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Originally Posted by KevinC
I don't see how the RFID tag (which it most definitely is) can be using the battery - it would be "on" all the time. An "active" tag merely means that it can be written to (which this one can as info is stored in the fob), not that it's battery-operated. The FCC info is there for the normal remote transmitter, which is a separate beast.
There's no reason the fob shouldn't work forever w/o battery replacement, at least the RFID portion of it. Hitting a button to unlock the "old-fashioned way" is another story.
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Both "active" and "passive" tags can be written to. An "active" tag has it's own power supply, a "passive" tag gets it's power from being "excited" by the receivers radio signal. "Active" tags are on all of the time, but use very little power. They, however, are not really transmitting a signal unless a receiver asks for it kind of like your computer in hibernate mode and then you hit the resume button.
Talking with our RFID export at work his best guess it that the FOB is a combination of a tranmitter and "passive" RFID tag. The transmitter to open the doors, and the RFID tag to send the "unlock" signal to the car once it is excited by the receiver in the dash..