Originally Posted by Popup
I tried to connect an external power supply instead of the battery, but with very limited results. As mentioned above, it keeps consuming power (~120-130mA) but without any on-screen activity. (For at least fifteen minutes.)
That actually sounds quite normal - if the system is busy with one of its background tasks, such as rotating log files or indexing books.
If during all of this you made any changes to the contents of the user storage partition (the one you see over the usb cable - A.K.A: /mnt/us) it is very possible that the indexer is running.
That run time can be long, and if it hits one of the publications that it can not index, it is known to get stuck in a loop.
Suggestion: Empty the USB storage area (to be certain you don't have a "bad book") - re-boot.
Expect it to actually go to sleep within a few minutes of the screen indicating it is "asleep" (it isn't, not when the screen is first shown).