USB charging is exactly that, the GX850 provides it in the body itself, but you can always buy a separate battery charger that's simply powered via USB rather than AC, like this or this other one. I bought the OAproda couple years ago but solo (I don't particularly trust 3rd party batteries outside of maybe Watsons, but that's very much a YMMV thing).

Those are for the BLN-1 of the E-M5 II btw, I don't recall if the E-M10's battery charges off the same charger or not (I know it's a different one).