Micro USB Cable Power Tests

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” are ribbon cables, “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. 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.

Micro USB cable current capacity
Cable Length Length' Resistance Load Type 0mA 250mA 500mA 1.1A 1.6A 2.3A
Anker 10cm10 cm0.14 ar88 mΩPassive 5.105.075.065.045.024.95
Anker 10cm10 cm0.14 ar88 mΩActive 5.105.065.045.035.034.96
Anker 30cm29 cm0.41 ar78 mΩPassive 5.105.065.045.004.974.90
Anker 30cm29 cm0.41 ar78 mΩActive 5.105.065.035.035.014.93
Anker 1m88 cm1.24 ar88 mΩPassive 5.105.055.034.974.934.82
Anker 1m88 cm1.24 ar88 mΩActive 5.105.065.034.984.964.87
JuiceEBitz 1mcmar mΩPassive 5.105.055.034.974.914.80
Anker 2m180 cm1.54 ar132 mΩPassive 5.105.055.004.934.864.75
Anker 2m180 cm1.54 ar132 mΩActive 5.105.045.014.934.884.78
Ugreen 1m102 cm1.43 ar131 mΩPassive 5.105.055.014.934.874.74
Ugreen 1m102 cm1.43 ar131 mΩActive 5.105.055.014.934.884.76
BlitzWolf 1m102 cm1.43 ar151 mΩPassive 5.105.055.004.934.844.72
BlitzWolf 1m102 cm1.43 ar151 mΩActive 5.105.055.034.964.934.78
JuiceEBitz 2m200 cm2.81 ar133 mΩPassive 5.105.045.014.914.834.70
JuiceEBitz 2m200 cm2.81 ar133 mΩActive 5.105.045.014.944.884.77
Anker 3m298 cm4.20 ar159 mΩPassive 5.104.984.904.784.694.55
Anker 3m298 cm4.20 ar159 mΩActive 5.105.034.984.874.784.61
Generic B48 cm0.68 ar207 mΩPassive 5.105.034.974.854.754.58
Generic B48 cm0.68 ar207 mΩActive 5.105.014.964.854.734.53
Generic A200 cm2.81 ar182 mΩPassive 5.105.014.954.824.704.53
Generic A200 cm2.81 ar182 mΩActive 5.105.034.954.824.714.47
Generic I210 cm2.95 ar181 mΩPassive 5.105.054.934.804.684.51
Generic I210 cm2.95 ar181 mΩActive 5.105.014.954.794.684.46
Generic E130 cm1.83 ar192 mΩPassive 5.105.014.934.814.684.50
Generic E130 cm1.83 ar192 mΩActive 5.105.004.934.784.644.42
Generic C100 cm1.41 ar200 mΩPassive 5.105.004.934.804.674.50
Generic C100 cm1.41 ar200 mΩActive 5.105.014.934.784.644.41
Logitech100 cm1.41 ar216 mΩPassive 5.104.974.714.644.544.34
Logitech100 cm1.41 ar216 mΩActive 5.104.824.724.674.534.39
Ribbon B 1m100 cm1.41 ar145 mΩPassive 5.105.034.964.854.754.60
Ribbon B 1m100 cm1.41 ar145 mΩActive 5.105.034.964.854.754.60
White 1m98 cm1.39 ar257 mΩPassive 5.104.964.854.664.50
1.4A
4.25
2.0A
White 1m98 cm1.39 ar257 mΩActive 5.104.964.864.614.424.08
White 2m203 cm2.86 ar230 mΩPassive 5.104.974.864.684.54
1.4A
4.30
2.0A
White 2m203 cm2.86 ar230 mΩActive 5.104.984.884.694.524.23
Ribbon A 1m100 cm1.41 ar280 mΩPassive 5.104.954.824.614.45
1.4A
4.17
1.95A
Ribbon A 1m100 cm1.41 ar280 mΩActive 5.104.954.834.574.353.98
Generic H100 cm1.41 ar291 mΩPassive 5.104.954.824.614.42
1.4A
4.15
1.9A
Generic H100 cm1.41 ar291 mΩActive 5.104.954.834.564.333.94
Generic J230 cm3.23 ar326 mΩPassive 5.104.934.814.584.38
1.4A
4.10
1.9A
Generic J230 cm3.23 ar326 mΩActive 5.104.934.824.504.243.83
Generic G200 cm2.81 ar340 mΩPassive 5.104.934.804.564.35
1.4A
4.08
1.9A
Generic G200 cm2.81 ar340 mΩActive 5.104.924.794.474.233.79
White 30cm30 cm0.42 ar517 mΩPassive 5.074.824.614.25
890mA
3.94
1.2A
3.55
1.6A
White 30cm30 cm0.42 ar517 mΩActive 5.104.834.574.013.53-
Generic F150 cm2.11 ar515 mΩPassive 5.004.664.504.224.03
1.3A
3.72
1.7A
Generic F150 cm2.11 ar515 mΩActive 5.114.564.454.073.76-
Switched100 cm1.41 ar540-810 mΩPassive 4.93

