I wasn't quite sure where to post this question as it relates to the car and tetchy stuff too...  
I've a Galaxy S3 and like many others I find that even when charging it discharges the battery when being used as a sat nav.  My old set-up in the car was a fairly typical one - 12V to 5V (1mA) converter and then a USB cable trailing across the dash to my phone.  To try and 1) make it neater and 2) increase the current available I bough an inverter (http://www.ebay.co.uk/itm/DC-DC-Converter-Module-12V-To-5V-3A-15W-USB-Output-Power-Adapter-/130980965053?ssPageName=ADME:L:OU:GB:3160) and mount (http://www.ebay.co.uk/itm/In-Car-Marine-Dashboard-Flush-Mount-USB-2-0-Extension-Lead-Cable-Mounting-Panel-/360499624332?ssPageName=ADME:L:OC:GB:3160) which I fitted into the central storage box using FUBAR's excellent guide for installing a dash cam (http://www.briskoda.net/forums/topic/265343-dash-cam-12v-multi-adaptor-hidden-install-driver-storage-compartment/).
Annoyingly the phone now thinks it's connected to a PC and only pulls 500mA.  I can get around this by upping the maximum current limit in the kernel (I've a custom ROM) but I'll surely forget to set it back and fry my PC, so, my questions are how does the phone determine if it's USB or mains connected, and 2) how can I correct it for the in car set-up?

Cheers . . .

It's your charger/cable combo dictating the rate.

What rating is it? Is it the cable that came with the charger or another?

For a phone like the s3,if your screen is on full, you'd really need 1.5 or better 2mA supply

The charger is rated at 3A, so plenty of head room there...  and the cable was a retractable reel type, but the Samsung OE one gave the same result. 

Confusingly the phone pulls 1A with both cables when used with the previous inverter.  A quick Google seems to reference bridging the middle USB pins to fool the phone.  I'm note sure how, or if I want to do this.

Just a wee update.  Having further Googled and also searched a bit more on this site (http://www.briskoda.net/forums/topic/281704-usb-charger-car-home/?p=3338763) I look the plunge and shorted out the 2 central pins (using high tech tin foil) and hey presto the phone now charges at 1A.