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On my Lexus it's both active and passive RFID. If you start the car and the key is 'active', you have to keep it in the cabin, or the car starts complaining (I've not been brave enough to drive with it not in there...).

If you start the car with the key as passive (ie: dead battery - you have to hold it on the ignition button to be detected), it recognizes the difference and doesn't complain.

[kreneskyp]

Just because it's "active" doesn't mean the FOB needs a battery for the RFID to work. The car could periodically check for the key.

If your RFID required a battery, and required that it constantly be on, then the battery would need to be replaced often. Couple this with your car potentially shutting off in the middle of the highway and you've got a very unsafe car.

I know for certain that VW keys do not need the battery for the RFID to work. The battery is only used for remote lock/unlock. Perhaps other cars are different, but I'd consider that a design flaw.

edit: forgot to mention that my VW does not even have physical locks except for the glovebox. A dead rfid battery, if it had one, would mean i was stranded. I'm actually not sure what happens if my car battery dies while the car is locked.

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The fob battery does die fairly often (replace it once/twice a year?).

My car has no visible physical locks - there's a small notch under the drivers door handle you can pop off a piece of the handle using the fob, and then there is a physical lock you can get into the car with (no way to open the trunk until you turn the car on).

I've had to do this a few times :)

Like I said, you can still use the key to start the car, but if the battery is dead, you have to hold it up to the start button.