You should not let the battery get below 20% on a consistent basis as that can damage a lithium battery. Have you done that?
How old is the battery?
How much are you using the device on battery power? What apps are you using and which are running all the time and perhaps in the background?
How many hours are you getting everyday?
We have a section in our FAQ dedicated to battery usage. You might want to check that out as well. Here is one article: Maximum Battery Life - BlackBerryFAQ