My demo unit only powers on when the power cable is plugged in. It automatically powers on, but it shuts off as soon as I remove the cable. When I switch it off, it comes on again after two seconds. Battery charges normally until it's full. I checked the battery voltage, everything is normal. I tried with another battery - same problem, so it's not the battery what's causing this.
I guess Nintendo made some kind of modification to the mainboard. Maybe a jumper, maybe a chip that needs to be replaced, I have no clue.
I know that there are people out there with the same problem and I would really appreciate if someone knows how to fix it.