These days, more and more people are complaining about Nokia problems, and I do believe the company is going through hard time. A lot of Nokia 800 users are complaining about the battery life. I've looked into the problem and it seems that the hardware part is ok, and there is a software issue you need to address. Nokia released a statement a year ago about these problems, and they are fixed with the software update.

In order to verify if you have the problem in the first place, you need to enter Nokia Diagnostic Tool and check the battery status from there. You should look for the number that indicate "Full Charge Capacity". If that's number indicates less than 1000mA, then you have the problem and you should probably look for a firmware update for your phone. If the problem is still there after the firmware update, you should send it to Nokia, and they will fix it for you.