5.08

5.08
5.09
4.58

4.88

4.87
4.92
4.42

4.71

4.68
4.80
4.20
850mA
4.42
950mA
4.38
4.53
3.98
1.25A
4.26
1.35A
4.17
4.34
3.87
1.8A
3.99
1.85A
3.92
4.10
Switched100 cm1.41 ar540-810 mΩActive 5.104.824.664.334.083.69
Generic D82 cm1.15 ar2,382 mΩPassive 4.934.093.50
350mA
3.10
600mA
--
Generic D82 cm1.15 ar2,382 mΩActive 5.103.97----

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.

Mini USB Cable Power Tests

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.

Mini USB cable current capacity
Cable Length Length' Resistance Load Type 0mA 250mA 500mA 1.1A 1.6A 2.3A
Generic B80 cm1.13 ar164 mΩPassive 5.105.065.024.934.854.68
Generic B80 cm1.13 ar164 mΩActive 5.105.034.984.914.834.68
Ugreen155 cm2.18 ar179 mΩPassive 5.104.994.884.744.654.45
Ugreen155 cm2.18 ar179 mΩActive 5.105.014.934.784.684.47
Generic C102cm1.43 ar243 mΩPassive 5.104.874.814.714.604.44
Generic C102cm1.43 ar243 mΩActive 5.104.934.814.664.634.39
Generic A99cm1.39 ar212 mΩPassive 5.105.014.934.754.614.38
Generic A99cm1.39 ar212 mΩActive 5.104.984.874.634.484.23
Canon122 cm1.72 ar223 mΩPassive 5.105.004.924.744.584.32
Canon122 cm1.72 ar223 mΩActive 5.104.854.734.534.304.04
Braided109 cm1.54 ar315 mΩPassive 5.104.934.804.554.354.05
Braided109 cm1.54 ar315 mΩActive 5.104.914.744.404.143.71

USB-C Cable Power Tests

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.

USB-C cable current capacity
Cable Length Length' Resistance Load Type 0mA 250mA 500mA 1.1A 1.6A 2.3A
White 1m101 cm1.43 ar134 mΩPassive 5.105.075.034.964.884.73
White 1m101 cm1.43 ar134 mΩActive 5.105.085.064.984.934.78
Unitek103 cm1.45 ar134 mΩPassive 5.105.055.014.924.844.68
Unitek103 cm1.45 ar134 mΩActive 5.105.034.984.914.844.70
Ravpower99 cm1.39 ar145 mΩPassive 5.105.014.954.884.834.68
Ravpower99 cm1.39 ar145 mΩActive 5.105.034.984.884.824.66
Generic A203cm2.86 ar212 mΩPassive 5.105.024.944.784.654.45
Generic A203cm2.86 ar212 mΩActive 5.104.984.914.734.584.34
White 2m203cm2.86 ar204 mΩPassive 5.105.014.924.754.614.41
White 2m203cm2.86 ar204 mΩActive 5.105.034.944.774.634.34