If you were able to make ~100K with the original battery it is time to change it. Mine has 58K and I just had my first jump start last week. I think I will do the same. The dealer quoted me $210 for the battery and $130 for an hour of labor. I think $340 is a good deal for a good night's sleep for another 8 years.
BMW X5 2005 Sport E53 M62
4.4i V8 DOHC 32V AWD