I had the same problem and when I made a few enquiries about a new key I just got told you MUST go to a Stealership and its $675. So I searched Ebay and found a company in Hong Kong who supplies genuine keys with the internal computer and battery, it was about $20 I think. So when it arrived I had to send it to a special locksmith with the vehicle details and VIN to have it cut, a further $15 and had it back in a week.

Then I had a new key to fit the car that needed to be coded to the X5, which I was told by the Stealership could only be done by them for $75. Searched this site and found the directions to have the X5 do it itself in 30 seconds, all done and have two keys that work in all modes.

Be careful with keys on ebay, some do not have the internal electronics.