Increasing my phone's charging rate while tethered 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?
Charging rate is set by hardware and software, and can't be modified by software only. 

Charging rate depends on the following: 


resistance between D+ and D- (determines is it a official, unofficial or USB charging)
hardware technology and limitations (QuickCharge 2.0, 3.0)
cable length and quality (bad cables usually have more resistance)


For the S3, as far as I know, you can charge slow (USB, 5V 500mA) or fast (wall charger, 5V 900mA). 

The charging type is set depending on the resistance, which is different for wall chargers (fixed to activate fast charging functionality in phones chip) and standard USB charging. If you want to charge quickly while tethering, the only valid option is to have your phone connected to an official Samsung wall charger which will ensure a higher charging current. There may be an option to set the USB connection to "Charge only" but that doesn't guarantee that the fast charging will be turned on.