I have been having battery problems for the last couple of months. What I have found is battery will drop suddenly from 52%-53% to 0.3% and I get a low battery warning. I have continued using N800 to see how long does it really last and found that I could use it for another couple of hours before battery really goes dead and N800 shuts off. In those two hours battery level can jump back up after few hours of non-use.

Someone else had similar problem and solved it by replacing the battery. My battery is almost two years old. So it could be just the battery needs replacement.