First, you should not let you battery get below 20% on a regular basis. that can damage a lithium battery requiring replacement. Sounds like that could be part of your problem. It is best to keep the battery pluggedin and topped of until you need to run on battery power.
As for apps running in the background, read this from our FAQ: Maximum Battery Life - BlackBerryFAQ
And if you are in a signal fringe area where the device is constantly searching between EDGE and 3G, that is a huge drain.