USB port amperage?

Does anyone know the output of the factory USB port? My phone (Nexus 4) will charge very slowly when plugged into the factory USB port. If I am listening to music from my phone through the head unit, it still discharges even when plugged in. If it is limited to 500mA, that would explain it. Has anyone tried using a charge only cable with the data pins shorted to charge at a higher rate?

 

This post seems to concur with the 500 mA cap: http://www.tacomaworld.com/forum/2nd-gen-tacomas/276991-usb-power-consumption-error.html#post6862728

In fact some users see this message:

 

Dude, i had the same problem... The Aux USB sucks for charging. If I ran GPS or webex apps, my battery would still drain while charging. I found a sweet low profile charger called Volt Ranger (Stupid name, i know). It offer two usb ports, one which is 2.1 output amps.

 

Well that's a bummer. Guess I will have to get a 12V to USB adapter. It sucks because the 12V outlets are right over the cupholder. I might try mounting some USB charging ports in one of the blanks next to the factory USB port. Thanks for the responses.