Recently I bought an iphone 3gs from a friend, but my battery seems to run down quicker than I thought it would. I have no Apps running in the background, having turned them all off. I have gone through the settings and turned off most things, mail, wifi, push, etc etc. you get the idea... however I charged the phone up last night, went to bed and obviously left the phone switched on for the alarm to wake me up for work (i quickly discovered with the iphone, unlike other mobiles, you need to do this or the alarm does not go off) and the battery was down to 86% from 100% in just 6 hours. A 14% drop off with no real use. After I got to work today it had lost another 3% between 7am and 9.30am after only sending 1 text message. Is this normal or should I be worried, if the battery is on its way out can it be replaced as I see no way to open the iphone up as with other mobiles and is it expensive to replace, if it can be done? Thanks for your replies.