This writeup was brought about by someone's braindead decision to keep using a cellphone charger interface for Raspberry Pis long after it became obvious that this was a really dumb idea. Problem is that the vast majority of micro USB cables are incapable of passing the level of current required to run a Pi, which means you either need to go and buy a custom Pi-specific power supply, thereby defeating the point of using a cellphone charger in the first place, or spend forever trying to find a cable that can feed enough power to it that it won't glitch or fail.
Since I have to maintain a couple of Pi-based systems despite preferring not to (at least some packages can be moved to an ODroid even though they're nominally meant for a Pi), I ended up stuck in the position of feeding power to them through something that was never designed for it, or at least not the levels of power that a Pi requires. Buying a bunch of Pi-specific power supplies was out of the question both because of the cost and because I don't have room for a row of wall warts to power the Pis.
So it was down to finding a USB cable that you could run low-voltage, high-current DC power through, pretty much the worst way to power a device. For reference, USB power is specced to 5V±5%, so 5.25 - 4.75V. The Pi has a primitive undervolt sensor that trips at 4.63V but it's a one-shot so if at any point in the past the voltage ever went below that level it'll stay tripped, making it difficult to diagnose when the problem occurred.
The following table shows the results from various USB cables tested with two types of dummy loads, a passive resistive load and an electronic load. Input was 5.1V from a lab power supply, voltage and current was measured across a dummy load set to draw 250mA and 500mA (for the original USB power rating), 1.1A (about 5W with any potential drop factored in), and then 1.6A and 2.3A, with the 2.3A being a common maximum-current figure for many USB power bricks. The figures in the columns are the output voltage measured at the load, and in a few cases where the voltage drop with the passive load was extreme, the current. The cables without names are generic unbranded cables, the cables labelled “White” are charging cables that came with a phone or tablet, “Ribbon ” is a ribbon cable, “Switched ” is one with an inline switch specifically meant for delivering power to a device. To help those who aren't used to metric measurements, I've also given the length in arshins. You're welcome.
There are two sets of figures because of the way the measurements were taken, the first line is for the passive load which was connected through a USB power meter that added two USB connectors and the meter draw while the second line is for the electronic load which was connected straight to the source. This means that for the good-quality cables voltages are a bit higher on the electronic than the passive load. In contrast the passive load uses the basic R=V/I to try and draw the current it needs so will switch in the appropriate resistance to draw 1.1A even if it's only getting 900mA while the electronic load will draw an actual 1.1A. This means that for the poor-quality cables the voltages for the electronic load drop a lot more than for the passive load.
If you don't understand the above paragraph, go with the values for the electronic load.
