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I have a set of USB cables (data and charge) and I noticed that, for a given charger, the charge time differs by cable.

What would be a good way to measure the charging fitness of a USB cable? Possibly with equipment which is not NASA-grade?

I know that I could measure the charge current for a set of charger and cable but I would prefer to assess the cable alone.

Would this measure be dependent on the charger characteristics? In other words, are some cables more or less efficient depending on the charger (not taking into account the current the charger can sustain)?

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If the only difference in charge time is the cable, there are two possibilities.

1) A specific USB charging cable might have resistors inside one of the connectors that set the voltage on the D+ & D- lines to voltage levels that tells your device how much current it should take in while charging. Although this is not common, I have seen cables just like this.

2) USB cables from different manufacturers can have smaller or larger sizes of wire for the Gnd & Vdd lines. Thin wire gives more voltage drop, which tells your device to reduce the amount of current being taken in. Thicker wire gives a lower voltage drop and your device takes in more current.

In terms of measuring your particular cables, you need a meter that can resolve resistances in fractions of an Ohm. Although I have such a meter, I would use an accurate current-limited power supply set for 500 mA with a voltmeter connected right across the terminals.

Short the Vdd & Gnd pins at one end of the cable and connect the Vdd & Gnd pins of the other end of the cable to your current-limited supply. If you set the current accurately to 500 mA, you can calculate the wire resistance. Do this for all of your USB cables.

I'm suggesting a current of 500 mA because pretty much all USB devices can handle at least that amount of current. Some power supplies and devices can, of course, handle much more. But 500 mA gives you enough current that you can easily measure the resistance.

I have some very nice flat USB charging cables that have very heavy wires for the two outside conductors (Gnd & Vdd) and a very thin twisted-pair in the middle for the D+ & D- conductors. They work very well at charging my phone - better than some of the other cables that I have.

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  • \$\begingroup\$ It's very unlikely that cables do anything with the data lines, other than fail to connect them through. A cable which fails to connect through will prevent the device being charged from determining that it is connected to a high capacity charger, and thus (for a well designed device) limit charge rate. \$\endgroup\$ – Chris Stratton Apr 8 '18 at 18:03
  • \$\begingroup\$ @ChrisStratton: I'm not sure exactly why you say what you said. Most USB cables for phones connect all 4 lines through. Many (most) phones will look at the data lines while charging. The USB standard says that chargers should short D+ to D- to let the phone know that it is connected to a charger but many phones / chargers actually put different voltage levels on the D+ & D- lines to specify non-standard charge current capability. \$\endgroup\$ – Dwayne Reid Apr 9 '18 at 14:49
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If you have Samsung phone you can find an app that measures the current being provided, which is (often drastically) affected by charger and cable combination and the cleanliness and tightness of fit sockets on the charger and phone. I've not found an app to do this on other phones.

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  • \$\begingroup\$ It doesn't seem like this differentiates between ohmic losses in the conductors, and various impediments to charge rate signaling such as unconnected data lines. \$\endgroup\$ – Chris Stratton Apr 8 '18 at 18:05
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You can use an inexpensive ammeter that goes between the USB and the USB cable. The ammeter displays output voltage and amperage.

You can get something like this: https://www.aliexpress.com/item/32997886876.html?spm=a2g0s.9042311.0.0.1c514c4dLiqw6U

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