In my case, the key will open the car using the lock cylinder on the door handle and will start the car just fine. The remote functions on the key dont work at all. This occurs no matter how far I am away from the vehicle.

It realy does sound like the electronics in your key has died. Either the batteries or the radio transmitter... if the key won't recover after a longish drive then I would say you are out of luck and need to go visit a dealer to order a new key. I doubt that you will be able to achieve anything by attempting to open the key. The battery is not intended to be a user changable item and its not going to be easy to get to without probably wrecking the key housing! :(

Sorry I take that back about the impracticality of opening the key after reading this post by mrbmwx5...

http://www.xoutpost.com/bmw-sav-foru...y-key-fob.html

I have messed around with my key. I opened it put a new battery in and it worked great for about 6 months. The same issues came back again. Finally it just quit working (remote function). Then I bought a Key off "Drew" on this forum and it works like a charm. He could hook you up with one.