Some intersting info
After reading everything around the web re fast charging and seeing that Sony want €30 for their fast charger I decided to do some tests.
To test I cut a usb "fast charging" cable and removed the usb plug - this cable has a much thicker power cables (red and black) so should be able to take any current I throw at it.
Next I got out my digital bench power supply and set it to exactly 5.0v - this power supply has a digital amp meter built in that is very accurate - note this power supply can output up to 5 amps - easily enough for "fast" charging.
Next got my Xperia S and attached it to the bench power supply.
Phone on, usb pins 2/3 not connected to anything - Current: 280 to 500mah - appears to fluctuate depending on the processor usage.
Phone on, usb pins 2/3 shorted, Current - 700ma
Phone off, usb pins 2/3 open - Current: 500ma
phone off, usb pins 2/3 shorted - Current: 1.45 amps
Switched phone on while connected, pins 2/3 shorted - power used - 1.45 amps
unplugged and plugged back in the cable to the phone - power used - 700mah
Therefore, it sees to get fast charging to work you have to have the phone powered off initially (though you can power it on) and a usb power supply that supplies at least 1.5 amps - though probably needs 2 to be safe. The phone appears to be "intelligent" i.e. checks to see if pins 2/3 are shorted and whether the phone is on or off.
Now assuming the battery is 1700mah this means that 80% of it is 1360mah in a perfect world it would take 55 minutes to charge it 80% using a 1500ma current.
Will do more tests tomorrow (30 min test) and report results.
Okay, done some more tests:
Appears to work via a curve:
charging starts off at 1.4 amps, this increases to 1.51 amps in the first 10-15 minutes.
After 30 mins - 45% charged average 1.5 amps
After 45 mins - 63% charge average for 15 mins 1.4 amps
After 60 mins - 80% charge average for 15 mins - 1.3 amps
After 75 mins - 91% charge average for 15 mins - 1.3 amps
After 90 mins - 97% charge for last 15 mins - 0.55 amps
At 90% it dropped back to 0.55 amps.