Thanks to Kzang for showing me where the battery should be. Looks like he has a gas model with two batteries. The 35d has one battery located on the driver's side, underneath the plastic cover that holds what I suppose passes for the lug wrench. It is held in place with Five (5) 8mm hex-head bolts. A socket and long extension work best to remove these bolts. 3 are easy to get to, but the two down in the bottom of the deep well on the passenger side, are best removed using a socket and extension. In a pinch, a small crescent wrench or a nut driver would do the job. My battery would be shown below, but this site would not accept my jpegs. I have no idea how to determine if it is a new battery, or the original, since I never saw it before today. I assume the codes on top would tell me, if I had the desire to find out. As large as the battery is, I can see someone easily straining their back getting it out of the hole it rests in. From the appearance, it does not seem to be a standard truck battery. It says it is made in USA but does not have a mfg name on it that I could find. I did not remove the red plastic attachment and connection on top to find out. I have the extended warranties on this X5, so I assume I am good till I hit 100k miles in 2018
Amanda