I went through a similar exercise recently and I learnt a few things…

 

Firstly, this free software is INVALUABLE… it tells you the charging rate, and will save you hours of time! https://play.google.com/store/apps/details?id=com.abmantis.galaxychargingcurrent.free

Secondly, it is IMPORTANT to short the USB data lines if you want full charge current. (I didn’t notice any pull up or pull down resistor(s), but if you Google info you will find that different pull up and pull down resister combinations tell the phone if it is placed in a genuine car dock etc). The phone seems to limit the charge rate if the data lines are not connected!

 

Background… I modified a USB 5V cigarette adapter to charge my S3 from the car (I wanted to leave the actual USB out jack on the cigarette adapter available for other devices, so hard wired the 5V only to the phone plug.

Problem was I never achieved more than 460mA charge current as indicated using the software mentioned above. As I always have my phone screen ON whilst driving (I use several programmes to monitor cellular signal strengths / cell tower info as well as using WAZE whilst driving - I'm an Ex cellular radio technician - and yes, my wife thinks I'm crazy, but what the heck - old habits die hard!), I found that whilst the phone indicated it was charging, the battery level actually dropped, very slowly - in other words, the phone was using a little more current than I was able to charge it at. (I guess you could consider that the phone was charging DOWN rather than charging UP!!) Anyway, I tried much heaver cable from the 5V adapter to the phone, assuming the problem was volt drop, but never managed to get a charge rate over 460mA!

 

I later noticed that when I plugged in a USB cable from the adapter's actual output jack to the phone I got a charge current of 1AMP. This really had me puzzled as this cable was much thinner than the one I hard wired from inside the adapter, so I investigated, and discovered the USB data lines are shorted together insider the adapter.

 

So that seemed to be the answer – NOT just the cable wire size as I had previously thought.

 

So, now I have all 4 USB wires run back rm the phone to the cigarette lighter adapter, I get the full 1AMP charge current, and the phone charges up whilst I drive about with the screen on.

 

CONCLUSION: You need to take into account the data lines even if you are just charging, as well as the potential volt drop caused by cable size.