The Nokia Lumia 800 comes with a non-user-replaceable battery. And even when this Windows Phone manages to maintain a pretty good battery life, some of the users of Lumia 800 recently reported a poor battery life problem of their phone.
However, Nokia recognized the problem and promised to fix the issue soon. The remedy involves just a small fix in the software of the phone thankfully. But those who do not wish to wait for the fix( which is coming in the beginning of the next year) can claim for replacement of their phone. The phone users can dial ##634# to know if their phone is facing this problem under the battery status. If the fully charged battery is shown to be less than 1000mAh, then probably you are among the affected ones.