It seems most cars/PCs are limited to the USB 2 ports to supply 5 volts at 500ma.
From another thread of mine:
While my son was driving our Volt, I was using my phone (
RAZR-MAXX-HD) when it was pretty low (7%). I plugged it into the USB connector in the arm rest. I noticed that while it indicated it was charging it could not quite keep up with the phones charging needs. In fact the charge dropped and the phone ended up powering down. Mind you I was doing several things on the phone.
Out of curiosity I wondered if that USB had limited power. I bought a 'USB port voltage current tester' and did some test. I compared it to charging in my house and to using a Dynex 12v to USB charger in all of the 12v ports in the car. Here is the results.
https://docs.google.com/spreadsheet/ccc?key=0AplA7Kfp45ULdGlpOFFSdEZzdko2WTlnNGR6cjgwMWc