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Can I feed another MBP by USB-C cable from a port on the first laptop?

Power packs can deliver 1.5A (at 220V = 330W) over USB-C to the MBP. MBP power requirement is 4.3A (@ 20.2V = 87W). So seems the power pack can handle it.

Ah but port delivers 900 mA (Milliamps) at 5 V = 4.5W. So guessing answer is no. But my amateur electronics may be wrong so confirmation appreciated.

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  • 1
    This is a good question but both the calculations in the question are based on faulty reasoning. I don't know where the 1.5 A figure comes from but USB-C chargers don't deliver power at 220 V - the mains input might be specified at 220 V, 1.5 A but that doesn't tell you what the output will be (and it doesn't even mean the input power is 330 W, because of power factor with AC current). Meanwhile the 900 mA figure is for a standard USB 3.0 port, not a USB-C port with Power Delivery.
    – nekomatic
    Jul 10, 2018 at 13:10

5 Answers 5

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Allan's answer is not correct, I was able to daisy chain my Mac. I am currently at a conference without my charger and I have daisy chained my Mac to another Mac (both USB-C 2017 Mac Book Pros). Both Mac's are closed, so there is not much variance in usage as both are idle. I opened the screens and checked the power level on both every 10 minutes. The second Mac charged much slower and seemed to charge faster once the first Mac was fully charged, but it still seemed slower than normal to charge.

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| Time    | Mac 1 | Mac 2 |
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| 0  mins | 81%   | 22%   |
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| 10 mins | 89%   | 24%   |
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| 20 mins | 96%   | 26%   |
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| 30 mins | 100%  | 29%   |
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| 40 mins | 100%  | 33%   |
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| 50 mins | 100%  | 37%   |
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| 60 mins | 100%  | 42%   |
---------------------------
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  • 3
    Empirical evidence beats theory.
    – geotheory
    Jul 11, 2018 at 9:52
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    FWIW I just tried this with two 2017 Macbook Pros, and they did not charge while being used.
    – tom
    Jul 10, 2019 at 20:42
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    @tom could be, I had both closed.
    – FMD
    Jul 11, 2019 at 21:24
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    Seems that the conference was not very interesting if you had time for those experiments ;)
    – hans
    Sep 17, 2020 at 13:11
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The USB-C Specification specifies power delivery of up to 100W.

Increased power levels from existing USB standards up to 100W.

You won't be able to "daisy chain" or more technically, charge MacBooks in series as 100W will be less than the required 174W (87W * 2). However, you will be able to charge them in parallel if your adapter has the capability to do so.

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  • USB Power Delivery means that the device and the charger can negotiate the voltage and current that will be supplied and drawn. Whether two MacBooks can both charge when daisy chained depends on exactly how the MacBook implements the Power Delivery spec but the reasoning behind your calculation is not correct - for example the upstream MacBook's port could tell the downstream one 'you can draw up to 29 watts' and the downstream one should obey the instruction.
    – nekomatic
    Jul 10, 2018 at 13:05
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I'm doing it right now. I have an 87W wall plug plugged into a macbook pro 16 and then another usb-c cable running to a 2018 macbook pro 15. System Information reports 87W on the 16 and 15W on the MBP15.

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You can do this (I'm currently charging a MBPro while running another off of it), however not all USB-C ports on the MBPro are the same.

I was only able to daisy chain using the left-hand ports. There have been Thunderbolt 3 performance differences between left and right ports on some models, so it seems there are also power delivery differences as well.

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    Interesting. I've found on two successive 2016 MBPros the right hand ports have all stopped working for recharging. In fact for both macs only the rear end left hand port ended up working.
    – geotheory
    Dec 28, 2019 at 23:11
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I support @Fasani 's answer.

After tests of daisy-chaining newly bought MacBook Pro (model A1990 EMC 3215) with ASUSPRO (b9440UA) I concluded that you can't daisy chain 20V USB-C power supply, however you try.

But

MacBook Pro is very aggressive in getting whatever power is available to it, so it will eagerly use 5V if that is all it can get. Of course, with reduced wattage. And of course, daisy-chaining USB-C must deliver at least 5V, because it is in the minimal USB-C specs.

ASUS B9440UA tolerates only 20V power, so no daisy-chaining those.

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  • How do you charge a 12v battery with 5v?
    – xaxxon
    Nov 20, 2019 at 7:11
  • " How do you charge a 12v battery with 5v? " By using a "boost/buck" power supply that takes the 5 volts in to supply 12 volts out. The power in on the 5 volt side will be equal to the power out on the 12 volt side plus losses to resistance and such. The USB-C ports on a MacBook Pro can supply 15 watts at 5 volts so the battery will see as much as 15 watts minus conversion losses from the battery charger. The reverse is true with 20 volts coming in on the port but its operating in "buck" mode than "boost" mode to lower the voltage.
    – MacGuffin
    Jan 31 at 0:03

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