Battery Life

AT&T NYC. I purchased the Siemens S46 three weeks ago. I am happy with most of the phone features & sound quality etc. Some accessories a bit of a problem, though I did find a reliable supplier. (Previous phone Nokia 8260). I am not happy with battery life, especially when manually imputing info into organiser. Drained the power real quick. Even after input was over, call/battery life does not seem that good. I noticed battery goes from full slowly to Half & then seems to go down fairly rapidly.
I read all the raves about great battery life, mine does not seem to be so. AT&T offered to change the battery. Trouble is I am not sure if battery or phone.
Advice anyone.