[Nexus 6P Accessories] Elnrik:
1)

Does USB3 offer faster charging than USB2?

USB 3.x and USB 2.0 on legacy Type-A devices meant different things compared to when we are talking Type-C connections. On Type-A connections, USB 3.0 meant more data pins/wires and a higher base charging rate. To maintain backward compatibility, those extra pins/wires had to be kept separate which resulted in that funny Type-B USB 3.x connection.

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On Type-A USB 2.0 you get the standard 4 pins and .5A power.

In the Type-C world, using the terms USB 2.0 and USB 3.x now only signify which pairs of data pins in the Type-C cable/device are being used. Regarding our Nexus devices, the term USB 2.0 / 3.x does not change the way in which the device receives power, nor does it change the data transfer rates your phone is capable of. The port on your phone is only able to use the USB 2.0 set of data pins. See below.

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- Type A USB 2.0 vs 3.0 on any recent device will probably be the same, as almost everything should support BC2.1 for 1.5A charging. In cases where BC2.1 protocols aren't supported, 2.0 defaults to .5A and 3.0 defaults to .9A.

- It *ALWAYS* entirely depends on what the device can negotiate for with the charger. The Nexus will only use USB certified methods to do this which means 1.5A, .9A or .5A charging, regardless of what the charger is rated for.

2)

Why is USB Type-A to USB Type-C charging (using a compliant cable) limited to 1.5A instead of 2A/2.4A?

In short, your device will only use USB complaint specifications when negotiating for power - thus it is limited to 1.5A when being converted to a legacy Type-A connection. Here is what happens when your phone is connected to a host: The Nexus looks at the CC pin/wire and sees the 56kΩ resistor, and thus knows the connection is being converted to Type-A. It then uses the USB 2.0 data pins to negotiate for power via BC 2.1 (max 1.5A). If it can't negotiate using BC 2.1 it defaults to .9A or .5A charging. If you think about it, it must do this. The lowest common denominator, or if you prefer, the most compatible method for delivering power here are the USB specifications. The review he left on that specific cable seems a little misleading regarding 2.4A capabilities.

3)

Fast Charging? How does it actually work?

This term has actually become a little pet peeve of mine. Fast Charging is NOT Quick Charging. The first is a generic term, the second is Qualcomm's technology.

USB Type-C devices connected to Type-C ports will "fast charge" at 3A. But do we need a special term for that? 3.0A charging was designed into the Type-C specifications the start. It is a native capability of the spec. Nothing extra or special (like dedicated QC chips in devices, or certified chargers) are needed to get it to work.

Before Type-C came along, about the only thing that could charge above the default USB power delivery specs was Qualcomm's Quick Charge. Please read this article to understand Quick Charging. As you can see, Quick Charging uses variable voltage and amperage levels to charge a device faster. On your Nexus, the voltage is never allowed to vary beyond 5V. (Technically, I think it's 5.5V, but that's picking nits.) "Fast charging" is kind of a way of saying that it's charging rate is similar to Qualcomm's Quick Charge - but this is a stock ability of Type-C. It was designed that way. Regardless, they couldn't advertise this product without having something that matched Quick Charge. So, they coined the term "fast charging", which is just a descriptive and generic term for native Type-C capabilities.

Ok, so you asked how does Type-C charging work if it does not vary the voltage like QC does? BECAUSE SCIENCE! If you think of electricity as water, certain aspects become easier to understand. Voltage is the pressure driving the water forward. Amperage is the quantity of water being moved (known as current). The pressure multiplied by the amount = work that can be done (aka- watts). In short, volt * amp = watt. Or, watt / volt = amp. Or, watt / amp = volt

Let's do a little math exercise.

12 * 1.24 = ?

5 * 3 = ?

Qualcomm's Quick Charge is able to increase voltage to 12V, but it drops the current down when it does this. In total, it is capable of supplying about 15W. (18W I think is the top end of QC's abilities? Not sure it matters.)

Type-C delivers a constant 5V, but the current can change from 0 to 3A, supplying up to 15W.

So, I hope that helps. Let me know if you have more questions.