On my old car, I used to have a push button based key for my car. I push the button in the key, and the radio transmitter opens the locks. The battery lasted for 5 years. Easy to understand why the battery lasts, as the radio is operated only when pressing the push button.

On my new car, the key is a smart key: I put my hand inside the door handle, the car detects my hand, then it detects that the smart key is nearby and the car thus opens the locks. I don't even have to remove the key from my pocket.

Now, to me it seems that the radio transmitter in the smart key would need to be continuously operating (or if not continuously, at least in bursts every second or so). This has left me puzzled: how can a small coin cell battery power a continuously operating radio transmitter for years?

So, how are the smart car keys actually implemented? How is it possible that the small coin cell battery lasts for an acceptable duration of time?

What is the transmit power and duty cycle of these smart keys? Or do they instead of transmitting operate as a receiver, detecting the signal transmitted by the car, and start to transmit only if the car is detected nearby?

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    \$\begingroup\$ The car pings continuously not the key. If the key is in range it replies. So the key is listening....not transmitting, which consumes much less power. \$\endgroup\$ – Jack Creasey Mar 5 '17 at 7:20
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    \$\begingroup\$ Also, I don't have proof of this, but it's a fairly safe assumption that the microcontroller in the key is normally in a very low power (nanoamp to microamp) sleep state, and some analog circuitry (a simple tuned filter) wakes it up only when the query signal is present. So most of the time it's not even "really" listening. \$\endgroup\$ – hobbs Mar 5 '17 at 7:47
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    \$\begingroup\$ The key is probably some sort of RFID tag. \$\endgroup\$ – Lundin Mar 6 '17 at 7:41
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    \$\begingroup\$ In 2014, TI made a wireless keyboard that requires no battery using batteryless NFC. \$\endgroup\$ – David Schwartz Oct 21 '17 at 0:46

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