^ That is my thought completely. My tiny USB charger (DIY) assumes the source voltage is 6-30VDC. There is no way I would use my AAs (say) to charge electronic gizmos with built-in batteries, not when all my more important (depending...) devices use the AAs (and one uses AAAs). I carry any one of my myriad of RC battery packs for the charge source if I don't expect to be near a car etc. for a while.

My experience with USB chargers with built-in LiPo batteries is they're not very reliable. And expensive if they have a substantial battery, luckily not my $$. Typically one cell goes bad, good luck replacing a cell the way they package them (won't work with a single bad cell), I have loads of extracted good cells already, but not much use for anything *safe*. They tend to use really cheap LiPos, and anybody who uses LiPos for other stuff knows this could be a very bad scene, quality of cell is critically important. Not the same thing as lithium-ion.