Test no. 9 is pretty easy to run, and should be tested several times with the engine on and off. SlickGT1 has the test listed in his signature. It's also listed in the X5 Cool Hidden Features thread.
After you looked at how to do it (if you have not already) and need help let me know I can walk you through. Once you 'unlock' the test, you won't need to go through the unlock process again. You can simply scroll thought to Tr.9 and run it to check your voltage.
Get a few readings with the engine off ( one before you have driven, one after you have driven). Take a drive and note what the voltage readings are over the drive ( when you first turn the engine over, when you are on the highway, when you are parked after a drive).
Report the readings back and someone will be able to give you their two cents...
If it ends up being the battery you will want to take the look before you leap approach and just to check on the threads to ensure you swap out in the most beneficial way.