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 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. Lui Gough also has a very good writeup on the problem with plenty of technical detail.

Micro USB Cable Power Tests

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.

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
Anker 10cm10 cm0.14 ar88 mΩActive
Anker 30cm29 cm0.41 ar78 mΩPassive
Anker 30cm29 cm0.41 ar78 mΩActive
Anker 1m88 cm1.24 ar88 mΩPassive
Anker 1m88 cm1.24 ar88 mΩActive
JuiceEBitz 1mcmar mΩPassive
Anker 2m Gen1180 cm1.54 ar132 mΩPassive
Anker 2m Gen1180 cm1.54 ar132 mΩActive
Anker 2m Gen2180 cm1.54 ar117 mΩPassive
Anker 2m Gen2180 cm1.54 ar117 mΩActive
Ugreen 1m102 cm1.43 ar131 mΩPassive
Ugreen 1m102 cm1.43 ar131 mΩActive
BlitzWolf 1m102 cm1.43 ar151 mΩPassive
BlitzWolf 1m102 cm1.43 ar151 mΩActive
JuiceEBitz 2m200 cm2.81 ar133 mΩPassive
JuiceEBitz 2m200 cm2.81 ar133 mΩActive
Anker 3m298 cm4.20 ar159 mΩPassive 5.104.984.904.784.694.55
Anker 3m298 cm4.20 ar159 mΩActive
Generic B48 cm0.68 ar207 mΩPassive
Generic B48 cm0.68 ar207 mΩActive
Generic A200 cm2.81 ar182 mΩPassive
Generic A200 cm2.81 ar182 mΩActive
Generic I210 cm2.95 ar181 mΩPassive
Generic I210 cm2.95 ar181 mΩActive
Generic E130 cm1.83 ar192 mΩPassive
Generic E130 cm1.83 ar192 mΩActive
Generic C100 cm1.41 ar200 mΩPassive
Generic C100 cm1.41 ar200 mΩActive
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
Ribbon B 1m100 cm1.41 ar145 mΩActive
White 1m #1103 cm1.43 ar218 mΩPassive 5.104.984.914.724.584.38
White 1m #1103 cm1.43 ar218 mΩActive
White 1m #298 cm1.39 ar257 mΩPassive 5.104.964.854.664.50
White 1m #298 cm1.39 ar257 mΩActive 5.104.964.864.614.424.08
White 2m203 cm2.86 ar230 mΩPassive 5.104.974.864.684.54
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
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
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
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
Generic G200 cm2.81 ar340 mΩActive 5.104.924.794.474.233.79
White 30cm30 cm0.42 ar517 mΩPassive 5.074.824.614.25
White 30cm30 cm0.42 ar517 mΩActive 5.104.834.574.013.53
Generic F150 cm2.11 ar515 mΩPassive 5.004.664.504.224.03
Generic F150 cm2.11 ar515 mΩActive 5.114.564.454.073.76
Switched100 cm1.41 ar540-810 mΩPassive 4.93






Switched100 cm1.41 ar540-810 mΩActive 5.104.824.664.334.083.69
Generic D82 cm1.15 ar2,382 mΩPassive 4.934.093.50
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
Generic B80 cm1.13 ar164 mΩActive
Ugreen155 cm2.18 ar179 mΩPassive 5.104.994.884.744.654.45
Ugreen155 cm2.18 ar179 mΩActive
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
Generic A99cm1.39 ar212 mΩActive 5.104.984.874.634.484.23
Generic D103cm1.45 ar236 mΩPassive 5.104.974.884.674.534.34
Generic D103cm1.45 ar236 mΩActive 5.104.984.884.634.514.23
Canon122 cm1.72 ar223 mΩPassive
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. The Essager cable actually claims to be E-marked, but then it also has a supposed USB-C connector that looks like no actual USB-C connector so you need to take that claim with a grain of salt, alongside the one of putting 7 amps through a USB-A outlet. In their defence they make the best-performing USB-A to C cable even if their marketing is a bit creative).

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
Essager 1m101 cm1.43 ar73 mΩPassive
Essager 1m101 cm1.43 ar73 mΩActive
Baseus 1m110 cm1.55 ar92 mΩPassive
Baseus 1m110 cm1.55 ar92 mΩActive
White 1m101 cm1.43 ar134 mΩPassive
White 1m101 cm1.43 ar134 mΩActive
Generic C104cm1.45 ar114 mΩPassive
Generic C104cm1.45 ar114 mΩActive
Unitek103 cm1.45 ar134 mΩPassive
Unitek103 cm1.45 ar134 mΩActive
Ravpower99 cm1.39 ar145 mΩPassive
Ravpower99 cm1.39 ar145 mΩActive
Generic B87cm1.22 ar145 mΩPassive 5.104.814.784.724.664.53
Generic B87cm1.22 ar145 mΩActive
Generic A203cm2.86 ar212 mΩPassive
Generic A203cm2.86 ar212 mΩActive 5.104.984.914.734.584.34
White 2m203cm2.86 ar204 mΩPassive
White 2m203cm2.86 ar204 mΩActive

So why would you care about the higher-current capacity of USB-A to USB-C cables? Well, after the swing-and-a-miss of trying to power devices through micro USB cables, the current trend is another miss by trying to power them through USB-C cables, as opposed to taking something like a 20-year-old TPS54350, cutting and pasting the reference design onto your device, adding a barrel jack for input, and declaring victory.

The problem with USB-C is that it's no longer a dumb wire like USB-A was, but designers still treat it as such, entirely omitting the required CC pulldown resistors and other power signalling, or failing to copy the design (two resistors) from the USB-C spec and getting it wrong (and that's not getting into the complications of downstream ports, SDP and CDP, charging ports, DCP, and USB-C PD). As a result, any number of devices with a USB-C power connector are telling the power source that they can't handle more than 500mA or perhaps 900mA, leading to mysterious crashes, reboots, and glitches when they're plugged into a USB-C power source.

The only way to fix this is with a USB-A to USB-C cable, which ignores all of the incorrect power signalling that the downstream device is performing, and for that you need a USB-A-input cable that can carry sufficient power to the USB-C connector at the other end.

USB to Barrel Jack Cable Power Tests

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.

USB barrel jack cable current capacity
Cable Length Length' Resistance Load Type 0mA 250mA 500mA 1.1A 1.6A 2.3A
White 0.6m68 cm0.94 ar49 mΩActive
Adafruit103 cm1.45 ar73 mΩActive
Unitek152 cm2.15 ar71 mΩActive
Black 1m103 cm1.43 ar231 mΩActive 5.104.934.784.484.293.94
Black 1m103 cm1.43 ar234 mΩActive 5.104.934.784.474.293.92
StarTech103 cm1.43 ar251 mΩActive 5.104.944.824.334.143.83
Generic C103 cm1.43 ar256 mΩActive 5.104.604.354.124.083.65
Generic D121 cm1.70 ar455 mΩActive 5.104.474.093.683.222.89
Generic B151 cm2.12 ar2,972 mΩActive 4.923.552.00
Generic B151 cm2.12 ar2,695 mΩActive 4.933.562.00

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 way 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 title like Is this the world's worst USB power cable? or You won't 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.