The smallest 2 ampere USB charger I've come accross so far is from Samsung and my Galaxy S4 makes almost full use of its capabilities by drawing 1.6 amperes when the battery is almost empty. In case you are wondering how I know, have a look at the measurement tool I used for measuring the power consumption of a Raspberry Pi. What I was quite surprised about, however, was that all other devices I tried it with, including a new iPhone 6, only charge at 1 ampere at most. I wondered why that is so I dug a bit deeper. Here's a summary of what I've found:
One reason for not drawing more than 1A out of the charger is that some devices simply aren't capable to charge at higher rates, no matter which charger is used. The other reason is that USB charging is only standardized up to 900 mA and everything above is proprietary. Here's how it works:
- When a device is first connected to USB it may only draw 100 mA until it knows what kind of power source is behind the cable.
- If it's a PC or a hub, the device can request to get more power and, if granted, may draw up to 450 mA out of us USB2 connector. And that's as much as my S4 will draw out of the USB connector of my PC.
- USB3 connectors can supply up to 900 mA with the same mechanism.
- Beyond the 450 mA USB2 / 900 mA USB3, the USB Charging Specification v1.1 that was published in 2007 defines two types of charging ports. The first is called Charging Downstream Port (CDP). When a device recognizes such a USB2 port it can draw up to 900 mA of power while still transferring data.
- The second type of USB charging port defined by v1.1 of the spec is the Dedicated Charging Port (DCP). No data transfers are possible on such a port but it can deliver a current between 500 mA and 1.5A. On such a port the D+ and D- data lines are shortened over a 200 Ohm resistor so the device can find out that it's not connected to a USB data port. Further, a device recognizes how much current it can draw out of such a port by monitoring the voltage drop when current consumption is increased.
- With v1.2 of the charging specification, published in September 2010, a Dedicated Charging Port may supply up to 5A of current.
And that's as far as the standardized solutions go. In addition there are also some Apple and Samsung proprietary solutions to indicate the maximum current their chargers can supply:
- Apple 1A (by supplying a certain voltage on the D+/D- lines)
- Apple 2.1 Ampere
- Apple 2.4 Ampere
- Samsung 2.4 Ampere
There we go, quite a complicated state of affairs. No wonder, only one device I have makes use of the potential of my 2A travel charger. For more information, have a look at the USB article on Wikipedia that also contains links to the specifications and the external blog posts here, here and here.