I had a 3h30 run yesterday, started it at 100% and plugged the the Apple Watch 7 41mm when it reached 17% to avoid interrupting navigation and recording.
On my previous outdoors run (in cooler ambient temp) it shut off at ~13%.
Today I'm looking at ways to improve the battery life without significant compromises in experience and I found something.
With an External HR sensor paired in WOD, the internal HR sensor is still activated:
- With active connection and receiving HR data
- When the connection is lost
When the External HR connection is lost, despite the internal HR sensor is continuously active, WOD does not display any heart rate.
Expected behavior
- When an External HR sensor is paired, selected as source in WOD, and connected when starting the activity, the Apple Watch optical heart rate sensor should be disabled in order to save power
- If the activity is started without External HR connection, optionally: Internal HR gets turn on ON and becomes the active source
- During an activity with External HR as active source connected, if connection is lost: WOD should turn on and fall back to the Apple Watch internal OHR as source until the External HR sensor is re-connected.
Partial workaround
Connecting the External HR sensor to Watch OS from the Bluetooth Settings already provides the expected behavior, including the internal HR fallback.
Drawbacks
- Watch OS doesn't notify of External HR disconnection
- Watch OS doesn't reconnect to the External HR sensor automatically, keeping the internal HR sensor as a source. (breaks my use case, AW can't measure HR from the wrist reliably during runs)
- WOD loses the battery % of the heart rate sensor anymore.
Could WOD request WatchOS to re-connect to External HR sensors, or is it not unaware of the underlying sensor status anyway?
Conclusion
Although the battery drain of running the internal Apple Watch HR sensor unnecessarily is likely small compared to the display, GPS, speaker and haptics, it would be preferable to keep it off when unused and use it as optional fallback on External HR sensor disconnection.
It's the behavior Garmin and I believe other sports watch follow today.