Patience is important.
I use a TekPower 1540 which can output 40A. Takes about 14 hours to go from 2.50 to 3.600 starting with 40A Constant Current for an EVE 280AH cell. That drops to Constant Voltage around 30A and seems to sit FOREVER between 3.20 until it tickles 3.40 then the rest from there to 3.65 is quicker. This is doing a SINGLE CELL AT A TIME. If you are going 2 or more cells in Parallel then it will take that much longer.
Remember the main power curve is from 3.000 to 3.400. Voltage drop is quick from 3.00V, it's the cliff fall. From 3.400 it's Climbing the cliff as the end is a steep curve as well. These "ENDS" actually only represent less than 10% of total gross capacity and most folks reserve this off anyways to prolong lifecycles and maximize cell life.
AN IMPORTANT TIP !
When charging the cells and once they reach 3.400+ EAGLE EYE THE CHARGER !
As the cells start to fill up, their impedance & resistance changes and the amps taken, starts to drop as it should.
At this point recheck VOLTAGE at the Charger Terminals and at the CELL Terminals, you will see a difference in voltage between the two, this is expected.
As the amperage taken drops, the voltage will start to align between the charger & cell. THERE'S THE GOTCHA !
Watch Carefully, because this is where things can go pear shaped.
You need to be careful because the voltage will slowly climb up at this point and can exceed 3.65V quite quickly and you will have to fine tune the charger output to ensure you do not cross 3.65V.
* It is NOT a crisis if you stay below 3.800. Above 3.8V the damage potential begins, by 4.0V there is harm and that is VERY QUICK !
Even if charged to 3.8V, once charge is removed the cells always settle and will typically settle around 3.500 +/-0.050 within 2 hours of charge. If allowed to sit static, they will ultimately settle closer to 3.45 +/- a tiny bit and this is fine as that is the top of the power curve.
THIS IS NORMAL FOR LFP !
I was doing "sets" and that is so tedious and not practical for me in my situation. I chose to use an alternate method to accelerate the process and make it workable for me. Cell Topping and then Parallel Set Topping.
1- Charge each cell to 3.65V at maximum Amps possible till Amps received is <2.0A (not much gain waiting till it drops to less)
2- Allow cell to settle on it's own, while charging other cells.
3- Once final cell is charged, let it sit NO LESS than 2 hours to properly settle out.
* Other cells will have continued to settle and will likely be static around 3.450 +/- 0.020. This is OK.
4- Then set all cells in Parallel and reapply charge at High Amp to 3.65V and wait till Amps Taken drops to <4A *this is collective for all the cells so it's perfectly fine.
5- Once the Parallel Set is done, remove charge and allow to settle at least 1 hour. The cells will likely be around 3.500 to 3.550 or so. Each cell WILL match that voltage. If allowed to sit in Parallel, even for a few days, the cells will continue to settle to close to 3.450 +/-. It is normal.
You can now assemble your battery pack with fully topped Cells.
SEE HERE:
General LiFePO4 (LFP) Voltage to SOC charts/tables 12/24/48V | DIY Solar Power Forum (diysolarforum.com)