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I have an M1 MacBook Air with its original power supply/charger and original USB-C cable. All working great.

I also have a cheap Android phone (Redmi 10C) with USB-C.

Sometimes when I'm on the road I need to charge both. I usually swap the cable between the two devices since the Mac's charger is beefy and good quality and its original cable should also be good quality. This also works great.

But I thought it would be handy to be able to charge both at the same time instead of swapping the cable over. So I bought a USB-C to USB-C cable rated at 100W.

To my surprise, when I connect the phone to the Mac with the new cable when the Mac is already charging using its original cable, the result is that the phone also seems to be charging the Mac!

I tried unplugging the cables and plugging them back in different orders and it made no difference. I couldn't see any options on either device, but might not know where to look.

To my greater surprise, I then connected the Mac to the charger using the new cheap cable and the phone to the Mac using the Mac's original USB cable. This time the phone started charging!

My question is: How to understand what is happening? Is the Mac detecting the properties of the two cables somehow? Is there a setting on one or both devices to enable me to set the flow direction? Is it just a fluke and might flow in the other direction next time? Is there a specification somewhere for how this works/should work?

Here's the USB cable notification as given by my phone when I connect the phone and MacBook:

The USB cable connection notification on my phone

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    I think you's right, this'd be a better fit for Ask Different. Cross-posting is generally frowned upon. Mar 22, 2023 at 9:55
  • @Peregrino69: I went with here since it was both an Apple question and an Android question and often Apple questions attract non-technical answers. I also anticipated that posting on the Apple site might get "You should've posted to the Android site" responses (-: I don't think there's a way to migrate it without deleting it here, which also doesn't seem ideal. So I might see how it goes for a couple of days? Mar 22, 2023 at 9:59
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    LOL yeah, Apple fanboys might do that - and Android Enthusiasts might kick it off as an Apple issue :-D I'd probably use the 100W charging cable on the laptop and cheapo for the phone :-) Mar 22, 2023 at 10:01
  • So far they're both charging. The phone seems to be charging faster and I'm interested to see if the Mac will start charging faster when the phone hits 100% Mar 22, 2023 at 10:04
  • Sometimes it works using the Apple cable to the MacBook and the cheap cable from there to the phone, but almost always it needs the other way. I haven't discovered what the differentiating factor is. Could be which port I use or the order I plug the cables in, but I suspect it's how much charge each device has. Apr 4, 2023 at 13:47

2 Answers 2

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The solution: pull down the notifications on your phone and tap the one related to cable connection. Then change the charging direction.

The new cable probably has a different set of wires. USB-C promises a single connector for everything but unfortunately not that it will always work - you need support for the feature you need in both devices and the cable. Cables aren't required to be wired fully.

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  • Unfortunately my phone doesn't offer to change the charging direction in the cable connection notification. It only offers file transfer or photo transfer \-: Mar 24, 2023 at 13:55
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My question is: How to understand what is happening? Is the Mac detecting the properties of the two cables somehow?

The Mac is detecting the properties of the cables.

USB-C cables will have a resistor network or small IC to indicate to connected devices the capabilities of the cable. A 60 watt cable will have a resistor network or C to tell connected devices it is capable of up to 20 volts and up to 3 amps. A 100 watt cable will have an IC to indicate up to 20 volts and up to 5 amps.

Is there a setting on one or both devices to enable me to set the flow direction?

I'm not aware of any setting to specify the flow of power. By knowing some of the rules on USB there's ways to at least indicate preferences on the flow of power.

Is it just a fluke and might flow in the other direction next time? Is there a specification somewhere for how this works/should work?

This does sound like something of a fluke since the phone should not take precedence over a power brick on supplying power to the MacBook. There are specifications on how this works, Apple has resources on their website laying out the rules their products follow on charging and USB has published specifications on how certified products should function. I have to wonder if your cable and phone are following the spec.

Perhaps the first and most important rule to know is that Apple devices will pull power from only one source at a time, and will always choose to draw power from the device advertising it can provide the most power. I'm not sure what the Apple rule is on if there are two power supplies of equal power connected, perhaps it draws power from the first device connected or there's some randomness to it.

My best guess on what is happening is somehow the phone is reporting itself as capable of providing more power than the power brick. While that is unlikely it is possible. I have to wonder if the Android phone is somehow lying about its ability to supply power, the non-Apple cable is somehow interfering with the power negotiation, or both.

One possible way to specify the flow of power is to do what you happened upon in your experimentation, use the cheap cable to connect the MacBook to the power brick and the Apple cable to connect to the phone. My guess though is that the direction of power flipped not because you moved the cable but because you disconnected everything and then reconnected.

Another rule to keep in mind is that when connecting two USB-C devices, and both are capable of supplying or sinking power, then the flow of power is picked at random. But to reverse the flow of power all one needs to do is disconnect the cable for a short time and then reconnect. There's a short memory on the direction of power flow, and it is the process of disconnecting and reconnecting that changes the flow of power. It is this behavior that can give the impression of moving cables about or flipping a cable around to direct the flow of power but it is just the disconnect and reconnect that is flipping the power flow.

As far as I know the USB-C charging cable that Apple includes with their devices are USB 2.0 20 volt 5 amp versions. Maybe that has changed with the new Apple products and the update to the USB-PD spec but last I checked all Apple "charge only" USB-C cables are rated for 100 watts. I use "charge only" in quotes because that is what Apple calls them in spite of being fully capable of USB 2.0 data, my guess is they use that nomenclature to discourage tech support calls on why data transfer is slow. If what I suspect is true then it is a bit odd that the MacBook would prefer to draw power from the phone than the power brick. If both cables are rated for 100 watts, and meet the USB spec, then the MacBook thinks the phone can supply more power. It is possible that the phone can provide more power than the power brick but unlikely unless the power brick is one of the 18 or 20 watt bricks.

Here is how I solved this problem...

USB-C can supply power in both directions but USB-A cannot. By pairing a USB-C to USB-A adapter with a USB-A to USB-C cable I can connect two USB-C devices together and dictate which device supplies power by which device has the adapter connected. This will limit power to what USB-A can handle, which is 12 watts, but given the situation I suspect this is sufficient.

Since a power brick will only supply power, not sink it, there's no directing power flow with a USB-A connection in the middle. It will supply power or not. It will limit power flow to 5 volts at 2.4 amps, and some people do this as they believe a slower charge will extend battery life. I have my doubts that is all that effective, and with better battery management software in phones and such there's likely no longer any need for such hacks.

Perhaps I digress too much with unnecessary detail, what the point is that the power flow is limited by the weakest link. By intentionally putting in a weak link you can direct how the power flows with your USB-C devices. I got a USB-C to USB-A adapter for $8, and a USB-A to USB-C cable for another $8, so for $16 I know I'm not going to have power flow from my iPad into my MacBook while my MacBook and iPad is plugged in to charge. I don't know what the adapters and cables sell for now but the point is that the fix is something relatively inexpensive and readily available.

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