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Apple's controversial decision to convert all the ports on their 2016 MacBook Pro lineup to USB-C ports contributes to blur the difference between ports that supply power (such as a power adapters) and ports that can power external devices (such as USB keys, mice or other gadgets).

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This made me think about the following scenario: you sit at a table where your laptop is plugged in on the right, and don't notice that it is plugged in, so you decide to charge it with a second nearby charger that is laying around (i.e. belonging to someone who lives with you and who recently sat at the same spot).

What would happen then?

Would the computer charge twice as fast, would one of the chargers be dismissed by the system (which one?), or would it generate a surcharge that would destroy the computer?

I couldn't find anything about this online, but since I am always trying to force a USB in the MagSafe charger of my MacBook Air, I think that I could easily end up making this kind of mistake.

Note: My question asks what happens when two chargers are inserted into a MacBook Pro, but I am also wondering what would happend if 3, or 4 chargers were inserted. Although this scenario is unlikely to happen by mistake, it would constitute an obvious vulnerability to an exposed device, i.e. in a public space.

  • 33
    I wonder why they kept the headphone jack. – immibis Nov 3 '16 at 3:12
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    @immibis Courage. :-p – ceejayoz Nov 3 '16 at 14:27
  • 2
    @immibis undoubtedly an oversight, or perhaps some technical reason. I'm sure they'll remove it in the 2017 model. – Doktor J Nov 4 '16 at 14:32
  • 1
    2018 model here and still has jack :o – Petr Dec 18 '18 at 20:26
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    Unanswered here is what happens when two smaller power sources are connected at the same time. I have left my charger at the office and am using an iPad charger instead, two of them. Doesn't appear to be helping, but I am unsure I could tell. – ktamlyn Feb 26 at 14:54
110

The system will choose the power source that provides the most power, and it will not draw power from the others.

Apple has released a support article describing this:

  • Your MacBook Pro draws power from only one power supply, even if more than one is attached—so using multiple power supplies will not speed up charging.

  • If you connect multiple power supplies to your MacBook Pro, the one that provides the most power will be used, regardless of the order in which you connected them.

  • You should not connect any power supply that exceeds 100W, as it might damage your Mac.
  • 16
    huh. now i am wondering why it won't just draw power from the power source that provides the most power under 100W... – Michael Nov 3 '16 at 1:07
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    Or why it won't only draw 100W from the charger that's capable of supplying more than 100W. – immibis Nov 3 '16 at 3:11
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    Or maybe it can handle more than 100W they just want to be off the hook in case something goes wrong. – Roman Reiner Nov 3 '16 at 6:07
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    Adapters don't "push" the power to the device. – dunni Nov 3 '16 at 9:49
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    @JJJ If that were true, every appliance in your house would fry as the outlet "pushes" 1800W. – Random832 Nov 3 '16 at 22:51
-1

They don't push power (watts) but they do push VOLTAGE. And if a charger pushes the wrong voltage, bad things can happen to your device.

The voltage is negotiated between devices (e.g. charger/mac), but if the charger is non-compliant, bad things can happen.

  • 1
    For everyone downvoting Craig W, the mathematical formula is Volts multiplied by Amps equals Watts (V*A=W). In other words, they push voltage, and too much voltage will cause an increase in Watts, damaging the equipment. Linus Tech Tips explains this much better than I can in a TechQuickie episode here: youtube.com/watch?v=mvuHsu8S6v8 – Joshua Richards jr2 0x6A7232 Nov 24 '17 at 23:12
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    Downvoted because this is not an answer to the question and should have just been a comment on the actual answer. – dossy Sep 26 '18 at 20:59

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