Progress...
We've made some significant progress, in the last couple of days, and I just submitted what I think is the "final" version of the layout, to have boards made. The changes we have from this point forward will most likely just be some resistor value tweaking, and/or maybe a change to a slightly higher current rated NPN transistor.
Here's what the boards look like:
What is different from the first layout is that Richard added a gate driver chip to square up the pulses going to FET gate, and we got rid of 2nd TC54 chip that we were using to to detect when the shunt was fully on. Richard figured out a clever way to do this without the TC54 that actually works better.
Basically, the way the circuit works is that once a cell's voltage reaches about 3.65-3.70V, the LM431 programmable zener starts to conduct, which then drives a transistor/power resistor shunt. When the shunt first starts to bypass current, the green portion of a bicolor LED starts to come on. When the shunt is close to maxing out (about 150-200mA...), the red portion of the same LED comes on. This makes the color a yellowish-orange hue. Wed the red portion comes on, so does the opto output for this channel, which then pulls down the FET gate, via the gate driver, which then cuts the charger current. The cell voltage starts to drop when this happens, which turns off the opto, turning the FET back on. This oscillation continues, just keeping the high cells at the cutoff, while the rest catch up. Once all the cells have their shunts working (i.e. -- all the LEDs are on and "yellowish"...), the current will start to drop. When a cell can't accept anymore current than what the shunt can handle, its red portion of the LED and its opto go off, leaving just the green portion of the LED lit. Once all the LEDs are green, the cells are about as full as they are going to get.
Richard is still playing with voltages and the bypass current, which is why there might be a couple of tweaks, here and there. We want to make this as bulletproof as possible. Once the tweaks are all done, I will post the schematic. At that point, the kits should be ready to go as well. I've already got all the parts.
-- Gary