Charge with sun only completely drained battery

Carry a portable solar charger?

i.e. https://thewirecutter.com/reviews/best-portable-solar-battery-pack/

Or https://www.tomsguide.com/best-picks/best-solar-chargers

https://forums.garmin.com/outdoor-recreation/outdoor-recreation/f/fenix-6-series/171763/does-f6x-solar-charges-by-light-while-shutdown/952747#952747

Ray from DC Rainmaker tried this and this and got from 0% to 4% after 11 hours of sitting in direct sunlight.  So.. I guess so?  But whether it's practical or not is a different thing.

Comment #775 on his review here www.dcrainmaker.com/.../comment-page-2

You could also carry a *small* backup battery, the smallest ones will charge your watch several times.

https://www.tomsguide.com/best-picks/best-portable-chargers

https://www.pcmag.com/feature/368589/the-best-portable-chargers-and-power-banks-for-2019

Absolutely!

The 6X Solar (and regular 6X) lasts long time on a full charge. But if you think you might be in the wilderness for more than a couple days, you should always have a portable charger of some sort. Not just for your watch, but for all your other USB-chargeable goodies too (phone, flashlight, etc).

When I'm gone for more than 3 days somewhere, I even carry a portable solar panel charger for absolute emergencies. I could be "missing" for 6 months, and still be able to keep my stuff charged.

I'd also recommend setting up a low battery alert on your watch, so that you never let it drop to 0%. If it gets below, say, 10% charge, then you should switch to Expedition mode, which should be able to stretch out what's left on the battery for at least another week.

With a little planning (charging the watch before your trip), and a small investment in a portable charging setup (battery pack and/or solar charger), there's no reason anyone's watch battery should ever drop to 0%.