Why would a USB cable give less power after being used for a while?

I'm asking this from the perspective of wanting to understand what actually happens inside the cable.

I use this app to measure how many mA are being received by my smartphone while charging. Some cheap cables seem to be unable to transmit enough power to charge my phone. So I always test cables with the app before buying them. Last time I was in the shop, I tested one that only gave 200 mA. The one I ended up buying (two pieces of) gave 1100 mA. I used the same power adapter (it's the one that came with the phone) for all the tests. The two cables I bought would sometimes it gives as much as 1300 mA.

Today I was charging with one of those cables (one that had consistently been giving me around 1000 mA for weeks, and I noticed that the battery level was going down even while I was charging. I tested with the app and I saw that I was only getting 150 mA. Using the same cable in another adapter gave a similar result. I tried the other (identical) cable that I had purchased at the same time, and I was back to getting 1000 mA.

Clearly, something bad had happened to the former cable.

Speaking in terms of the inner workings of the cable, what would cause such a change in the cable? (Its ability to transmit power dropping drastically.)

Additional information:

  • The change appeared to happen suddenly. It was sort of like one day the cable was fine, then the next day it was at 150 mA.
  • I live in an area with a very unstable power supply. There are often surges and fluctuations in the current here. Many people have stabilizers, etc, on their electronics to protect them from such things.
  • Several of my cables have just stopped working altogether after some weeks of use.
  • The observations of the app are somewhat corroborated by the fact that when the app displays a low mA, the battery level of the phone goes down if I use it while charging, wheras when it displays a high mA, the battery goes rapidly up, even if I'm using the phone. I'm not saying the app is exact (it may even be off by several hundred mA), but it gives you a rough idea. I'd say 150 mA vs 1000 mA is a significant enough difference to make the app relevant.


Since you have two cables to compare, one of which is performing badly, grab a multimeter and compare the following measurements between cables:

  1. Resistance along the 5V conductor, from one connector pin to the corresponding pin on the other connector
  2. Resistance along the ground conductor, measured as above
  3. Resistance along the D+ conductor, measured as above
  4. Resistance along the D- conductor, measured as above
  5. Resistance between D+ and D-, measured at the USB A connector end (the connector that plugs into the power supply)
  6. Resistance between D+ and D- on the phone connector end

Such a drastic reduction in charge current is most likely down to failed high charge current negotiation, normally achieved by either shorting D+ and D- or setting particular voltages on them. Connector corrosion or partial screen detachment is unlikely to have caused such a large drop without having disconnected entirely.

Category: usb Time: 2016-07-29 Views: 0

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