Little late but just chiming in here. This is relevant to my interests as one of the X5 keys is starting to act up.

You can open up the key by taking an X-Acto knife and cutting along the valley separating the two halves of the key. Go slow and cut away a little at a time until it splits apart. I did this job with my E39 M5 key and replaced the battery with a non-rechargeable one. Big no-no in the eyes of many, but I had no issues of a battery exploding and it worked fine for the 1.5 years I had the M5.

Well I had to find out if this job was as easy as I would like - I bought the car from the second owner and he didn't tell me quite a few things about it when we exchanged the registration papers; so I have one key that works perfectly (I don't know then if it is the 'Number 1' key or supplementary when you re-program it from iside the car), and another that will start the car, meaning that the induction circuit is Ok, but the remote door lock never worked.

So I carefully split this one open to check if it had a battery that didn't take a charge, since it hadn't been used for a year or two, annnndd... 3.0 Volts exactly. So if the battery is good, and the induction circuit is good, does that mean that only the central locking transmitter is fried??