I almost always have a power bank handy. I've learned to carry two, due to one of the folks at work seemingly always needing to borrow one when we're together.
An Xtar PB2... powerbank with 2 replaceable 18650s. Can swap batteries about with other 18650 gear (flashlight). Also can charge 18650s with a USB power source. If need more capacity, just carry more 18650s.
Newer version has been announced, the PB2S. Will be able use 21700 batteries aswell as 18650s, and has a USB-C input as opposed to micro USB of the PB2.
I used to back when I had my S5 and you could buy replacement batteries.
MUCH better than an external battery bank -- a few seconds and you're back up to 100%, no having to plug in and wait. Charge up the drained battery later that night.
Now manufacturers prefer built in batteries for simplicity of design and planned obsolescence, there's no downside.
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