My solution is to have a battery box with a USB connection that I can plug my cellphone into - this gives me a way of charging my phone when away from power.
I can go camping for a week and not run out of power on my phone.
The battery box I have takes 4 x 18650 Li-Ion cells which I can replace as needed, currently 2.5Ah cells but I might buy some new 3.4Ah Panasonic cells since my S4 has a much higher capacity battery than my old phone had.
For anyone wanting these battery cases for their phone - are you unable to run a whole day on the phone's standard battery? I've just been charging each night and haven't had a problem.