usable cycles will never come back.
Chapter 2 starts on page 57 (of the pdf) and is titled "Capacity Fade".
On page 64 (page 66 in the pdf) there is a good comparison of graphs between the charge/discharge curves of LFP and NMC cells. The curves are completely different, and I can see why balancing LFP cells in the middle of their charge curve is basically impossible. These examples are at low charge and discharge rates, similar to what we use in solar storage. At a full 1C rate, they do swing a bit more.
The NMC cells (like my LG Chem ones) are only flat between 20% and 50% SoC, where the voltage only changes from 3.8 to 3.85 volts while charging. From 5% to 20% the voltage goes from 3.4 volts up to 3.8 volts. And from 50% up to 100% the voltage goes from 3.85 to 4.25 volts.
During charge, the LFP cells are nearly flat from 10% up to 85% SoC where the voltage only changed 125 mv from 3.25 to 3.375. Discharging the battery from 3.25 down to 2.0 volts per cell is only getting 10% of the capacity. And at the top, charging up from 3.375 to 3.65 volts, is only pushing 15% more in. And once at 3.65 volts, removing the charge current and letting the cell rest, the voltage will settle back down to about 3.35 volts when any load is applied. So under discharge, the cells start at 3.35 volts. After you use 85% of the capacity, the cell is still at 3.15 volts. That is incredibly flat. At a C/5 rate, it is a little easier to see the cut off point as it does start to roll down and hits 2.7 volts at a little less than 20% remaining.
If (when) I replace my batteries with an LFP pack, I will certainly want a good coulomb counting battery state of charge meter to tell the system how much is left. My BMS will do it, but it can't send that information to the Schneider inverter/charger. For now, I am only using battery voltage control, which works just fine on the NMC cells, but it is clearly not near ass accurate for use on LFP cells.
I know I went a bit off the original topic, but I wanted to show that in normal use, there just is no reason to really push the cell voltages up there. The initial top balance should just get all of the cells to hit that top knee area. You don't want to hold the voltage up there too long, but a full day sitting at 3.65 is not too bad. But I think holding them all at just 3.5 for longer would top balance just as well, and not put as much stress on the internal structure of the cells.