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Comment by emgeee | Hacker News Reader

Anecdotally I was able to get this command to return 8 watts by using a usb-A -> usb-C cable meant for charging a mouse.

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That sounds like you're hitting the limit from non-charging ports. Normal A to C should allow much more. But either way that's a special case here, because the limits are entirely on the USB A end. That's not a proper C cable.

An A-to-C cable is limited by what an USB-A host can supply, since that's the limit which a compliant C-device sensing an A-to-C cable or adapter would observe.

Depending on the combination of host and device, that would be 4.5W for standard USB 3, 2.5W for USB 2, up to 7.5W for USB Battery Charging (BC), or possibly more for non-USB-C PD (highly unlikely, since it would require legacy PD signalling on the voltage pins, which almost no USB-C devices support as far as I know).

I strongly suspect it's USB BC, rounded up to 8W by Apple.

As an aside: Apple's USB power controllers used in their laptops are surprisingly flexible based on my observations: In addition to the obvious USB-PD, they support at least USB BC and Apple's proprietary USB-A resistor-based protocol. I was able to successfully charge a MacBook Pro at 12W and even 5W supplied by an old iPad/iPhone charger and the appropriate combination of adapters once.

Great post thanks for the information. Seems to verify my experiences.

I’m actually able to run my m2 MBA from a 5w usb a iPhone charger. I thought that was cute.

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