|Anker 10cm||10 cm||0.14 ar||88 mΩ||Passive||5.10||5.07||5.06||5.04||5.02||4.95||Anker 10cm||10 cm||0.14 ar||88 mΩ||Active||5.10||5.06||5.04||5.03||5.03||4.96|
|Anker 1m||88 cm||1.24 ar||88 mΩ||Passive||5.10||5.05||5.03||4.97||4.93||4.82||Anker 1m||88 cm||1.24 ar||88 mΩ||Active||5.10||5.06||5.03||4.98||4.96||4.87|
|Ugreen 1m||102 cm||1.43 ar||131 mΩ||Passive||5.10||5.05||5.01||4.93||4.87||4.74|
|Ugreen 1m||102 cm||1.43 ar||131 mΩ||Active||5.10||5.05||5.01||4.93||4.88||4.76|
|BlitzWolf 1m||102 cm||1.43 ar||151 mΩ||Passive||5.10||5.05||5.00||4.93||4.84||4.72|
|BlitzWolf 1m||102 cm||1.43 ar||151 mΩ||Active||5.10||5.05||5.03||4.96||4.93||4.78|
|JuiceEBitz 2m||200 cm||2.81 ar||133 mΩ||Passive||5.10||5.04||5.01||4.91||4.83||4.70|
|JuiceEBitz 2m||200 cm||2.81 ar||133 mΩ||Active||5.10||5.04||5.01||4.94||4.88||4.77|
|Generic B||48 cm||0.68 ar||207 mΩ||Passive||5.10||5.03||4.97||4.85||4.75||4.58|
|Generic B||48 cm||0.68 ar||207 mΩ||Active||5.10||5.01||4.96||4.85||4.73||4.53|
|Generic A||200 cm||2.81 ar||182 mΩ||Passive||5.10||5.01||4.95||4.82||4.70||4.53|
|Generic A||200 cm||2.81 ar||182 mΩ||Active||5.10||5.03||4.95||4.82||4.71||4.47|
|Generic I||210 cm||2.95 ar||181 mΩ||Passive||5.10||5.05||4.93||4.80||4.68||4.51|
|Generic I||210 cm||2.95 ar||181 mΩ||Active||5.10||5.01||4.95||4.79||4.68||4.46|
|Generic E||130 cm||1.83 ar||192 mΩ||Passive||5.10||5.01||4.93||4.81||4.68||4.50|
|Generic E||130 cm||1.83 ar||192 mΩ||Active||5.10||5.00||4.93||4.78||4.64||4.42|
|Generic C||100 cm||1.41 ar||200 mΩ||Passive||5.10||5.00||4.93||4.80||4.67||4.50|
|Generic C||100 cm||1.41 ar||200 mΩ||Active||5.10||5.01||4.93||4.78||4.64||4.41|
|Logitech||100 cm||1.41 ar||216 mΩ||Passive||5.10||4.97||4.71||4.64||4.54||4.34|
|Logitech||100 cm||1.41 ar||216 mΩ||Active||5.10||4.82||4.72||4.67||4.53||4.39|
|White 1m||98 cm||1.39 ar||257 mΩ||Passive||5.10||4.96||4.85||4.66||4.50|
|White 1m||98 cm||1.39 ar||257 mΩ||Active||5.10||4.96||4.86||4.61||4.42||4.08|
|Ribbon 1m||100 cm||1.41 ar||280 mΩ||Passive||5.10||4.95||4.82||4.61||4.45|
|Ribbon 1m||100 cm||1.41 ar||280 mΩ||Active||5.10||4.95||4.83||4.57||4.35||3.98|
|Generic H||100 cm||1.41 ar||291 mΩ||Passive||5.10||4.95||4.82||4.61||4.42|
|Generic H||100 cm||1.41 ar||291 mΩ||Active||5.10||4.95||4.83||4.56||4.33||3.94|
|Generic J||230 cm||3.23 ar||326 mΩ||Passive||5.10||4.93||4.81||4.58||4.38|
|Generic J||230 cm||3.23 ar||326 mΩ||Active||5.10||4.93||4.82||4.50||4.24||3.83|
|Generic G||200 cm||2.81 ar||340 mΩ||Passive||5.10||4.93||4.80||4.56||4.35|
|Generic G||200 cm||2.81 ar||340 mΩ||Active||5.10||4.92||4.79||4.47||4.23||3.79|
|White 30cm||30 cm||0.42 ar||517 mΩ||Passive||5.07||4.82||4.61||4.25|
|White 30cm||30 cm||0.42 ar||517 mΩ||Active||5.10||4.83||4.57||4.01||3.53||-|
|Generic F||150 cm||2.11 ar||515 mΩ||Passive||5.00||4.66||4.50||4.22||4.03|
|Generic F||150 cm||2.11 ar||515 mΩ||Active||5.11||4.56||4.45||4.07||3.76||-|
|Switched||100 cm||1.41 ar||540-810 mΩ||Passive||4.93
|Switched||100 cm||1.41 ar||540-810 mΩ||Active||5.10||4.82||4.66||4.33||4.08||3.69|
|Generic D||82 cm||1.15 ar||2,382 mΩ||Passive||4.93||4.09||3.50|
|Generic D||82 cm||1.15 ar||2,382 mΩ||Active||5.10||3.97||-||-||-||-|
Some general observations:
If anyone has any Pi cables they'd like tested, feel free to fax them to me. In particular if you think you've got something that can beat the Anker, I'd be interested in seeing it.