The problem is most generic car chargers are designed to work with Apple products. Apple uses a non-standard way of telling their devices essentially "hey, I'm a charger." This makes "standard" devices not recognize the charger properly. The device tends to think it's plugged into a computer and only draw around 500mAh.
You can convert a USB cable into a charging cable by shorting the data pins (on the device side), or you can buy charging only cables. I've been using these cables on my S2, and now my S5: http://amzn.com/B007F1DV76 . I used a Galaxy Charging app to verify charging rate and it indicated 1.2A on both the Samsung wall-wart and cable as well as my Scosche 2.1A car charger w/ charging only cable.
I'm not amused at all! End users shouldn't have to deal with this nonsense. This should work out of the box!
I have two alternative:
1. Buy this charging cable.
2. Change kernel to trick the phone to draw more power from the USB port.