i have a battery pack (6xx 18650 in pararell) whit output of 3.7V and a total capacity of 17400mAh(6x 2900) and i wish to make a charger from scratch. So far after reading a lot of documentation and CC/CV procedures when charging and came to a conclusion that i can make this thing relatively simply using my 2A 5V wall USB charger.
So i made a circuit that will do this:
First it connects the batteries directly to VCC output of wall charger. This will effectively put whole thing in a CC mode, since charger can only provide 2A(2.1A) and the batteries would pull a much higher current if the current would be available. All the time it is measuring the voltage on the batteries using a attiny and after the batteries voltage reaches 4.1V, it closes the VCC line feeding the batteries (using a mosfet/relay) and feed to batteries output of buck converter providing 4.2V whit 2A max output. It keeps it so until the current feeding batteries(it measures it whit a votlage drop on a low Ohm current sense resistor) drops below 5% of rated current. After, it disables any power going into batteries and just measures batteries voltage every 10 second until it falls under 3.3V and then starts charging again. It also constanlty measures temperature whit 3 temp sensors glued to pack on 3 different places whit thermal conductive tape and if any temperature exceeds threshold (currently set to 40°C) it closes any power line going to/from batteries and start flashing alarm LED.
Would this procedure be safe to the PCB itself and also the batteries?
Thanks for any help
EDIT: I realsied i can not charge battery directly from wal lcahrger so that is why now i will put in a BQ2057CSN a 4.2V/2A output Li-Ion charging IC whit cc/cv/conditioning modes to charge the batteries. Now i am just trying to figure out all the possible additional protections for battery that i can implenent.