My Lumia 521 won't charge from car USB - when plugged in, an error comes on the audio screen saying check USB. That's ok, bluetooth works fine, but for some reason it won't charge. Android phones charge just fine on the USB.
I don't think it's a problem of not enough voltage/amperage, I'm able to charge the Lumia on an old LG charger that says it's 4.8v, 0.4amp, which is less than most. IIRC this was working fine before the Windows 8.1, possibly it is not charging when it detects incompatible usb?