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 ” 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.

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 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
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
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
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
Ribbon 1m100 cm1.41 ar280 mΩPassive 5.104.954.824.614.45
1.4A
4.17
1.95A
Ribbon 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 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.