I have a Samsung Galaxy S3 and a laptop with 2 USB 3 ports. When tethered to my laptop (to provide Internet access via GSM/HSPA data), the phone charges very slowly, especially if I am using it for anything else in addition to data tethering. While connected to a USB 3 port using the OEM cable, it took about 5 hours to charge while tethered but not otherwise being used. Given the 2100 mAh battery, this works out to a bit under 500mA¹. It has a micro-USB 2 port as its sole means of power input.
The S3's USB-to-AC Adaptor indicates that it provides 5.0 V at 1.0 A.
The Battery HD app, when running on my phone, tells me that an S3 will charge just under twice as fast via AC power as via USB.
I haven't found any apps which indicate, or any other way to determine, the phone's current draw from the power supply.
If I'm reading wikipedia correctly:
Maximum current draw for a normal USB 2 port is 500mA.
Maximum current draw for a normal USB 3 port is 900mA.
Dedicated Charging Ports (which I gather is what the Samsung USB-to-AC adaptor has) can provide over 500mA but do not support data transfer. I didn't see any indication in the article of a maximum current.
Charging Downstream Ports can also provide over 500mA along with high-speed data transfer. It sounds like the maximum current was originally 900mA in the 2007 spec but was upped to 1500mA when version 1.2 was published in 2010.
¹: Wikipedia gives battery specs for the S3 as "2,100 mAh, 7.98 Wh, 3.8 V". The rate of charge of the battery over 5 hours works out to 420mA at 3.8V. At USB voltage of 5V, this is only 320mA. I'm not sure what the efficiency ratio for charging a Li-Ion battery is—80% perhaps?
How can I increase my phone's charging rate when connected to my computer? Is it possible for a USB 2 device to draw 900mA from a USB 3 port? Is it possible (and safe) to connect my phone to two ports at once to increase the total current?