These can be handy if you are concerned if everything (cables, ports, devices) are performing properly. I have similar devices for just type A ports and I have found them to be useful in diagnosing ‘dumb’ USB cable issues.
There are A-B cables which are sometimes called ‘charging’ cables instead of ‘data’ cables. They basically only have the two power lines and omit the two data lines. Other than to make them cheaper, I don’t know why they do this. They’re problematic in that even the oldest USB charging specs require the data lines to be present because the load has to signal that it wants all the current. This can be done with dumb devices by tying the two data lines together–no microprocessor needed.
To understand why this is a concern, a ‘proper’ USB host port (like you would find on a computer) is only supposed to provide 100mA of current (at 5V) to a device until the device ‘enumerates’ which is a process where the host will query the device for what it needs. As part of that, the plugged in device (called the target) can say how much current it needs. That’s up to 500mA for the original USB spec (up through USB2.0). A proper host also has a programmable current limit device in the power line of each port. This allows it to set current limits for downstream devices which the hardware will enforce, so if a target attempts to use >100mA before enumeration, the current switch and turn it off and signal a fault to the host. The host can detect that as an error and try to power cycle the device. Not all hosts are this pedantic, but they can be and would be completely inside the spec.
But, there was a ‘battery charging’ spec added at some point. To enable it the target can tie the data lines together and the host is supposed to detect this and proivde up to 2.4A, IIRC. ‘Charging’ cables don’t have the data lines and could therefore limit charging to 100mA or 500mA depending on how strict the host is. Or a lazy host design just may have some higher current limit that its designer felt like using.
So, you can run into a lot of situations where you will have no idea what kind of charge currents you’ll get and a device like this is good to see what’s really going on. Hope that helps.
