Battery capacity sets the energy budget
A miniature enclosure limits the physical volume available for the cell. The resulting energy budget has to support the microphone, analogue front end, processor, synthesiser, RF power amplifier and any control circuitry for the required operating period.
Average current matters more than peak current
RF transmission may draw relatively high instantaneous current, but operating life depends mainly on average current over time. A device that can sleep for long periods or transmit only when required can run much longer than one that keeps every subsystem active continuously.
RF power is expensive
Increasing transmitter output generally increases DC power consumption and can reduce operating time significantly. Because antenna efficiency and receiver sensitivity also affect range, simply using more RF power is often a poor substitute for a well-balanced link design.
Digital processing has a cost
Encryption, audio coding and radio control require computation, but modern low-power processors can perform these functions efficiently. The greater challenge is often the RF section and the requirement to keep oscillators or receivers active for timing and control.
Voice activation and duty cycling
Systems designed for authorised professional use may reduce average power by changing state when there is no relevant audio or by using scheduled operating modes. The benefit depends on the required responsiveness and operational requirement; aggressive power saving can add latency or complicate link management.
Why published battery-life figures vary
Operating time depends on cell condition, temperature, RF duty cycle, audio activity, transmit power and firmware mode. Meaningful specifications therefore state the operating assumptions rather than presenting one universal number.