That is what the circuit in post 17 does. The Tl431 is a voltage reference that can be set to turn on the shunt transistor to "steal" the current from the battery that is already at 4.2 volts. These work okay for low currents like you were talking about (100 - 200 ma. or so).
One for each battery, correct? I would probably need to bump up the transistor if I were to balance at 1 Amp, or substitute a FET. I was only doing low currents because I dont want to drain the battery with it sitting there, but then again the balancer will only be active when its above 4.2V. The Batteries are actually 4800mAH per Back (2 in parallel per 4.2V back-so basically 2P 2S). Is there any difference between the Discrete version and one using a Op-Amp in terms of function?
So I have many options here:
1) Discrete Like Post #17 and #21
2) Switch in between the Pos + neg of the two batteries being charged (each having their own charger)
3) Comparator Just dumping the load if the cell is above 4.2V
4) Dedicated IC (Small But Might be not solderable..I havent delved into really small parts yet)
5) Just make One huge Battery pack, with all the cells in Parallel. Avoids complicated Charging (I could Change my design for it-But I think its taking the "easy" way out)
6) Separate Buck Converter and Charger for Each Battery.
All of this has me wondering, how is this done on a commercial scale? I mean there has to be something Done on a commercial scale to balance cells thats cheap and effective, Unless they just design everything to run on single cell's. I mean, how is balancing done in a laptop?
ADD: Ok Linkage time. Im adding some Links since I know others might come here for the same reason. Looks like someone else had the same idea of using a comparator:
Comparator Balancer:
https://www.rcgroups.com/forums/showthread.php?t=157641
Discrete Balancer -looks like the schematic was updated a few times.
https://www.rcgroups.com/forums/showthread.php?t=270580