I think that the 12 seconds behaviour (only when connected to a charger/the PC) is some very very low level behaviour.
I have another board from a Kindle that was heavily water damaged whilst powered up and was then dried and left. Then the bare board was rinsed/scrubbed and near as dammit cremated in an attempt to dry/reflow. When you connect a battery to this board, the LEDs flash green briefly then nothing. When connected to a PC there's nothing showing either. (SE Blank Ringo or Kindle) So I assume that there's a separate power/charge supervision to the main CPU/software.