Not all USB cables are created equal. Charging your phone using a cheap high resistance cable takes for ever because large part of the energy gets lost in the cable as heat.

This is a simple micro and mini USB break out board, that allows easy resistance measurement by passing a current through the cable and measuring the voltage drop across the cable.

What do you need

• An (adjustable) power supply between 1V and 2V capable delivering 1A (less is OK too)
• A multimeter to measure voltage in mV
• A USB cable to test

How to test

• Plug both ends of the USB cable under test to the board
• Set your power supply to 1V-2V and connect the power supply to the board
• Adjust the voltage on the power supply so exactly 1A current passes (or set current limit to 1A)
• You can check the current by measuring the voltage across the 1 Ohm resistor at the Calibrate terminal, you should get 1000mV
• Measure voltage across the cable on the Measure terminal, 1mV corresponds to 1mOhm if you use 1A current
• Both measuring terminals use Kelvin Connection for accuracy.
• Caution: The resistor gets hot
• Note: Do not plug any power supply or charger to the mini or micro USB port, those ports are shorted
• Note: You are measuring round trip resistance of the USB cable including connectors resistance

What resistance to expect?

After measuring all of my USB cables I found that there is a huge difference between good cables and bad ones. Good 20 AWG cables have resistance between 150mOhm and 250mOhm depending on the length of the cable. Really cheap ones can have resistance over 1000mOhm, that's a voltage drop 2V if you try to pass 2A current through them, no wonder you can't charge anything with them.

Optional Alligator Banana Clips

Convenient Alligator Banana Clips are available for purchase for extra.