I have the GZ Yeti 400, an excellent power source for small stuff. It does have a 12Volt output port, but I plugged that with a USB adaptor so the unit now has four USB power ports. I also have two Anker Powercore power banks (my aforementioned 20KmAh & I recently purchased a 10KmAH unit for increased portability). The two Anker power banks are very portable and can keep my flip-phone and GPS receiver running for a long time.

As for Li-ion battery life — I’ve had a Nitecore UM10 charger for a while now. It is small, only does one Li-ion battery at a time and it charges exclusively from a USB port (or AC adaptor). The UM10 also has an USB output port under it’s cover (the bottom hinges open to access the port). The nice thing about the UM10 if you have a number of 18650 batteries, is that rather than a couple bars to allow you to sorta kinda guess the charge level, it has a percentage read-out so that when you get to 60% (or in my case 65%) I can stop charging and put the battery in storage; or I can run it up to 90% and drop it in one of my flashlights. As I understand, that last 10% charge causes heat to build up and that reduces battery life.

FWIW, $.02