New iPhone 4S battery problems?

So I just got an iPhone 4S, and put it in to charge last night, cause it says to put it for 10 hours. I took it out this morning at 10am fully charged and left it there while I was at school all day. It's Wi-Fi, 3G and everything was off, no apps where open (I still haven't downloaded any)
Anyway I came home now at 3pm and its down to 81% with no use at all! After like 15 minutes its 64%
Is this normal? Is something wrong?

Is Wi-Fi on? That can drain the battery

Only thing I can think of that would do that is leaving the brightness up full, the 3G internet on and the phone unlocked, or having a powercut while your at school and one of your parents sorting it before your back home.

Try turning the brightness as low as you can next time then lock the keys and let it charge, it should work fine.
(3G internet uses more battery than Wi-Fi, screen brightness is the main thing that uses up iphone batterys

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