Came home yesterday from a short business trip to find the battery in my M5 had gone tits-up. Did a history check and found it was last replaced in 2011, so after some quick research, was off to O'Reilly to pick up a SuperStart 95R Platinum AGM battery. You wouldn't believe the size & weight of this hulk - 63 lbs! Good price for AGM though at $179.95, and super-easy to replace. Unlike the E46 and other E39's, the M5 battery is in the center-rear area of the trunk, not in the right rear fender well. Now I'm ready to endure the brutal Phoenix heat. I always go AGM for that reason.