I'm considering charging a bunch of my iPod Touches with a USB 3.0 PCI add-in card.

In my research, I've found that USB provides 5 volts per device.

The differences come in charging speed. The more amperage, the faster a device charges.

USB provides "Unit Loads" of amperage for each device based on need.

USB 2.0 provides 100 milliamps per Unit Load, with a maximum of 5 unit loads per device. USB 3.0 provides 150 milliamps per Unit Load, with a maximum of 6 unit loads per device.

When a device is connected, it draws 1 Unit load until it asks for a higher amount based on need. That means a device can draw maximum 500 milliamps (or 0.5 amps) from USB 2.0 and 900 milliamps (0.9 amps) from usb 3.0.

The Apple wall charger for iPhones provides 1 amp (1000 milliamps).

Does that mean that an iPhone or iPod Touch plugged in to a USB 3.0 port will charge nearly as fast as one plugged in to Apple's AC plug?

3 Answers 3


I have tested an iPhone 5, and on my system it charges from a USB 3.0 port at the same rate as a 500 mA USB 2.0 port. As you would expect, the included 1 A wall charger is about twice as fast.

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  • 1
    Bam! coneslayer joins Ask Different with a vengeance!
    – Kalamane
    Sep 26, 2012 at 0:00
  • 1
    Thanks, @Kalamane. I now have a followup that might be of interest to Windows PC users who are looking for a faster way to charge over USB: ASUS Ai Charger
    – coneslayer
    Sep 27, 2012 at 0:14
  • If USB 3 and USB 2 charges at the same rate on your setup, that implies to me that the drivers for fast USB charging aren't installed or aren't working. I can charge twice as fast on USB 3 as I can on USB 2 so there's something wrong with your test set up if your USB 3 is acting like USB 2.
    – NickG
    Sep 24, 2013 at 12:52

Actually the power delivered by USB does not really depends of the generation. Some USB 2.0 will deliver more power than others. It depends of the hardware and how the USB is powered by the motherboard.


Apple has created something of a superset of the USB charging spec for devices with a dock connector, like the iPhone. It's compatible with USB 2, so you can plug it into any USB port or charger and draw the standard 0.5A. However with a bit of voodoo (I believe it has to do with resistors on certain pins and so forth), it can draw a 1A charge.

However this doesn't mean an iPhone can draw the full power from a USB 3 port. Currently iPods and iOS devices aren't compatible with the USB 3 spec, only USB 2. So, like any other USB 2 device, they're limited to drawing 0.5A, unless the charging device speaks Apple's custom protocol. This could of course change in the future, but at the moment, that's how it is. You might be interested in these pinout details of the dock connector, which offers some insight into how the charging works.


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