This is the progress for today:
I finally transferred the dead-bug build on a more permanent perfboard:
To test it, I make a quick-and-dirty 2S 18650 pack:
The yellow and the blue traces are opamp's output and half-bridge's output respectively.
Good news: it is running in closed-loop. Bad news: the switching bridge is noisy. :(
The 50% duty cycles indicates that it will make both of the cells to hold the same voltage, i.e. 50% for cell 1, 50% for cell 2.
Result so far:
-Good: Cell 1 and Cell 2 both is approaching the middle voltage.
-Bad: It takes too long.
Note: Cell 2 has more 340kHz ripples because it has more internal impedance.
Next plan are:
-Reduce the switching noise
-Make it balance the voltage faster!