Dedicated GPS that wouldn't suck to lose as much as an actively used phone? Jury-rigged offline smart home dashboard (if you use that sort of thing, I don't)?
If nothing else, at least one backup device with a working battery would be important to have if your primary fails, so that you have something to fall back to.
If nothing else, at least one backup device with a working battery would be important to have if your primary fails, so that you have something to fall back to.