The goal is to not have the battery go dead in the middle of a flash. As tasks go flashing is pretty power intensive. I successfully flashed mine with the AC connected and a well charged battery. If you power down for some reason the N810 won't start with the power button while connected to AC. If you reboot from the command line "sudo reboot" it will reboot on AC power. It's my suspician that the power switch lock when charging is the basis of thinking you can't flash on AC. Perhaps an N810 predecessor was that way. I can only speak to my personal experience. No doubt flashing on fully charged battery will work.