I have three of the New Trent external batteries. I choose the size based on the length of the trip. I also own a Goal Zero solar charger and Nomad 10 battery pack. The New Trent battery is by far the way to go in my opinion unless you plan on being in the backcountry longer than 10 days.
Goal Zero has worked well for me. BTW- besides all the multiple uses described here already, I actually use my phone for a phone when I have service. Getting away doesn't necessarily mean I don't want to talk to Little Man or my wife.
I use a Rayovac portable back-up charger it works with multiple devices and only weight 2.125 oz. It is like plugging your phone into the wall you can use your phone instantly and do not have to wait for the device to charge.
My solio charger is pretty lightweight, but doesn't have a lot of capacity, either. Worked well for me last season though.
I've been considering trying a New Trent out, but I still haven't figured out if I can charge the batteries on my Garmin using the usb cable. Does anyone know if this is possible? If I can use the new Trent to power my GPS and relieve my use of AA batteries, I could justify the weight... otherwise I'll likely stick with the solio and extra batteries