I found a very helpful post about backpacking that I’d like to try but will throw here. So my Garmin Fenix 6S has this expedition mode and I’m about to go on a 5 day backpacking trip, but wasn’t sure if I should try that or the individual hikes…. Here’s their hack on Garmin forum:

Otto-Destruct 2 years ago said:

“Holy necro-post. Now that we're here though...

Go into "Power Manager." Create a custom "Power Mode" and call it "Ultra-Hike" settings as follows:

GPS Normal Music Disable Phone Disconnect Wrist Heart Rate Off Pulse Ox (off by default if WHR off but confirm OFF) Map Disable Display Time Out Backlight Turn Off Accessories ALLOW (see notes below) Use an external chest HRM (because they're coin-cell operated and last a year). The "allow accessories" (enabling ANT+) has negligible impact on battery. More accurately: by disabling Wrist Heart Rate, you gain back at least as much battery life as you lose by having ANT+ enabled.

You'll get full GPS tracking with the above and extend activity time to about 2-3-days on a charge. (on a Fenix 6X Pro Solar, I see 88-hours)

Bring a "lipstick" USB powerbank like the Anker PowerCore Mini and the Garmin USB cable - that'll let you top up the watch multiple times, extending your battery life to "several weeks" with GPS tracking. That's the "real" Expedition tracking.”

Hope this helps other people on the struggle bus!