Happened to me exactly like that (GN appears to be charging, when unplugged, battery consumption was awfull and low battery notifications all time)
And after a long day surfing on every android forum i found a solution that worked, at least, for me:
It was just the usb port that was dirty or bend over down, when it happens, it makes a contact that makes it thinks its plugged, not charging, just plugged. (it appears the bolt, but in battery settings, it said "not charging")
I just grabed a needle and bend the little pice up, just a little and carefully. Tried to clean it just in case, no dirt over there.
When I charged it over again, it worked as always, aparently its solved.
Hope it works for you, excuse my english, latin speaker.