I would still consider the battery, only from firsthand experience.

This Saturday, I went to Danville as a day trip (1 hr steady state drive) and after filling up at a fuel station, the X3 began to have trouble starting. Slow crank, and the DSC + ABS light would illuminate for 2 secs longer before extinguishing. Because this all occurred immediately after filling up, I suspected bad fuel. Battery was #2 on my list but the X3 has been starting fine recently and all had been normal, plus I do mostly highway driving.

I continued my errands, stopping/starting another 5 times, each time with the same symptoms. The X3 carried me home; I'd intended to fix my bad parking job so I tried to start it again but it would not start. Sufficient power to run everything (10 volts) but not enough to crank. Basically the X3's battery gave up the ghost in one of the more convenient places.

I suspect the heat killed the 7,5 year old battery.