This type of BMW key fob uses rechargeable battery soldered to the PCB board. Charging take place passively when you are driving with the fob in the ignition slot.
As all rechargeable batteries degrade over time, it is a matter of time it will no longer charge. Unfortunately the design of this key fob is such that you cannot simply pop it open and change the battery as it is completely sealed with ultrasonic welding.
In other words, if it reaches the end of the life, you should get a new one. However, if you do not wish to spend so much for a car that is going to expire soon, you have a cheaper alternative! We can replace the rechargeable battery for you at a much lower cost than complete replacement.