I assume you are using a charger instead of just the low power USB line. The problem is something else, and I can't remember because on the rare times it came up it only took some simple trial and error step to correct. It might be found in the archives here, but something like unplug and replug, or reboot, or something.
It depends on the level of firmware too. I think several of the more recent ones would pop that message up the first time you charge after updating the firmware. I suppose it could also be a hardware problem like the power plug, etc.
Find it hard to believe 4 of these things have the same issue. It is the latest firmware. It had been over a year since I last updated these tablets so I updated them this weekend and now charging gives this message. The tablets charge ok otherwise. When I have the time ill try a factory reset. Searching around that seems to solve 99% of issues with these things.