Hello,
I had the exact same problem, the battery that my phone came with was terrible!! With absolutely no use, only gsm on, and everything else off, it lasted 16-17 hrs. If I started using Wifi, nd the screen wud b on for like 45 mins...The battery wud die!!! In 45-1 hr, fully charged to no power!!! And with light use, few mins camera use, nd stuff like that it lasted about 10 hrs. So i decided to buy a new battery, now i bought one frm ebay (the guy said he got it from Nokia retail store in Singapore, looks genuine), but anyways, now even with about 2 hrs of wifi use, couple of phone calls and messaging, lasts me about 30 hrs. And with no use at all, around 50+ hours!!