[Nexus 6P Accessories] aaron_huber:
I don't want to complicate the conversation, but the issue is actually the reverse. No charger can "push" more amperage to a device than it can handle or blow it up. Now, sending too much voltage can certainly result in that but we're mainly talking amps here.

As an example, think of a light bulb and a vacuum cleaner you plug into your wall at home. Both devices work at the standard voltage so that's not an issue (I'm in the US so I'll say 120V). That circuit can handle way more amps than the light bulb can handle without blowing up, so why doesn't it? Because the device draws the amps it needs. A 100W light bulb will draw 100W of power (~120V x .83A) and the vacuum cleaner will draw more like 1200W (120V x 10A). Now say you plug in two vacuums on the same plug? What about three? You'll likely trip the circuit breaker (depending on how many amps the circuit can handle, but usually 10 or 15 for a standard US home). Why? Because if you draw more amps than the circuit can handle the wiring will heat up too much - high amps causes more heat which requires thicker wires or things get too hot. The circuit break prevents you from drawing more current than the circuit can handle and burning your house down.

Now let's translate that to USB. The standard is 5V, so any USB device expects when it gets plugged in it's going to get 5V so blowing up shouldn't be an issue. But how does the device know how many amps it can draw? If you plug your phone into a computer, the computer likely can only supply 2.5W on that port. That means that the port itself, the motherboard, the power supply, etc. all are rated to deliver 0.5A safely on whatever size wiring they use. What if your phone tries to pull more? Best case you'll crash the computer, worst case you'll release the magic smoke from your motherboard. It's the port that will blow up, not your phone.

So how does fast charging work up until now? When you plug in your phone it has to know it's safe to pull more than 0.5A of power at 5V which requires either some assumptions (not safe) or some communication. The earliest high-amp USB chargers shorted out the 2 data pins. Then when the device connected to the charger it knew it wasn't connected to a device like a PC that might blow up if it pulled to much amperage. For example, when I got my Nexus 5 I bought a high amp car charger to get fast charging and was really pissed when I plugged it in and my phone said it was doing "USB" charging instead of "AC". Some quick reading and a purchase of a "charge only" USB cable with the data lines shorted and I've been happily fast charging ever since.

So what else can devices do? Qualcomm Quick Charge actually sends out a specific pattern of signals on the USB wires (not sure on the details) that both the device and the charger use to communicate and negotiate what voltage and current to use. Things start out at 5V and low amperage until both devices agree on what to switch to. That is why it requires a chip on both ends.

Now all of that is really sucky for consumers. You've got a single port overloaded will all kinds of different voltages, amperages, and communications standards. Fast forward to USB-C and the Power Delivery spec and we've got some hope on the horizon. Now there is an industry standard spec that everyone should follow (I'm looking at you Apple, stupid Lightning cables), it's free (Qualcomm owns Quick Charge and gets fees for licensing it to others), and should be a bit future proof (I don't see myself buying a phone that needs more than 100W to charge any time soon).

The problem is the transition from the old to the new. What happens when I plug a 6P into a high-amp USB 2.0 plug with the data pins shorted? Google could do the same everyone has until now and pull more amps. Did they? We'll know that when someone plugs the thing in and tries it. Google could have supported QC 2.0, they chose not to. My guess is they just decided to follow the new spec and start fresh and leave the baggage behind, but we'll find out.

TL;DR - I'm not going to buy any cables or chargers for my 6P (other than the ones Google is selling) until someone actually has one and tests it out and confirms that the phone says "charging rapidly" if I expect to get rapid charging. I'm pretty sure until then I'll get charge at some speed on my existing 5V/2.1A chargers. How fast will it be? We won't know until we plug it in and see how Google chose to design the device.