Any advice on 'on the move' phone chargers?

I’m looking at getting some sort of charger for my iPhone while on the move. I want a back up plan if I am out with the group I volunteer with and I need to phone parents or look up the internet.

I’ve had a quick look at wind up chargers but can’t find out much about their effectiveness.

What about solar chargers?

Or the rechargable battery packs?

Any information or your experience would be helpful. I don’t have a lot of money so nothing expensive would be possible.