I have a suggestion for you guys... I don't know if it's only me, but sometimes when I disconnect my N900 from the charge, it writes on the screen something like ''to save energy, disconnect the charger from the device''. It tells me this when I already disconnected.
I think that sometimes, even though you disconnect from the charge, it still thinks it's connected without receiving any power. What I do is I restart my n900 each time I disconnect it.