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When two 2016 MacBook Pros are connected over USB-C, can they transmit power? Could an adapter look like a phone from one side but like a charger from the other, so that it would convince one MacBook to power another?

How does the 2016 MacBook Pro know to draw power when connected to a charger, but to provide power when connected to a phone? I see that wattage is negotiated when they are first connected, but is who-charges-who also negotiated??

8

Yes you can. I finally figured out how.

Use a dongle and hook them up like this:

power supply > usb-c cable > mbp > usb-c cable > usb-c multi-port dongle > mbp2

I've tested it with the UpTab displayport with power delivery adapter and with the Apple multiport adapter (A1621).

In my experience, 15w is enough to power and charge the 15" mbp. Here are discharge rates I've observed on my laptop:

  • Idle w/min screen brightness: 4.25w
  • Idle w/max screen brightness: 7.7w
  • Chrome open, no tabs, max brightness: 8.6w
  • Chrome open, 2 gmail tabs, 1 pandora tab, max volume: 16.5w
  • Chrome open, playing 1 YouTube video, min brightness: 9.4w
  • Chrome open, playing Spotify, min brightness: 6.05w
  • iTunes open (Chrome closed), playing song, min brightness: 6.1w

The second mpb will only receive 15w from the first. An app like coconut battery can help you see how quickly your laptop is charging.

Using this method, you can even trick your laptop into thinking that it is charging itself. (And then you could upload that video on YouTube, claiming to discover infinite energy, and profit on the ad revenue)

The OEM power supply for the 2016 15" mbp has 3 output modes:

  • 4.3A @ 20.2v = 86w
  • 3A @ 9v = 27w
  • 2.4A @ 5.2V = 12.48w

So even the OEM power supply will occasionally supply only 12.48w to the mpb.enter image description here

Update: After playing around with this more, the above method will work if you want to charge 2 identical devices. But if you have a 15"mbp and a 13"mbp, it seems like the larger device always supplies power to the smaller device. I don't have one available currently to verify exact charging watts. Charging a 15"mpb from 0% to 100% battery takes about 8 hours at 15w. This assumes that the laptop is off or is sleeping. In a pinch, I've worked 8 hours with my 15"mbp plugged into my 18w usb-c phone charger. I was able to maintain the battery's charge during light to medium load. The move to usb-c for power has been awesome. You could conceivably charge your laptop with the 5w iPhone charger, but it would take about 15 hours to fully charge while shut down (assuming a 75wH battery and 5w charge rate and no inefficiencies due to charging), I'll have to test this out tonight to verify that it works.

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Technically it would be possible to trick the MacBook Pro to think another device is actually a different device to what it is (e.g. that a connected MacBook is in fact an iPhone).

However, this would not achieve the goal of charging one MacBook from another MacBook. According to Apple:

MacBook Pro (15-inch, Late 2016) and MacBook (13-inch, Late 2016, Four Thunderbolt 3 Ports) can power two devices that use up to 15 watts, and two more devices that use up to 7.5 watts. It doesn't matter which side of your MacBook Pro that they're connected to—power is delivered on a first-come, first-served basis.

Source: https://support.apple.com/en-us/HT207256

MacBooks typically require in the range of 55 watts to 90 watts of power in order to charge (depending on the model, e.g. the late-2016 13" MacBook Pros require 60 watts, while the 15" models require 85 watts).

As a result, the amount of power being delivered from the MBP via it's ThunderBolt ports would not be enough to charge another MacBook.

  • That page says "Using a power supply that doesn't provide sufficient power can result in slow or delayed charging. It's best to use the power supply that came with your Mac.". I've used a 2013 Macbook Air 45W charger to charge a 2015 15" Retina Macbook Pro, which ships with a 85W charger, and it works, albeit slowly. – pjvandehaar Nov 21 '16 at 1:57
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    Yes, using a 45W charger to charge a device that should have an 85W charger will result in a slow charge (as you've found). However that 45W charger provides about 53% of the recommended wattage, while the max amount of watts a late-2016 MBP can deliver via a single ThunderBolt port is 15W. That 15W is barely over 17.5% of what is recommended to charge your 2015 Retina MacBook Pro. This is likely to not slowly charge, but at best offer delayed charging (I suspect 'never' charge). Also, over time it's quite likely you will damage your MacBook Air charger. – Monomeeth Nov 21 '16 at 2:15
  • I have charged my 2016 15" macbook pro with my 15w google pixel phone charger. So technically it should be possible to charge a mbpr with another one. In my experience it slowly charges the laptop while in use. I've used an app to monitor the watt draw/charge and usually the laptop charges with 1 - 8 watts while in use. Since the battery is a 75wh battery and since the laptop has an advertised life of 10hrs, then average battery draw would be 7.5 watts per hour. – Jared Menard Sep 20 '17 at 18:18
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    This answer isn't even close to right, USB-C allows for bi-directional charging. The Macbook Pro is configured as a Dual-Role Port (DRP). If two identical DRP devices (both try.sync or try.source) are connected together which is configured as sync and which is source is random. Nonetheless the MacBook Pro charges fine at 12W/18W/etc. It won't be fast but it's certainly doable. – crlanglois Dec 4 '17 at 18:36
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I was missing my primary charger for my 15in MacBook pro for a couple days, and I successfully charged it with the charger for my USB-C tablet. I made sure to only have it plugged in when it was off, it was definitely slow, and I do not know the exact power stats from the charger, but it definitely does not take a full 85 watts to charge a 2016 MacBook Pro 15". I would not recommend a solution like this with any frequency, as it's probably not great for the battery.

  • I just wanted to re-enforce this isn't something you'd want to do with any frequency, as it's likely to cause long-term damage to the tablet charger. – Monomeeth Aug 29 '17 at 22:25
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Yes. Just use a good-quality TypeC-TypeC cable to connect the two Macbooks. I use one from Apple Store and also a Surface Book cable. The trick is, the USB-C cable is directional, although it may look symmetrical. If it doesn't seem to work the way you intended, reverse the cable and connect again.

I routinely use my Macbook Pro 15inch 2016 to charge my colleague's New Macbook during meetings and I also once charged another Macbook Pro (13inch) during a hackathon. The output doesn't always cover consumption so they may still lose battery life, but it does slows down power drain.

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