new mega 5.8 battery issue

Hi i'm new in here and I got a question with my new Galaxy Mega 5.8 i got only 2 weeks ago. Since the phone came out from box, its battery just so weird---once i unplug it, 3 hours passed, it downs to 80% without any heavy usage(all stuff like Wi-fi,bluetooth,gps,syncronization are off). I haven't calibrated it cuz i don't really have time... may it be the reason? But my former S2 has two batteries(stock one and additional one from mpj) worked perfect without calibrated. Just for two weeks n I didn't do anything. Can you or anyone tell me whether the battery life is normal or I have to go to the warranty ? Thanks in advance