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 you can power a device (see also the conclusion section at the end). 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 sections show 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. 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. If you want a lot more detail about USB cable and connector current-carrying capacity, Andreas Spiess has a good overview, note however that he's mostly working with very short cables where there's inherently less voltage drop.
The following are measurements for micro USB power cables as used in the earlier generations of Raspbery Pi. The cables without names are generic unbranded cables, the cables labelled “White” are charging cables that came with a phone or tablet, “Ribbon” are ribbon cables, “Switched” is one with an inline switch specifically meant for delivering power to a device.
|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 30cm||29 cm||0.41 ar||78 mΩ||Passive||5.10||5.06||5.04||5.00||4.97||4.90||Anker 30cm||29 cm||0.41 ar||78 mΩ||Active||5.10||5.06||5.03||5.03||5.01||4.93|
|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|
|Anker 2m||180 cm||1.54 ar||132 mΩ||Passive||5.10||5.05||5.00||4.93||4.86||4.75||Anker 2m||180 cm||1.54 ar||132 mΩ||Active||5.10||5.04||5.01||4.93||4.88||4.78|
|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|
|Anker 3m||298 cm||4.20 ar||159 mΩ||Passive||5.10||4.98||4.90||4.78||4.69||4.55||Anker 3m||298 cm||4.20 ar||159 mΩ||Active||5.10||5.03||4.98||4.87||4.78||4.61|
|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|
|Ribbon B 1m||100 cm||1.41 ar||145 mΩ||Passive||5.10||5.03||4.96||4.85||4.75||4.60|
|Ribbon B 1m||100 cm||1.41 ar||145 mΩ||Active||5.10||5.03||4.96||4.85||4.75||4.60|
|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|
|White 2m||203 cm||2.86 ar||230 mΩ||Passive||5.10||4.97||4.86||4.68||4.54|
|White 2m||203 cm||2.86 ar||230 mΩ||Active||5.10||4.98||4.88||4.69||4.52||4.23|
|Ribbon A 1m||100 cm||1.41 ar||280 mΩ||Passive||5.10||4.95||4.82||4.61||4.45|
|Ribbon A 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 USB 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.
Because there are still devices powered off mini USB, here's a much shorter table for those cables. These date back to 500mA max USB devices, so there's less expectation that they handle higher currents well. As before the cables without names are generic unbranded cables, “Braided” is a plastic-covered braided tinned-copper cable as was fashionable years ago, so with the conductor braided rather than a protective nylon braid.
|Generic B||80 cm||1.13 ar||164 mΩ||Passive||5.10||5.06||5.02||4.93||4.85||4.68|
|Generic B||80 cm||1.13 ar||164 mΩ||Active||5.10||5.03||4.98||4.91||4.83||4.68|
|Ugreen||155 cm||2.18 ar||179 mΩ||Passive||5.10||4.99||4.88||4.74||4.65||4.45|
|Ugreen||155 cm||2.18 ar||179 mΩ||Active||5.10||5.01||4.93||4.78||4.68||4.47|
|Generic C||102cm||1.43 ar||243 mΩ||Passive||5.10||4.87||4.81||4.71||4.60||4.44|
|Generic C||102cm||1.43 ar||243 mΩ||Active||5.10||4.93||4.81||4.66||4.63||4.39|
|Generic A||99cm||1.39 ar||212 mΩ||Passive||5.10||5.01||4.93||4.75||4.61||4.38|
|Generic A||99cm||1.39 ar||212 mΩ||Active||5.10||4.98||4.87||4.63||4.48||4.23|
|Canon||122 cm||1.72 ar||223 mΩ||Passive||5.10||5.00||4.92||4.74||4.58||4.32|
|Canon||122 cm||1.72 ar||223 mΩ||Active||5.10||4.85||4.73||4.53||4.30||4.04|
|Braided||109 cm||1.54 ar||315 mΩ||Passive||5.10||4.93||4.80||4.55||4.35||4.05|
|Braided||109 cm||1.54 ar||315 mΩ||Active||5.10||4.91||4.74||4.40||4.14||3.71|
Finally, a few USB-C cables thrown in just for comparison. These are all USB-A to USB-C, there are no tests for USB-C to USB-C cables because those do USB-PD and that seems to interact in odd ways with the electronic load I'm using. None of the USB-A to USB-C cables are E-Marked (not that you'd expect them to be, but you never know).
