I have a Garmin 800 and love it for touring.
Every year for the past 30 or so, I've done a 10-day backpack on the Pacific Crest Trail (PCT). It's been interesting to watch the technology used by thru-hikers evolve. These days, most carry some sort of smart phone. A couple years ago, everyone had a solar panel on their backpack to keep their phone charged. Last year, no more panels, none. Why? Inexpensive high capacity batteries which are cheaper, lighter, and more convenient. Same thing with a Garmin. You can run them just fine while on an external battery, just don't do it in the rain. Being a real cheapie, I use a cheapo battery pack with 4 AA rechargables, which makes the required 5 volts and combined with the Garmin's battery, will last several days. I do have to carry a charger for the AAs, though.
That said, I also have a separate cheapo wired computer so at least I'll have distance if something goes haywire with the Garmin. No matter what systems I've used, I always have had 2 odometers.
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