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The Proof: Poor USB cables slow phone charging by 77%

Inexpensive cellphone charging cables, commonly sold online, at drug stores, and at discount stores can exhibit poor phone charging speed. Some times the charging performance is so poor that people think their phone has gone bad. Here, I'll show how cheap cable construction can compromise charging performance.

Verifying Cable Quality

The following experiment is designed to show how bad cables ruin charging speeds. I will be measuring various USB extension cables to experimentally show how poor cable construction can have a significant negative impact on smart phone charging rates.

In order to standardize the experiment and eliminate variables, I will be using a genuine Apple "1 Amp" USB charger along with a genuine Apple USB lightning cable. These components were delivered new by Apple with my iPhone 11, so I know they are "legitimate" (not clones or counterfeits). They are name brand components sold by a reliable retailer.

But I'm not here to test these well-known cables. Instead I am testing general purpose USB extension cables in order to show how a  poor cable can negatively impact charging rates.

Two Experiments

  • Experiment 1 will report the power delivered by the standard charging setup, without any USB extension cable in the system.
  • Experiment 2 will report the power delivered by the standard charging setup, but with a USB extension cable in the system.

Will a simple USB extension cord added to the charging setup change the amount of power delivered?  Many people might conclude that there will be negligible difference between the above charging setups.

To figure this out, we will look at a sample of the energy delivered (Watts) through the power meter. Watts is calculated my multiplying amperage times voltage. 

Data Collection

Experiment 1 is simply measuring the power being delivered to my iPhone 11 through the Apple-provided AC adapter and cable. As you can see in Photo 1, this test shows the charging system delivering 0.96 Amps at 4.90 Volts. This seems reasonable given the specifications of the charger (1 Amp at 5 VDC).

Experiment 1 data: 0.96A x 4.90v = 4.704 Watts

Photo 1: The baseline test delivers 4.704 watts

 

Experiment 2 has the exact same setup, except that a USB extension cable has been inserted between the Apple AC power adapter and the power meter. All other aspects of the experiment are identical. As you can see in Photo 2, this test shows the system delivering a mere 0.26 Amps at 4.08 Volts.

Experiment 2 data: 0.26A x 4.08v = 1.061 Watts

Photo 2: with the cheap USB cord in the circuit, 77% less energy is provided

 

Results and Analysis

Experiment 1, without USB Extension..: 4.704 Watts Delivered
Experiment 2, with the USB Extension.: 1.061 Watts Delivered

The results from this simple test show that there is a 77% reduction in energy delivered if the USB extension cable is in the charging path.

One explanation for this is that the USB extension cable is poorly constructed. Thin conductors would make the cable notably less expensive to manufacturer, but would result in higher electrical resistance. 

In particular, the short USB extension cable adds 10.5Ω of resistance to the charging circuit.  A well-constructed short USB cable would have a resistance of less than 1Ω.

Ultimately, this results in very slow charging times.

Other low-end USB cables, including cables with microUSB and lightning connectors, may also be constructed with low-quality conductors, leading to charging times that are much lower than expected.

Conclusion

It is impossible to say how good a cable is without directly measuring its qualities. However, shorter, thicker, stiffer cables are probably better in terms of charging speed. Thinner, longer, or highly flexible cables are far more likely to be lousy cables.

Only buy legitimate brand-name cables from a truly reputable source. Otherwise, you might be buying high-resistance cables that are not able to charge your phone as quickly as it drains. 

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