Discussion about the hypothetical scenario of a zombie apocalypse.
Unless stated otherwise, by zombie we're refering to The Walking Dead -kind of undead creature.
Serious discussion only.
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A smartphone can potenttialy be quite useful even without cellural/internet connection. Entire wikipedia can be downloaded for offline use and many maps too. I'm sure there are plenty of survival apps that work offline too that contain a ton of useful info about edible plants, shelters, traps, plant medicine, knots etc.
None of this is useful though if you haven't downloaded it before the apocalypse and have some kind of way of charging your device.
As far as collecting immediately useful intelligence of zombie movements, I think a foxhole radio would be more maintainable than a cellular phone.
spoiler
If you're lucky, you can find a ham radio operator with such a device. You could use the radio to coordinate with other distant groups who may have resources like the smartphones you are referencing. Yes, smartphones are technically radios, but I donʼt think the required cellular towers can be relied upon during an infrastructure collapse.
A commercial handheld radio would be more efficient, more capable and easier to use than one built with scraps. Still, you cannott download offline maps on a radio