Letting a Lithium battery run down below 20% can cause damage to the battery.
It sounds like you have several data intensive apps running in the background. When you are finished with an app, make sure you close it down, not just exit out. Keep the battery topped up when you are not using it.
By update, do you mean an OS update? Check the 8500 section for comments about the OS version you are running. That could be part of the problem. I never recommend doing an OS update unless there is a real need.
And from our FAQ: Maximum Battery Life - BlackBerryFAQ