Wiring new protection circuit board
I didn't know how to do all the things a1 suggested and was unclear what "this post" in last sentence referred to. I did buy and install one of the aforementioned protection boards w/ 15 A working capacity. I paid $12.50 and got fast shipment from KY (https://www.amazon.com/gp/product/B00JJIZYSC
). Same YH11047A can be bought for $6 if you get shipped from China.
I removed the R4's thermistor, checked that its response curve is identical to one from an old NiMH pack (it is), and put it between output negative terminal and the sensor terminal (wide strip on the side) -- same location as in NiMH packs. I did not add a heat sink. All the original R4 cells tested good, and were left the same. Photos show it wired in place.
Charging in the Roomba 400 seems fine. When charged in 15 min. segments (6 hrs rest between segments) the Roomba case gets slightly warm to touch, the batt stays room temp. When green light indicates full charge, the output terminals show 16.55 V without load. Run time is slightly better than with the original protection/balancing circuit.
This circuit appears somewhat less complex than the original; whatever protection and balancing it's doing is probably less sophisticated. I'll run it for a few cycles of use / recharge, then open case and load test each cell to get a better idea if it's charging evenly. With the careful recharging method I use, in the Roomba, in short periods on timer so as never to overheat the batt or charging parts, I think this simple solution may be adequate, tho admittedly not as safe as the original circuit. [also I never leave batts in the Roomba once they fully charge.]