Science project: measuring USB current

My friend Soren recently gave me a Super Power Bank, a nifty portable Lithium battery pack with two USB ports for charging devices on the go. The Super Power Bank has a 6600 mAh battery with two USB ports for charging devices; one providing 1.0 A, the other 2.1 A. He asked me a question: do my devices charge faster on the 2.1 A port? Down the rabbit hole I flew.

Here is a disorienting upside-down shot of the Super Power Bank so you can read the technical details:

Super Power Bank

I knew I could answer Soren’s question by simply measuring the recharge time between known points (say, 10% to full) on first one port, then the other. But where’s the fun in that? I wanted to understand why and how some USB devices exceed the 500 mA limitation of USB 2.0 ports.

So, here is the USB test harness I built:


This is a 50x35x20 mm project box from Maplin. Through it runs a common 1m USB type A male to type A female cable, typically called a “USB extension lead”. In the project box I’ve installed six 2mm test sockets in various colours. I cut and soldered the wires to the test sockets, and maintained the earth connection between the outer plug shields. These are wired as so:

USB pin Wire colour Socket colour Name Description Notes
1 Red Red VCC Power +5 V Two sockets in series
2 White Yellow D- Data – One socket in parallel
3 Green Blue D+ Data + One socket in parallel
4 Black Black GND Power Ground Two sockets in series

I’m interested in measuring the current and voltage of the power pins, so there are two sockets in series for each line. To measure current you must insert your multimeter in series. However, these series sockets interrupt the power flow, so to actually use the cable I must bridge the gap with a test lead or multimeter. I’m only interested in voltage of the data pins, so there is one socket for each line, in parallel. I’m also interested in shorting the data pins, as some USB devices apparently take that as a sign they can draw more than 500 mA (PC motherboard data pins would never be shorted). To do this, I can use the blue test lead to bridge between the yellow and blue sockets.

Here is the USB test harness in use, not charging a bluetooth keyboard (it has a full battery):


Testing with my multimeter, I learned that the Super Power Bank does not short the data pins. Instead, it outputs different DC voltages. Both ports output 0.64 V on the data pins when idle. Under load, the 1.0 A port outputs 1.92 V, and the 2.1 A port outputs 2.35 V. There doesn’t appear to be a standard for this, although Wikipedia has some ideas. The Super Power Bank doesn’t actually begin charging a device until you press its charge button. The power pins, as expected, show 0 V when not charging, and +5 VDC after you press the button.

When you connect a USB device to charge and press the button, the device seems to negotiate for a moment before drawing current. My HTC Desire HD draws about 0.2 A at first, then if it needs charging this jumps to 0.45 A. This is regardless of which port (1.0 or 2.1 A) I use. If the data pins are shorted, however, it instead draws 0.54-0.58 A.

With the help of Travis, I tested various Android devices charging behaviours. All devices had at least partially discharged batteries. All results show Amperes at 5 VDC, as measured by my multimeter’s inline 10A circuit. Tests were performed on each port of the Super Power Bank, with data pins either normal or shorted.

Update 2013-02-11: Gathered Nook Color and iPhone 5 data.

Device Manufacturer 1.0 A, normal data 1.0 A, data shorted 2.1 A, normal data 2.1 A, data shorted
Desire HD HTC 0.45 0.58 0.45 0.54
Galaxy Nexus Google/Samsung 0.44 0.58 0.44 0.58
Nexus S Google/Samsung 0.45 0.50 0.45 0.55
Nexus 7 Google/Asus 0.43 0.43 0.43 0.43
Nexus 4 Google/LG 0.50 0.50 0.55 0.55
Nook Color Barnes & Noble 0.46 0.46 0.46 0.46
iPhone 5 Apple 0.52 0.52 0.57 0.57


  • Some devices (Nexus 7, Nook Color) don’t exceed 500 mA even if the port indicates it can provide more.
  • The Desire HD, Galaxy Nexus, and Nexus S all accept shorted data pins as permission to draw more than 500 mA.
  • The Nexus 4 and iPhone 5 accept data pin voltage of at least 2.35 V as permission to draw more than 500 mA.

I gave the USB test harness to Soren today, at the moment this post is scheduled to appear on my blog. Surprise, Soren! Thanks for the cool toy and for the inspiration to do science!

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  1. Ryan’s avatar

    So it looks like while they’re technically drawing “more” power from either port when the data pins are bridged, it’s still nowhere near 2A, let alone 1A, which the phones/tablets should be able to take.

    I’d really love to find one of these 2A / 2.1A power banks that has the data pins shorted already, according to the spec, so that it’ll quickly charge standard USB devices (Basically Android), and not just Apple products.

    It’s really cool that Apple is smart about using the data pins to figure out how much amperage they can take, but unfortunately it has made manufacturers of these batteries lazy, assuming the device will do everything. It’s also not to the USB spec.


    1. Tyler Wagner’s avatar

      It’s very easy to short the data pins. Just take a USB cable and put a tiny bit of tin foil over the middle two pins, with a spot of superglue.

      I share your frustration over the totally disorganised state of USB charging.


      1. Ryan’s avatar

        Some of these power banks are manufactured to be very small, and I don’t know if I want to be taking them apart to expose the USB connector pins.

        I’ve actually found that the PS Vita USB charging adapter does just this, and is much less permanent. eBay link
        It is a little big, though.

        Be careful with this, though – if the charger isn’t meant to provide 1A (Or more), and you’re telling the phone that it can get it through the charger, you may be overloading the charger.

        Now one of the ones that advertises 2A, and doesn’t have them shorted, that’s probably fine, but you may be damaging electronics if they’re only meant to send 500mA, and forcing them to send twice that.


      2. Matt Heck’s avatar

        USB power negotiation is interesting. On an active bus, your device powers up only authorized to draw something like 35mA; you have to negotiate for more. The hub can say “no” if it’s going to exceed its total rated output across all ports.

        However, recent versions of the spec– as well as certain “creative” vendors– provide extensions to allow charging when the machine is in standby. This is interesting, in as much as the operating system normally has a say in whether or not you’re going to be allowed to negotiate for higher current or not. The chipset can solve this issue without involving the OS, but it means the notification process is going to be different during a resume from standby (notification after the fact) as opposed to running (participation in the negotiation process, then notification).

        There’s also USB “On-the-Go”, which is this weird extension for embedded devices.

        And as for USB 3.0… oh, man, don’t even get me started. I think I own *ONE* USB 3.0 device that works right.


        1. Tyler Wagner’s avatar

          My laptop has a USB2 powershare port. It seems to feed directly from the +5VDC power supply. For instance, it charges anything plugged into it even when the laptop is off, and it doesn’t seem to mind devices drawing more than 500 mA.

          How do devices “negotiate” for more power?


        2. Matt Heck’s avatar

          Also… I don’t know about you, but I’d rather see mini-USB than micro-USB on any high-current device. Micro-USB jacks are nowhere near as mechanically robust, which is why digital camera manufacturers shunned it. But, that’s what all the phones use, so now we’re stuck with it.


          1. Tyler Wagner’s avatar

            Ah, but the micro-USB cable is usually far more prone to break than the socket side. So the socket is reasonably protected. Almost like they planned it that way. The cable takes one for the team.



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