marwatk wrote:... My ... unit ... shuts down ... and won't respond to commands ... when it is plugged into the charger...
I would first suspect your Fast Charger. If it were mine, I would test the charging power supply by first measuring its output voltage (no load, not connected to anything).
If I measure about 22Vdc, open-ckt, that would suggest I do a load-test on it. One of the easiest ways for me to do that would be to connect two lamps in series across the Fast-Charger's output plug. Of course, that setup requires some hardware purchases. I would take the FC to Radio Shack and buy a jack to match the FC's Roomba-plug; then I would go to an auto-parts store and ask for one (I already have one) "211" cabin lamp (popularly used as automobile dome lights from the '60s to whenever, '90s (?)). The 211-lamp looks like a cartridge fuse (0.38" dia. X 1.6" LG), an axial filament in a glass-tube, and with a metal terminal at each end. The terminal shape makes it easy to clip onto, so no lamp-holder/socket is needed.
Other stuff like small-size hookup wire, solder, and a few clip-leads could also be used (all on hand at my place).
I do all the work of connecting the two lamps in series across the jack. Then connect the jack to the FC's pigtail and plug in the FC's AC cord.
If the FC is working, the lamps should be brightly lit (a charged Roomba-battery powering a single 211-lamp, pulled 1.1A, so powering two in series with about 11/15ths the voltage across each lamp should hold current up around 0.8A--IF the FC is working. Of course, I would also measure current through the load, and voltage across the load--more comforting to see the data!
Mind you, I'm not telling you that *you* should do all that, I'm just saying what I would do if it were my system with such a problem!