On most of the notebooks that I have taken apart (not too many, I don't usually repair notebooks) there is a small, lithium type, battery hidden deep inside. It is the backup CMOS battery. I have generally found it near the screen hinges, under a protective plate or a layer of PC Board. Thankfully, the ones I have dealt with have all been plugged in, not soldered on.
I would guess that you have something similar, hiding in the depths of your machine. Until you unplug it, along with all the other batties and AC, you are going to remain stuck.