Great question — answering as the UphillTech team:
The most simple solution is keeping them close to your body somewhere within your clothing. This shouldn't be a problem with small items like phone batteries or small power banks. However, to make sure that they don't get damaged as your clothing gets damp from sweating, I would advise to pack them into some lightly waterproof container. An old plastic bag or a freezer bag will do. Camera batteries (depends on the camera type) can be a different issue since they can be quite bulky, so carrying them in the pockets of your jacket might be a bit umcomfortable. Unfortunately they don't produce heat by themselves (if they do, get rid of them as soon as possible ;-) ), so just putting them...
Full answer by Benedikt Bauer: https://outdoors.stackexchange.com/a/9951 (CC BY-SA 4.0).
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