I'm having the same issue, i'm not using any sorta battery widget/app, just stock(non-rooted).
Its been charging for about 4 hours today and went from 20% to 30% and its hot, really hot.
I've had it since day one and seems to match what the OP is saying about once it goes below 20-30% it has a hard time charging but like i mentioned before, mine gets toasty when its failing at charging.
Also had an issue while streaming from google music beta, went from 100 to 20% in <2 hours and once again it was hot as the devil's dick then it wouldn't charge.
Last edited by cobrahicks; 8th August 2011 at 10:41 PM.