Have you just purchased a new phone and found out that the battery does not last as long as what the sales rep told you it would?  Maybe the problem is not the battery but the proper charging of it. Most of us think we know how to charge our cellular phone battery because we have had a cellular phone in the past and we charge the new phone just like the old one but being a tech for almost 15 years I can tell you that most battery problems occur when the battery was not trained to accept a full charge.  What most of us do not realize is that you will need to train your battery when charging to accept a full charge and not a partial one. The proper charging process is this:

Drain you new phone battery completely before charging it
Charge for a minimum of 10-15 hours before using it
Drain your new phone battery again
Charge for 10-15 hours
Drain again
Charge 10-15 hours
After this process then drain completely and charge 6-8 hours and continue this process as long as you own the phone and you will virtually eliminate battery charging problems!