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Yes new cells.Lets do some math.
I assume these are new cells. New cells generally come at ~40% SOC. 16 280Ah Cells in parallel will need .6 X 16 X 280 = 2688 Ah to fully charge. At 20A this will take 2688/20=134Hrs to fully charge. Yes, it is normal.
Keep in mind that at 3.3V 20 A is only 66W.
I typically advise folks to wire them in series with a bms and charge them till there is a cell over-voltage. This will charge in a reasonable amount of time. (52.8V x 20A = 1056W). After the BMS faults on a cell overvoltage, they can be put back in parallel to do the top balancing.
The most important tool for top balancing is patience.
Top Balancing LiFePo4 Cells using a low cost benchtop power supply.
To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...
wait.. you had one that hit "max charge? you mean it hit 3.65 volts? if one out of 16 is hitting 3.65 and the rest are at 3.3, then you need to check your connections and ensure that all of your buss bars are getting good contact.
while in series configuration I had one cell go to the max voltage the bms allowed and it shut down charging. The other cells were a lot lower so i broke it down and have been top balancing in parallel for 3 days now.
OK. Now it is a matter of being very patient. Set the target voltage on the Power supply before you hook it to the parallel cells and then hook it up and wait. No matter how tempted you are, DO NOT raise the voltage on the power supply.....
That cell started with a higher SOC or was a 'weaker' cell. This is normal.
As long as the Power Supply is correctly set to the target voltage, there should be no reason to worry about over-charging the cells. At around 3.45 volts the current will start tapering off and the voltage will start going up. This will accelerate till the charge is complete.
OK in series so as a 48 volt pack. then one of the cells was either not discharged properly prior to shipping, or the others were over discharged, or you got a bum cell or two. i would put the other 15 in parallel and bring them up to about 3.45 and then reconnect the suspect cell and see if they top balance out. it could take a couple of weeks depending upon where you are in the charge process.
Ive had all 16 cells balancing since Sunday. Pack is reading 3.385 now so its making progress. I hope I dont have a bad cell, 100* weather coming and I need this to be working LOL
Patience Grasshopper. Patience.
Voltage sat idle for days....
For how long?
About 4 hrs each with 20amps i was using my Icharger x8
Did you see current going in to these cells? If they were pulling current from your charger, they were still in the process of charging.
Charger was on but zero movement on voltage into the cells when i checked
