MacBook – How to find out which USB Type C port is supplying power

From the article, The iPad's USB charging caveat:

[Originally posted on Neowin.net by giga on 04 April 2010 12:26]

The iPad specifications page clearly state the ability to charge with USB (with no mention of OS requirements) but Apple have additionally provided a support document disclosing that "some USB 2.0 ports and accessories do not provide enough power to charge iPad."

Providing some further clarification to Macworld, Apple asserts that USB charging is supported but that it varies according to the state of the iPad and the power output of the USB hub:

• For the fastest charging, use the iPad’s included 10-Watt USB power adapter. This will fully charge the iPad in a few hours, even if you’re using the iPad at the same time.
• When connected to a high-power USB port—such as the ones on recent Macs and the iPhone Power Adapter—the iPad will charge, even during use, but more slowly. (We haven’t yet determined how much more slowly.) Some third-party powered USB hubs provide higher-power USB ports, but many don’t; similarly, the USB ports on most Windows PCs don’t provide this additional power.
• When connected to lower-power USB ports—those on older Macs, most Windows PCs, and most USB hubs (powered or unpowered)—the iPad’s battery is not charged while the iPad is awake, but is charged (again, slowly) when the iPad is asleep. What’s confusing here is that the message "Not charging" appears in the menu bar when the iPad is awake, which might lead you to assume that the offending USB port can never charge your iPad. But rest assured, Apple says: once you put the iPad to sleep, the battery will indeed charge. (If you could see the screen while the iPad was asleep, it might even display the charging icon. It’s the modern-day “Does the refrigerator light stay on when I close the door?” mystery.)

Further analysis from Apple's statements indicate that the iPad can only be charged off of USB connections that provide near 1 amp of current. The majority of USB ports on most older Macs and PCs only provide 500 mA and thus can't be used to charge the device.

The USB 2.0 specification provides 500 mA of current to high-powered buses and 100 mA for low-powered buses. But in April 2009, the specification gained a new "Battery Charging Specification" which provided additional current to high power devices and it seems that Apple has started to incorporate it in their newer Macs.

It's difficult to tell if you have a hardware issue or this is just how things are designed.

Have you read through this article on how Apple detects the first device that requests higher power and enables that power on some, but not all ports?

• Apple Computers and Displays: Powering peripherals through USB

I know you have linked to it, but the 500 mA is subject to measurement errors on the computer side - so plugging in a device can draw more current than the device says it should. Similarly, the measurement side where the Mac determines the actual power draw could be flakey or failing. This sort of thing isn't tested in the system diagnostics. You would need a test set / oscilloscope / engineering lab to really measure these currents with enough resolution to pin down exactly what is happening.

It could be just the order in which the system decides to detect a device attached. Once you have ruled that out and have a repeatable issue - you might need to have the Mac checked for service if it isn't producing power as expected. Now that you know that the order in which devices are plugged can affect things, perhaps this will help narrow down if the enhanced power is correctly available to some (changeable) subset of the ports and hubs made by Apple.

There is no easy answer other than being precise and narrowing down exactly which devices are causing errors and statistically determining/isolating whether it's the devices or the port that is out of spec and needs to be replaced.