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I think you have to evaluate your needs before deciding what type works best for you. Maybe they need more than a whole day away from an outlet.
I use a Mophie battery-case for my phone when I travel. Very convenient if you just need more phone time. At work I prefer a spare charger/cable at my desk than even the bulk of a battery-case.
I have a couple of big & heavy (13 & 22 lbs) battery-plus-inverters at home, from both Goal Zero and Duracell, for camping and emergencies.
I haven't had the need for the middle-range of portable power sources (1 to 4 lbs). I find them too bulky for my needs. Your needs may be different.
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