Just got a new MacBook with USB 3 ports. I thought it would be able to charge my Nexus 4 phone much faster using USB 3 900 mA current.

However, I haven't seen any improvements compared to charging on a USB 2 port. Obviously, AC chargers can charge way over 500 mA using USB 2 cables.

Which brings me to question: can a (micro) USB 2 cable hooked up to a computer USB 3 port charge a device at 900 mA current, or is it limited by the USB 2 cable to 500 mA?

  • @Ƭᴇcʜιᴇ007 Right, the question is really similar but I don't feel like it has been answered correctly. I've read all the comments but there isn't a clear answer. I feel like there should be some Internet source/reference/spec that precisely answers my question. I would rather not have to use a current multimeter to find it out.
    – Jules
    Dec 30, 2014 at 17:57
  • Just because the current answer(s) don't satisfy you doesn't make it a different question. If you'd like newer/different answer to the existing question, please consider placing a bounty on it. Dec 30, 2014 at 18:03
  • @Ƭᴇcʜιᴇ007 Fair enough, but I do think my question is asking for something more precise than the other one i.e. specifically asking for the maximum current possible.
    – Jules
    Dec 30, 2014 at 18:20
  • a Mac can output over 2 amps, much more than USB spec (even over USB 2), but the device has to know how to ask for it. [I very much doubt Apple will put much effort into making sure a droid matches the spec]
    – Tetsujin
    Dec 30, 2014 at 19:22
  • @Tetsujin Okay, that's weird though because even Apple says charging iPads through a computer is slower and recommend using the wall charger which provides 2.1+ A.
    – Jules
    Dec 30, 2014 at 20:51

1 Answer 1


Per Apple: http://support.apple.com/en-us/HT201163#12

USB 2 cable is usually limited to 500 mA. It's about how many wires are in the cable, and the Mac limiting power accordingly (power management). A wall charger generally lacks logic, so it's full power all the time.

I checked with a variable load, and it seems to hold true (480 mA typically)

  • I came across that page too and can't find a more reputable source. A few other sites seems to confirm this as well. So from what I understand (correct me if I'm wrong), the 900 mA current is only available by using both a USB 3 port AND cable. These power restrictions apply to reduce interference when data transfer is required. AC/wall chargers don't deal with data and thus can deliver much more current, regardless of the USB cable version. See en.wikipedia.org/wiki/USB#Charging_ports for more info.
    – Jules
    Dec 31, 2014 at 0:58

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