Mine are in my 2010 X5. I get the low battery warning and need to charge it up. Also 2012 335is does the same thing. A good hour in the slot and the keys are good for a month or more. There isn't even a door on the keys to open to replace a battery.
I may have mistaken comfort access option but that is the one where you don't have to put the key in and you can lock the door by pressing on the handle correct?