Alot of 12V powered USB chargers use a linear regulator. They turn the excess voltage into heat. Pull 2A through one and it will be producing 18W of heat at 14V input. Get one that uses a switchmode supply to stepdown the voltage instead.

Get 1 of these instead, I have 3 and they work really well

http://www.jaycar.co.nz/productView.asp?ID=MP3666&w=usb+car+charger&form=KEYWORD