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:
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|
|Nook Color||Barnes & Noble||0.46||0.46||0.46||0.46|
- 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!