I'm aware that my phone might only demand max 1A. Let's suppose it does. Still, the following is strange to me-

How is it possible, with 2 "20 AWG" cables each capable of 2 amps, for one to give 0.4A to my phone and the other 0.7-0.9A?

I have two USB cables, from different companies. Both are 20 AWG.

I tested them with my 2A capable charger and a usb load tester showing volts and amps, that lets me adjust amps it demands, and both cables are capable of 2A.

Putting the load tester aside, using my phone instead, so I then take my 2A capable charger, USB volt meter and phone..

I plug one cable in on my phone and my USB volt amp meter measures 0.4A

Then I change the cable.

So I plug the other cable in on my phone and the USB volt amp meter measures 0.7A

And i've tested charging my phone in two locations, (one location has one charger and usb monitor, the other location has another charger and usb volt amp monitor), so i've tested it with two different USB volt/amp meters and two different chargers. And i've seen my phone charge much faster with the one that shows 0.7A, so I believe the reading.

How is this possible.. What could it be about the cables that causes this, And what device could measure it and show it?

I've already looked when buying cables to make sure they're 20 AWG(not 28 AWG), and i've got the load tester.. i'm happy to get other devices to get to the bottom of this. What else could it be and what other devices might I need to test that that is the case?

And the cable that shows 0.4A is actually shorter than the one that shows 0.7A. So it's not a length issue.

  • 2
    \$\begingroup\$ I am too lazy to read all of it. But 20AWG is wire much more than 2A. On other hand, i have never seen any USB cable with 20AWG wires. Someone is cheating you. \$\endgroup\$
    – user76844
    Feb 18 '17 at 10:02
  • \$\begingroup\$ @GregoryKornblum they probably are capable of more thn 2A, i'm not doubting that. Google it, you can buy them, but even if they were less than 20 AWG, eg 28AWG, the load tester shows them as capable of 2A so the question stands. \$\endgroup\$
    – barlop
    Feb 18 '17 at 10:04
  • \$\begingroup\$ I would bet it's the phone's choise how much current to take. I mean, 0.4A and 0.7A is on one hand almost 100% difference, so it's not something that is affected by voltage drop. On the other hand both values are very low. \$\endgroup\$
    – user76844
    Feb 18 '17 at 10:09
  • \$\begingroup\$ @GregoryKornblum The phone might only consume 1A so the values of 0.4A vs 0.7-0.9A, aren't low and it's not relevant if they are low compared to other devices. The question is, how is the phone making a choice depending on cable.. Just saying it's not voltage drop doesn't answer the question. \$\endgroup\$
    – barlop
    Feb 18 '17 at 10:27
  • 1
    \$\begingroup\$ Cables and chargers do not "give", it is the phone who decides what to "take". Wire gauge has little to do with this process, it is "charger signature" that is important. See related questions, electronics.stackexchange.com/questions/271641/… and electronics.stackexchange.com/questions/130559/… \$\endgroup\$ Feb 18 '17 at 17:26

More advanced devices determine what current they can take from a charger by looking at what is happening on the signal leads of the usb charger. For details see https://www.maximintegrated.com/en/app-notes/index.mvp/id/5801 In this document there is detailed description of the charger types and how they are identified to the device being charged here is an extract from the document that may explain your problem.

Standard Downstream Port (SDP) This port features 15kΩ pulldown resistors on both the D+ and D- lines. The current limits are those discussed above: 2.5mA when suspended, 100mA when connected, and 500mA when connected and configured for higher power.

Dedicated Charging Port (DCP) This port does not support any data transfer, but is capable of supplying charge currents beyond 1.5A. It features a short between the D+ and D- lines. This type of port allows for wall chargers and car chargers with high-charge capability without the need for enumeration.

Downstream Port (CDP) This port allows for both high-current charging and data transfer fully compliant with USB 2.0. It features the 15kΩ pulldown resistors necessary for the D+ and D- communication, and also has internal circuitry that is switched in during the charger detection phase. This internal circuitry allows the portable device to distinguish a CDP from other port types.

I suspect that the signal leads (D+,D-) are missing or mis configured on the 0.4A lead and that your phone assumes it is a standard port only capable of supplying 500ma.

  • \$\begingroup\$ How can I test for this? Is there any device that tests for that and is a pass-through device? So for example, if I connect up my charger and 20AWG usb cable and load tester, then I get 2A, but if I connect such a pass-through cable misconfiguration device, then i'd get 0.4A? \$\endgroup\$
    – barlop
    Feb 18 '17 at 12:06
  • \$\begingroup\$ Or is there any device that just tests for it? Or what is the cheapest "advanced device" I might find? \$\endgroup\$
    – barlop
    Feb 18 '17 at 12:06
  • \$\begingroup\$ If you connect the 0.4A cable as a data cable rather than a charger cable can you send data over it? \$\endgroup\$
    – RoyC
    Feb 18 '17 at 12:08
  • \$\begingroup\$ If you can then the data wires are present and working. \$\endgroup\$
    – RoyC
    Feb 18 '17 at 12:13
  • \$\begingroup\$ I don't think it's dependent on data wires. I have a variety of cables.. Of the two that provide 0.7-0.9A, one is sync and charge, and one is charge. And of the other cables that I have, some are charge and some are sync and charge, and they don't. The ones from a company called portapow are giving more Amps. And all the others are giving less like 0.4A. (portapow make unreliable chargers so i'm not endorsing them, but their cables seem good!) \$\endgroup\$
    – barlop
    Feb 18 '17 at 12:16

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