Get a new battery and follow the instructions in your cars manual. There is nothing wrong with your X5.
¨If you plan to park the vehicle longer than 4 weeks, have the battery disconnected from the vehicle's electrical system by disconnecting the cable from the negative terminal. Then have the battery charged with an appropriate battery charger.¨

I've had this issue on my X5, parked every second month, charging and changing several flat batteries. A simple battery switch on the negative wire was my solution.