Sure that you have already tried all these measures in this resource:http://www.nokia.com/gb-en/support/product/lumia80
You don't say how long you have had your Lumia 800 as earliest devices now a year old and this is the maximum warranty period for built-in battery.
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!
Perhaps you should get your phone checjed out at your local Nokia Care Point. If there is a problem with the battery, they should be able to sort it out for you.