This is a Li-Po charger and balancer project for R/C hobby.The charger circuit is based on the circuit of Electron head and all folks in the DIY electronics topic on the rcgroups website.
U1 do the function constant current with limit current by R1 which calculated by
R1 = 1.25/Current in amps of current regulator.
for my project I need to charge 3 cell pack 620mAh.So R1 is
R1 = 1.25/.620 = 2.01612 Ohm
But I don’t need to charge at 1C because I’m not sure the actual C rate(actually may be 98-100% of battery specifications) .In my circuit I need to charge at 600mA or about 96% of 1C and R1 is 2.083 Ohm which formed by 3 X 1 Ohm in series and parallel with 100 Ohm 25 turns trimpot as the above circuit. Then adjust 100 Ohm trimpot to archive 600 mA (measure
by Amp meter at the output of U2)
U2 do the function constant voltage,for 3 cell pack adjust R3 to get the voltage 12.6 v at the output .
Resistor R3(5K) in the balancer section I use 25 turns trimport and adjust to get the across voltage for each cell is 4.2V (without cell connect).
Warning : Before you build this project please read all the posts in this topics
Pdf format PCB include positive, negative bottom and overlay