If you phone is going thru more than half the battery in 7 hours it is due to one of three things
1) you are using it alot, not just alot but ALOT!
2) you have something running in the background. IM programs and some other will suck your battery dead in no time!
3) you are in a poor reception area and you phone is struggling to maintain service. I have never seen this myself but I hear this is true. My normal area is covered very well.
I guess the phone could be broken or you could have a bad battery