Garmin Fenix 3: maxing out the battery life.

Some settings culled from a couple of forum threads and the official Garmin manuals. Go into the watch menu by pressing and holding Up, then choose Settings with the Go button. The following settings can affect battery life.

GPS tracks by Garmin user canopenerboy

Apps:

• Run (or whatever app you will use to record/navigate) →GPS →UltraTrack. In this mode, the GPS records, is switched off for a minute or so, on again, reacquires a signal, records, etc. This will extend the battery life, but considering how hard it is to get a fix while moving, it can result in really dodgy data. At least make sure your EPO is up to date. (http://www.javawa.nl/epo_en.html)
  Or better, skip UltraTrack and carry a charger (see below).  

Watch Face:

•  Seconds Style. Pick one without a seconds hand/counter, so the watch face only updates every minute.

Sensors:

Very thoughtful when you're ultra training.

• Compass →Mode →off. I doubt this will seriously dent the battery life, though. Altimeter and barometer can't be disabled.

Bluetooth: Status →off

Activity Tracking: Status →off. Duh. Unless you can't live without Insights.

System:

• Backlight: Mode manual, short timeout, reduce brightness.
• Sounds: No one needs key tones. I'd keep alert tones, but switch off vibration. Shaking a steel watch consumes a lot of energy.
• GLONASS: off. Unless you have GPS location troubles and know that the added GLONASS helps.
•  Data recording: "Smart" only reduces file size, not battery life. Garmin's .fit format is pretty compact (ballpark number: 100KB/h at the non-smart 1Hz recording rate) and you've got about 20MB of free space, so it's probably more useful to clean out/backup all .fit files from previous activities (GARMIN/ACTIVITY subfolder on the watch in mass storage mode)
• USB mode →Garmin. This will keep the watch data screens up if you charge during recording. The battery capacity is 300mAh, so even a compact 3000mAh lipstick power bank should be good for a couple of recharges and the watch can be worn with the charger clip attached.

My test run was about 10h in navigation mode, temperatures around freezing, all of the above settings except for UltraTrack (off), Compass (on), recording rate (1Hz, standard). Battery was down to 40%, so the advertised 16h battery life seems to be quite correct.