[Xperia Z3 Accessories] Entropy512:
Qualifying for QC2.0 status requires only one thing: Implement HVDCP, which the Z3 has.

Qualcomm appears to be trying to get QC2.0 to be semi-standardized, so is often referring to it in terminology consistent with the USB Battery Charging Specification ( http://www.usb.org/developers/docs/devclass_docs/ ), which calls "dumb" power supplies like most phone chargers "Dedicated Charging Ports" or DCPs. HVDCP = High Voltage Dedicated Charging Port - It's a semistandard method for letting a device tell a charger that the charger is allowed to increase voltage to 9v or 12v. Increasing voltage to 9/12v allows a given cable to deliver much more power without increasing the wire gauge of the cable or exceeding the rating of the connectors.

QC2.0 supports, for "class A" devices (I haven't seen any Class B devices yet), 9v@2A or 12v@1.5A, or 18W. Quick Charge 1.0 is really just 5v@2.1A (11W) which many chargers support as-is - most QC2.0 marketing uses 5v@1A chargers (5W) as a baseline for comparison - but nearly all Android devices for a while have supported well over 1A from a 5v source.

To protect the battery, Sony limits the current limit into the battery to 1.5A - The maximum battery voltage is 4.35v, so the maximum battery charging power is 1.5*4.35= about 6.5W into the battery.

Typical charging circuits inside the phone are only 85-90% efficient, so that means you need around 7.6W or so into the charging circuit.

Note that the 11W from a 5v power supply above assumes no cabling losses - in reality, you'll lose some power in the cables.

So you've got less than 11W available from the wall, and you want 7.6W into the battery. Unless system power consumption is below 3.4W (in the absolute best case), you'll never reach maximum battery current. In reality, you're likely losing enough power through the cable to give you a margin of only 1-2W. (Most devices I've seen reduce power consumption aggressively if voltage drops below 5v or so from the bus - see http://xdaforums.com/showthread.php?t=2065404 for example, you lose about 200 mA per 0.1v drop with that device)

Now with QC2.0, you've got 9v@2A = 18W from the charger. Also, for a given input power, current through the cable is lower. Cable losses are I^2*R So for a given power consumption from the wall, nearly double the voltage results in nearly half the current, and one quarter of the power lost in the cable. Plus the phone is less prone to go into input current limiting with 9v input. This provides enough power budget to feed 1.5A into the battery, keep screen brightness at full, and actually do stuff with the phone.

Most obvious example is Google Maps Navigation in daylight. With most non-QC2.0 chargers, the battery drains. With QC2.0, it charges at a reasonable rate.

(Not sure where this all fits in with the USB Power Delivery specification, which also involves running voltages higher than 5v on the V+ line - http://www.usb.org/developers/powerdelivery/ ) Edit: USB PD seems to require special connectors/cables and only involve "smart" devices. QC2.0 appears to attempt to implement USB PD Profile 2 (18W) using standard connectors and supporting "dumb" chargers with the HVDCP negotiation protocol. A lot of good detail on the charger side of HVDCP can be found at http://www.mouser.com/ds/2/328/chiphy_family_datasheet-269468.pdf