Logic check guys...OF COURSE there's a battery inside the key . Any radio transmitter needs a power source, even the weak 315mhz transmitters inside a BMW key fob (not that you'd want to destroy your $150.00 key just to get to it mind you, but yes - there IS a little battery inside in case you were wondering). A capacitor is not a subsitute for a constant power source, and would simply not have the reserve capacity to repeatedly transmit without being recharged after each use. Try locking/unlocking your doors a dozen or so times and you'll see what I'm talking about...