Why I'll stick to an external charger:
This is from back in the day, multiple years ago, but there were a fair number of cases of problems related to usb ports not having enough capacity to charge devices. Under powered wall warts going boom, people fried usb ports or boards on pcs they tried to use for charging.
If the device tries to draw more power than the usb power source can provide, there's going to be a problem. If the device draws so little current to be fine on even small wall watts it would take forever to recharge three big 18650
batteries. If the device is able to
sense usb power supply capacity, cut off if it isn't high enough, usb supplies that have enough oomph to charge that 3 battery set in a reasonable period of time are likely to be few and far between.
This is entirely ignoring the charger circuit implementation aspects. Is the circuitry in the pv charging the
batteries in a way that would be called good by the folks over at candlepower forums or other battery and charger savvy groups?
I would love to get one of these pv, but I would treat usb charging as a backup option, not a primary use.