I would leave the house with it already connected to a battery pack. Even one of the lipstick-sized 3,300mah batteries will keep a 520/820 running for 24+ hours with NAV on. And you'll have absolutely no worries about it, much preferable to nervously watching the battery display all day.
I don't rely on signage even when riding in areas I'm already familiar with. I've pre-ridden organized ride routes that have had incorrectly placed turn arrows. This is what cycling NAV units are built for.
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