Strange charging or confusion?

[once111]

Equipment:
8 x new Fortune 100Ah - 8p0s configuration
Kaiweets PS-3010F - 10a/30v variable power supply
BMS - None at the moment, overkill BMS going on after top balance and final cell config (0p8s)

I know some of what I am doing is not necessary with new matched cells. I also understand that some of this requires a lot of attention without certain safety measures like using a BMS. I am trying to use this opportunity to experiment and learn about these techniques I am studying before I have to put these batteries into production. I am trying to manually top balance my cells after a manual bottom balance. I have all the cells in parallel, so I have currently configured a 3.65v 800Ah battery.

From my understanding, the primary points of charging LiFePO batteries are to charge at Constant Current (CC) until the cells reach absorption at 3.65v (~90% SOC), then switch to charging at Constant Voltage (CV) until the current reduces to almost nothing (~100% SOC). The Kaiweets PS supply does 10a, so I was hoping to use the full 10a for the CC phase, as this will only be a 0.0125 C rate and slow enough at that rate. As I understand, this low C rate should not affect the charging other than taking a long time, 80 hours from zero to full. When I started my charging, I saw numbers I don't like and wanted to see if I have an issue with my understanding, technique, or equipment.

To start all cells were connected in parallel and allowed to equalize. The entire bank was 3.072v and all cells were within .001v. I configured the PS for CC at 10a, but the voltage of the PS rose to 3.94v instantly. After running a few minutes, I reconfigured and tried a 5a CC setup and the only came to 3.50v. While the 5a config looked like what I was expecting, the 0.00625 C rate is really low. I did not let either config run for more than a few minutes, hoping to get some clarification before doing anything that might hurt the cells. After these attempts, the bank is reading ~3.08v and all cells all show the same voltage.

These cells in parallel are capable of 800a at a 1 C rate. The suggested charge is a .5 C rate, or 400a. I am so far under these rates I don't understand the high voltage I am seeing. Looking at cell/bank voltages, not under load after a decent rest period, my ~3.08v cells are approximately 10% SOC. This leads me to assume the internal resistance of the cells is still on the low side, therefore it shouldn't take such a high voltage to push 10a. With that jump from 5a/3.50v to 10a/3.94v it would seem that moving to 20a+ chargers would run a crazy high voltage.

I feel like there is something I am missing. Will running the charger at 10a CC result in the voltage eventually dropping from 3.94 to the 3.65 target I am expecting? Is this technique wrong or only works with 5a CC? Do I need to run a base charge of CV @ 3.5v, then run CC @ 3.65, and end with CV @ 3.65? Is my 8p config giving me issues? My PS should be able to handle 8p0s (3.65v), 2p4s (14.6v), or 0p8s (29.2v) battery cell configurations, but I assumed 8p 3.65v would be simpler to start. Not sure what I am missing, and I don't want to overcharge by accident. Thanks in advance for any help.

 

[Steve_S]

100AH cells are ONLY capable of 100A output & 50A charge input, period. Single or in parallel with equal cells.
Cells will take CC Constant current from low point till they start to reach closer to full and then will take CV to finish off.
It's usual to charge at Hi Amp CC and the PS will auto-switch to CV and the amperage pushed will begin to drop as the cells fill.
100AH cell will take a Long Time at 10A. Put 8 cells in parallel and multiply the time by 8.

The PS will sense what voltage the cells are at, it will push at a slightly higher voltage as it charges, typically only 2V above cell volts.
As the cells cross 3.400, they will begin to rise quicker towards the 3.65V. EAGLE EYE TIME ! 3.4 to 3.6 can happen FAST !
You must watch carefully, use a DMM/DVOM and test voltage at cell and then at PS output terminal. You will see that the voltage difference will Narrow at which point you have to be ready to adjust the PS to NOT exceed 3.65V, amperage is moot at this stage. When the amps taken, begins to fall and voltage is remaining constant at BOTH the cell & PS terminals you have reached 100% SOC. The amps will continue to drop as the cell "Saturates" and the Amps taken will eventually drop below 2A and will be very slowly decreasing, it's pretty well done and the cell is saturated.
At this point, power off the PS and DO NOT ADJUST ANYTHING. Put the leads on the next cell and start again, the display numbers will be off and THAT IS OK, leave it alone (resist the temptation to play) the cell will charge up and do it's this and the PS will adjust itself automatically and will finish the cell at 3.65 and will lower the amps as the cell saturates.

Remember the cells nominal voltage is 3.200V.
The primary voltage curve is 3.450 to 3.000. Above & below that range only represents about 5% of overall capacity ONLY.
Runners & Lazies will usually do their unpleasant things beyond the main voltage curve.
Most people will TOP to 3.60V and allow amps to drop to <2A or a bit lower.

With a low amp charge source, most folks will Top Charge each cell independently and once all are topped to 3.60, they are then connected in parallel for a collective Top Balance which is cutoff once amps taken reaches 2A +/-. Then allowed to settle in parallel for a minimum of 4 hours. LFP will settle from 3.60 to 3.500 or even 3.450 within 4-6 hours and then sit there with a very slow self discharge, this is the "real" 100% SOC of LFP. Then set the pack into a normal 8S setup, add BMS Harness, hook the rest up and then plug the BMS harness into the BMS.

** NEVER EVER Connect the BMS harness to cells WITH the BMS connected to it. Fastest way to make Magic Smoke !
Always double check the connection with a DMM/DVOM for every BMS lead from cell to the terminal plug BEFORE plugging in.
BE SUPER CAUTIOUS to not mix up the harness leads, Magic Smoke Making if you do.

Please see links in my signature for more information which will be useful to you.
Hope it Helps, Good Luck
Steve