Avian’s Blog

After I made my USB cable resistance tester I naturally wanted to measure some cables. I searched my apartment and ended up with a big jumble of 18 micro and mini USB cables of various lengths and origins. I didn't imagine I would find that many, but I guess today just about everything comes with one and I apparently never throw away anything. In fact some cables were in a very bad shape and already had insulation flaking off from old age.

I measured the resistance of each cable at 1 A using the voltage ratio method I described in my previous post. The following table lists the results. For a lot of cables I don't know their origin and they must have came bundled with various devices. I've listed the brand if it was printed on or if I knew for certain which device the cable came with. I'm afraid this comparison isn't very useful as a guide which cable brand to buy, but it does give an interesting statistic of what kind of cables can be found out there in the wild.

Unsurprisingly, two short 30 cm cables came out as best in terms of resistance, measuring below 200 mΩ. A bit more unexpected was finding out that the 2 m CellularLine isn't far behind. This is a fancy and laughably overpriced cable I bought in a physical store not so long ago, the only one on this list that I'm sure didn't come bundled with any device. It appears in this case the price was at least somewhat justified.

I was also a bit surprised that some cables that came bundled with devices measured pretty high. The white Amazon was for charging a Kindle 3 and it had the highest resistance among the micro B cables I tested. On the other hand, it was also in pretty bad shape, so it might be that it was damaged somehow. Cables bundled with an HTC phone and Google Chromecast also measured more than 500 mΩ.

Other measurements I could find on the web seem to roughly agree with mine. martinm lists measured values between 289 and 1429 mΩ. Balaur on EEVblog forum measured between 276 and 947 mΩ on his cables. The only report that was really off was this forum post by d_t_a where most of the cables listed are lower than 200 mΩ.

Another thing I was interested in was how repeatable these measurements were. I mentioned several times in my previous posts that contact resistance can play a big role. Since each time you plug in a cable the contacts sit differently and have a slightly different resistance, contact resistance behaves like a random variable in the measurement results. When I was doing the measurements above this was quite obvious. Minimal movements of the cable caused the voltage displayed on the voltmeter to dance around.

I repeated the measurement of cable 16 from the table above 10 times. Before each repetition I unplugged and re-plugged both ends of the cable. Above you can see the histogram of those measurements. The results only vary for approximately ±1%, which is much less than I thought they would. This is about the same as the expected error of the measurement itself due to the accuracy of the reference resistor. Of course, this was all done over a short period of time. I'm guessing the resistance would change more over longer periods of time and more cycles as contacts deteriorate or gather dirt.

I also checked how the measurement is affected if I plug something between the tester and the cable. Gino mentioned in a comment they used an adapter and an extension cable in their measurement. So I repeated the measurement of cable 1 from the table with a short type A-to-type A extension in series. Just for fun, I also tested how much resistance a cheap USB multimeter adds:

As you can see from the results above, both of these added quite a lot. With the excellent 200 mΩ cable, both more than doubled the total resistance. Even with an average 500 mΩ cable, this multimeter would add around 240 mΩ or approximately 50% on top. Battery-powered devices like smartphones adjust their charging current according to the voltage drop they see on their end. Hence they might charge significantly slower when the multimeter is in series with the cable compared to just using a cable. This puts some doubt on the usability of these USB multimeters for evaluating USB cables and power supplies.