The coin cell is the power source behind BLE beacons, door and window sensors, tire-pressure monitors and many shelf labels. A lithium manganese dioxide cell such as the CR2032 offers a few hundred milliampere-hours in a package small enough to fit almost anywhere. Its 3 volt nominal output suits most low-power microcontrollers and radios directly, without a regulator.
Its weakness is pulse current. A radio transmission draws far more than a coin cell can supply efficiently, so the voltage sags, apparent capacity drops and the microcontroller may brown out; designers add a capacitor to buffer the pulse and keep the average current tiny through deep sleep. Where the device cannot be made that frugal, an AA cell or a larger lithium thionyl chloride battery is the usual step up, and the battery life calculation starts again.
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