i wouldn't pay much attention to percentage of battery you have left. It is based on how much battery you have been usung recently, so if you start doing something more battery intensive, your reading can be wildly inaccurate. I have on occasion gone from 100 percent full to completely empty in just a few minutes. -- It meant that the 100 percent full was bogus.

I almost never use the usb port except for flashing; I bought an external battery charger for maybe $10 ages ago and it has worked perfectly ever since. I hate having my N900 connected to an electrical outlet and just spend less than ten seconds swapping batteries whenever I need more power.

Others may have more details about battery usage; I don't care much.