I use the Fenix not only to tell time but also as backup navigation. I set up routes using GAIA GPS and send them to GPX files, which I upload to the Fenix to use the Navigate feature of the watch. I minimize my use of Navigate to reduce battery consumption.
I usually get 41 days before having to charge my Fenix.
Update Firmware
Keep the firmware on the Fenix up to date to take advantage of new power efficiencies. Use the Garmin Express app to update the firmware on the Fenix.
Turn On the Battery Saving Feature
The battery-saving feature turns off the connectivity features and reduces the display to time and date. This setting is very aggressive and does not stop the ability to do activities like Navigation.
Change the Backlight Settings
Hold MENU ⇒ System ⇒ Backlight ⇒ During Activity ⇒ Brightness ⇒ 5%
Hold MENU ⇒ System ⇒ Backlight ⇒ Not During Activity ⇒ Brightness ⇒ 5%
Hold MENU ⇒ System ⇒ Backlight ⇒ During Sleep ⇒ Brightness ⇒ 5%
Use UltraTrac
UltraTrac sets the update interval to 30 minutes for tracking and turns it off once completed to save battery.
Hold MENU ⇒ Activities & Apps ⇒ Navigation ⇒ GPS ⇒ UltraTrac
Turn Off Bluetooth
The Fenix uses Bluetooth-connected features with a smartphone with a cellular connection or Internet connectivity to upload activity data to Garmin, get weather, and connect to any supported Bluetooth sensors.
Hold LIGHT ⇒ System ⇒ Backlight ⇒ During Activity ⇒ Brightness ⇒ 5%
Turn Off Activity Tracking
When using an activity, tracking is on, so when done, stopping it is crucial. Tracking from actions should be discarded to free up memory on the Fenix.
Press Start/Stop ⇒ (wait a few seconds) Discard ⇒ Yes
Turn Off Smartphone Notifications
Delete the Fenix from the smartphone to stop the smartphone from trying to connect and eliminate any notifications coming from the smartphone.
One iPhone, Settings ⇒ Bluetooth ⇒ Tap the “i” next to the Fenix ⇒ Forget this device
Turn Off Wrist-Based Heart Rate Monitoring
Hold MENU ⇒ Sensors & Accessories ⇒ Wrist Heart Rate ⇒ Off
Turn Off Pulse Oximeter Readings
Hold MENU ⇒ Sensors & Accessories ⇒ Pulse Oximeter ⇒ Off