You can immediately see the difference between these and the mini USB's above, but then again the mini USB's were designed to pass 500mA max rather than the power loads expected of USB-C.
|White 1m||101 cm||1.43 ar||134 mΩ||Passive||5.10||5.07||5.03||4.96||4.88||4.73|
|White 1m||101 cm||1.43 ar||134 mΩ||Active||5.10||5.08||5.06||4.98||4.93||4.78|
|Unitek||103 cm||1.45 ar||134 mΩ||Passive||5.10||5.05||5.01||4.92||4.84||4.68|
|Unitek||103 cm||1.45 ar||134 mΩ||Active||5.10||5.03||4.98||4.91||4.84||4.70|
|Ravpower||99 cm||1.39 ar||145 mΩ||Passive||5.10||5.01||4.95||4.88||4.83||4.68|
|Ravpower||99 cm||1.39 ar||145 mΩ||Active||5.10||5.03||4.98||4.88||4.82||4.66|
|Generic A||203cm||2.86 ar||212 mΩ||Passive||5.10||5.02||4.94||4.78||4.65||4.45|
|Generic A||203cm||2.86 ar||212 mΩ||Active||5.10||4.98||4.91||4.73||4.58||4.34|
|White 2m||203cm||2.86 ar||204 mΩ||Passive||5.10||5.01||4.92||4.75||4.61||4.41|
|White 2m||203cm||2.86 ar||204 mΩ||Active||5.10||5.03||4.94||4.77||4.63||4.34|
As an addendum, some devices have proper barrel jacks instead of hacked-in USB power, proper DC power connectors that can carry 4-6A without breaking a sweat. However in a fit of breaking what isn't broken, vendors have produced USB to barrel jack cables that in theory would allow you to power devices with barrel jack connectors from a USB power supply.
You sort-of can. There are actually two classes of cables here, the name-brand ones and the generics. Have a look at the table below.
|White 0.6m||68 cm||0.94 ar||49 mΩ||Active||5.10||5.08||5.08||5.04||5.03||4.93|
|Adafruit||103 cm||1.45 ar||73 mΩ||Active||5.10||5.08||5.04||4.98||4.93||4.81|
|Unitek||152 cm||2.15 ar||71 mΩ||Active||5.10||5.08||5.03||4.98||4.93||4.78|
|Black 1m||103 cm||1.43 ar||231 mΩ||Active||5.10||4.93||4.78||4.48||4.29||3.94|
|Black 1m||103 cm||1.43 ar||234 mΩ||Active||5.10||4.93||4.78||4.47||4.29||3.92|
|Black 2m||202 cm||2.84 ar||2,972 mΩ||Active||4.92||3.55||2.00||—||—||—|
|Black 2m||202 cm||2.84 ar||2,695 mΩ||Active||4.93||3.56||2.00||—||—||—|
The name-brand ones, Adafruit and Unitek, perform pretty well, and in particular outperform every single actual USB cable because they use a proper power connector on at least one end. For example the 1 1/2 metre-long Unitek outperforms even the 10-cm long Anker, the best-performing micro USB power cable. However even then the current limit is about 2.5-3A while a standard barrel jack cable would run 4-6A and even more (there are ones rated at 8-10A, but you have to go out of your want to find those). The white cable is unbranded but came included with a device which means that the manufacturer would have made sure that it's actually fit for purpose.
The generic 1m cables, for which the USB connectors look like StarTech's
distinctive mouldings, perform about the same as low-quality micro USB cables.
The 2m ones on the other hand easily beat even the long thin resistor micro
USB cable, and couldn't supply any amount of current to the barrel jack
— if I was a Youtuber I'd probably be inspired to make a video with a
Is this the world's worst USB power cable? or
believe how bad this USB power cable is.
As should be obvious from the above, both the inability of USB power cables and connectors to carry any amount of current and the superior performance of the (name-brand) barrel jack USB cables against even the very best USB-only power cables, don't even think of powering anything that draws any amount of current through a mickey-mouse USB cable and connector. They're not power cables, they're data cables. Yes, USB-PD can deliver greater amounts of power, but then you need to put costly and complex USB-PD management circuitry in your device rather than just slapping on a barrel jack in place of the USB connector. And I don't keep mentioning barrel jacks because they're the perfect solution but because USB connectors and cables are such a terrible one, and barrel jacks are probably the simplest fix for the problem.