I'm trying to charge 18650 cells from LG which have a capacity of 3200 mAh. I read the datasheet of the cells and it said 4.2 V and 0.5C of nominal charging. I'm not an expert so I did some research and found the cells should be charged with CCCV.
So I picked up a LM2596 CCCV module. It has 3 settings for: current and voltage limiting and threshold current for charge-complete LED. I also picked a 1s BMS since, from what I understand, it is necessary for safety especially above 1s configs.
So this is what I understand of CCCV:
1.) We set the voltage limit on LM2596 to 4.2 V for single cell charging.
2.) We short the output terminals and set the current limit to 1.6 A as 0.5C.
3.) With these settings, cell would charge to 4.2 V at 1.6 A CC; when it reaches 4.2 V, CV mode would engage and keep the cell at 4.2 V by decreasing the current until it reaches the cut-off current which is 50 mA according to the datasheet. After that, which I also set with the potentiometer as 50 mA, the charge-complete LED will illuminate.
With this knowledge I hooked up my 19 V, 3.6 A laptop charger to the LM2596 as source and connected the output to the BMS, then connected the BMS to a single cell in a 18650 case.
Initially I started with a cell that was at 3.52 V and it charged in CC mode in the beginning.
Now here is where the problem begins: Unlike the graphs of CCCV explanations and demonstrations on the internet where current stays the same until 4.2 V and starts decreasing in CV mode, in my setup current starts to decrease long before 4.2 V, generally around 3.9 to 4 V.
I hook up my ampmeters and measure the current dropping constantly as the voltage increases slightly. At the end when it reaches the cut-off current of 50 mA, because it starts to decrease in current too soon, the charging completes with the cell voltage at around 4.05 to 4.1 V. At the end I can't charge any of the cells to 4.2 V, the current does not stay the same and decreases before reaching 4.2 V.
I tried to solve the problem by removing the BMS since I was already using 1s. However, it did not solve the issue, the pattern of charging and end voltage is the same: around 4.08-4.1 V.
What is the problem in here that prevents the CC to run until 4.2 V?