Htc hd7 battery

I received the HTC HD7 T9292 two days ago with windows phone 7.8 as OS but i note that the battery life is not good . ican't make one day sometimes i can't make 20h . i don't know why happen . I made some searches on google people talk about some disfunctionment due to WP7.8 upgrade. can you give me a solution to my problème ..... thank you