I have a generic car USB charging adapter, and it charges my phone just fine, albeit if the GPS is on, the charger can't keep up with the power draw. This remains the case even if I have the screen off.

The question is whether this an issue that there's simply not enough power available over the USB charging port, or if it's something more complicated, like the phone might be asking for more power but the USB adapter isn't smart enough to provide it.