| The pre-facelift R50 does not have a rechargeable battery in the key. The battery is a CR2032, replaced by prying open the key (there is a recess inside the keyring hole, put a small scewdriver in there to pry the key open). To initialise the key, turn the ignition to position 2 (ignition on) briefly, then off. Within 10 seconds press the "unlock" (MINI logo) button, then the "lock" button.
This is covered in the owner's manual (page 22 in the North American version). | 
