I'm getting worst time on my extended battery than I am with my standard. I get 10-12 hrs with my standard and only about 8 with my extended is my phone reading the battery wrong or something
Let the extended die to about 20 percent and then do a charge with the phone turned off overnight. Whenever you get these extended batteries you need to do a full charge a few times to get the results you are looking for. Usually they come with instrunctions. Whatever u do do not make it a practice to let the battery totally drain.
I get roughly 15 hours or so with pretty moderate use and still have at least 30% at that point. If it keeps if up you might think about getting it exchanged......