Monday (day 7) I plugged the cleaning robot back into the charger after use.
Today (day 10), I tried to restart it and it's completely dead. No lights, no motion.
We'll see now how good their customer service is. I e-mailed them via their support web site a few minutes ago, since their customer "service" department closes at 7 eastern time, and there's no information I could find about this particular problem.