Most power problems are often caused by a the network condition, a change in the users habits or a defective battery. In rare circumstances it can be faulty hardware in the phone or even the charger.
How can you be so certain that it's not the battery?
Batteries only have a finite life and can become defective at any time. Other factors could have also affected the batteries life span, such as exposure to high temperatures (for example left in direct sunlight for an extended period).
Network performance can also have a massive effect on battery life. For example if you are constantly in a medium to poor signal area your battery will drain much quicker. This can happen when using standard GSM networks or 3G networks, which cause even more of a drain.
Rebooting a phone also causes a large drain, so if you are switching your phone off several times a day that will be the cause.
Having the screen set brighter or lit more often will also cause a big drain.
