Originally Posted by HPIA4v2

From what I can gather on this AGM battery so far is that it's not far superior in cold temp but it has very small self-discharge curve (around 1-3 percent of capacity per month). That;s why BMW switch to this on newer models since those tend to have more electronics (i.e comfort access, that can drain battery while sitting in garage if the FOB is within 30ft in many cases).

The problem for older X5 would be charging (or over charging in this case) of AGM battery possibly shorthening the life of it. That's why on newer BMW there is inteligent charging algorithm that register the batt mfg date and the car adjust the charging current overtime (being more toward the end of batt life).