i had the same problem but not with n900,so what i do when not charging somes after the battry goes low..i found one thing that u all must try see your battrey clips if there is gape between them(i mean if you see that gap can not properly fix in mail part on ur moblie) make them to little fix in the mobile mail port.then try to charge.also i would recommned to buy a "battry chager" remove the battry and charge it using the battry charger.

sorry for the bad typing and bad english