There's been a lot of variety in "USB" level power sources in the last couple of years. Although they all seem to be lithium-something packs, I'd have to expect some to hold up better than others, since there's so much variety in battery
quality. Ignoring the ones that cheat on ratings, and the variation in capacity when you fast charge or slow charge.
And then there are some 'crank' options out there, and the solar
ones. Lots of choices! Or...just a car charger plug
. Those dc-dc converters are pretty efficient, and if you've got ship's power, they'll recharge all sorts of things very nicely too. For all of $5-10.