There could be a lot of reasons for this so I'll give you the usual suspects.
Bum battery - it seems unlikely but manufacturers make mistakes. Your battery could be a crap battery.
Background data - many applications such as facebook and email apps church for updates at specified intervals. You can change these so that they are not do often to save battery life..
Screen brightness - with a brighter screen you take up more power.
Lack of service - if you are in an area with bad service than your phone will try even harder to get a signal.
Bad app - some apps are buggy and for whatever reason hog up the cpu or they could be malicious in nature and are doing the same thing while something else that is evil.
There are several other reasons that might be causing your battery to lose power but they don't come to mind right mlle. I hope this helps
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