my solution has always been to buy phones with field-replaceable batteries and carry fully charged spares.
there is also a thing called e-charge ( I have one ) a charger that comes with a set of 4 ni-mh batteries and a lead and plugs for almost every phone under the sun.. you can charge the batteries that are in it and use them ( with another small part of the device and the correct plug ) to charge your phone via its usual charging socket ( micro USB )..or you can use 4 duracells or similar ( energizers) and put them in the same part of the device..and charge your phone from that..it also has a solar charger to charge its ni-mh batteries from ..or you can plug it into a wall...
Uses four LR6 batteries .."straight" or rechargable..whole thing with all the parts is about as big as two galaxy notes back to back..and without the batteries weighs very little ..the plugs and leads store in the cap..cost me about $50..oo
As my cameras use LR6 batteries, 4 at a time ..I just carry it and few cards of 4 LR6 Duracells ..Canon Superzoom bridge and phone..
Those with idevices ..may wish to ponder the ( non ) wisdom of their choice of shiny phone..