My windows phone is around 1.5 years old. But very recently I found my phone run out of battery within 12 hours. I guess tt could be one of the app that I installed is running numerous background tasks. I turned off all apps except Cellular Data under Settings -> Applications -> background tasks. But under Advanced I could see more apps including which I cannot control their background tasks. So, is there a way (or an app) to find out which app is trying to perform more background tasks at less interval?
No, there isn't. Background tasks are very limited in how many resources they can consume. Microsoft worked very hard to make sure they wouldn't drain your battery. What tasks do you have listed?
Odds are it is something else. How many email accounts do you have? How often do they sync? Are you talking alot? Playing music? It is possible the battery is just shot, and you need a new one.