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I have a 2018 iPad Pro and a recent MacBook Pro (with USB C). Is there any way I can control what charges what when I connect the two? If I connect the two with a USB-C charger, my MacBook automatically starts charging my iPad. Can I have it work the other way around? Can I have it not transfer power, and just be used to transfer data (like for Sidecar)?

This poses a more general question about USB-C: How can you control what direction power goes? I have a USB-C Battery that automatically charges my laptop or iPad when I plug it in, but as soon as the battery dies, the MacBook/iPad will start charging the battery!

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While the iPad Pro can charge other devices off of it, including other iPad Pro devices, I see no indication that charging a device like a MacBook is possible.

I don't see any feature in which you can disable power transfer while connected via USB-C, like while using Sidecar in a wired configuration.

Apple does have instructions on how to charge another iPad Pro from the iPad Pro, including how to select which iPad Pro charges which:

If you have two iPad Pro models with USB-C ports, you can connect them directly with a USB-C cable. One iPad Pro will draw power from the other. If you want to change which iPad Pro charges, unplug the USB-C cable and then reconnect them.

You could try the above with your MacBook and iPad Pro, but it's unclear if that would work.

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This poses a more general question about USB-C: How can you control what direction power goes?

Yes. Apple computers and devices have a kind of short memory on which direction the power flows. If you plug two Apple devices together with a USB-C cable and the power is flowing the "wrong" way then unplugging one device and plugging it back in will reverse the flow of power. I heard that in some cases if left too long the computer or device supplying power will stop supplying power, as the battery runs low, and ask for a charge.

One sure way to force direction of power flow is with a USB-C to USB-A male cable plugged into a USB-C to USB-A female adapter. The power will flow from the side with the female port to the side with the male jack.

The level of power flow should be no different with either setup as the USB-C ports will supply only 5 volts, and the maximum current will be 1.5 amps or 3.0 amps depending on the hardware and how much power other things are asking for.

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