Does turning on voice announcements affect battery life on a hike?

The short answer

Turning on spoken distance, pace or interval cues costs very little battery on its own. Producing a short sentence from a number and playing it once is a brief, occasional task, closer to the phone briefly waking up to do a small piece of work than to anything that runs continuously. It sits well below the GPS receiver's own steady draw, and nowhere near what the screen costs when it is lit.

Speaking a number is not the same job as recording one

It helps to separate two things that both end up coming out of, or going into, the same speaker and microphone. Recording a voice note means the microphone stays active, capturing continuous audio and writing it to a file for as long as the recording runs — real audio capture, encoded and saved, the same kind of work a voice memo app does. Speaking a distance, a pace or an interval cue out loud is a different job entirely: a short piece of text is turned into a few seconds of audio and played once, then the work is done until the next update is due.

A capture running continuously and a short clip generated and played occasionally are not the same size of task, even though a person standing nearby would hear both of them as "the phone talking" or "the phone listening." One holds a piece of hardware open and busy; the other asks for a brief burst of work every so often and then goes quiet again.

Why the frequency matters more than the feature

A spoken update fired once a kilometre, or once at the top and bottom of each interval set, is a handful of seconds of audio spread across an hour or more of walking. Multiply a few seconds by a dozen or so updates over a long hike and the total time the speaker is actually doing anything stays small next to the length of the walk itself. This is the same shape of cost as the built-in interval feature calling out a sprint and a rest — occasional, brief, and proportional to how often a cue is set to fire rather than to how long the hike runs.

What actually costs something on a long hike

• The GPS receiver, which runs continuously for the length of the recording regardless of whether anything is ever spoken out loud.
• The screen, which is the largest single draw on the phone any time it is lit — checking a number by eye instead of hearing it spoken costs more, not less.
• Streaming anything over a weak cellular signal, which is a real and separate cost that has nothing to do with a short spoken cue generated on the phone itself.

Against that list, a few seconds of speech every so often barely registers. It is a rounding error next to the receiver that was already running the whole time.

The part that actually saves something

The point of a spoken cue is not to add a feature on top of checking the screen — it is to replace checking the screen. The reason interval and pace cues exist at all is so a phone can stay in a pocket rather than getting pulled out, unlocked and read mid-stride. Every time a spoken update takes the place of unlocking the phone to look at a number, it is trading a brief, cheap burst of audio for the far more expensive job of lighting the screen. Over a long hike where checking in every so often is going to happen one way or the other, hearing the update tends to cost less overall than looking at it, not more.

Where TrailCam fits in

TrailCam can call out each sprint and rest in an interval set, and speak distance and pace updates as you go, in whichever of its four languages you use the app in — all without needing the screen on or the phone out of a pocket. TrailCam is free to download.