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European iPhones are more fun now
(www.theverge.com)
This is a most excellent place for technology news and articles.
How many often are you planning on replacing the battery in your phone that it would wear out the panel?
With my N900 I used to travel with 6 to 10 charged batteries to have a few days of runtime. Things got better now with powerbanks - but for something like hiking just carrying a few spares would still be smaller and lighter.
Honestly for hiking I’d suggest a power bank with solar charge capability. One thing to charge them all.
The space used by the smallest solar charger I've seen on Amazon seems to be similar to 6 or more batteries in the format the N900 was taking - so if you look at space, slow charging from solar charger, and reliance on sun conditions taking individual batteries seems to be the better option for a few days hike. It's also easier to stow individual batteries to wherever you still have space left.