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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.

MicroMachine
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3 Answers3

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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 (and another one) 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.

Community
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Alan Shutko
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    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 03 '16 at 01:07
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    @Michael Maybe the circuitry that detects the wattage burns out at 100? –  Nov 03 '16 at 02:55
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    Or why it won't only draw 100W from the charger that's capable of supplying more than 100W. – user253751 Nov 03 '16 at 03:11
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    It looks like the USB standard only goes up to 100W, so anything higher than that is fudging the rules a bit. So it's possible that a charger that is fudging the rules could do so in a way the MBP couldn't handle? – Alan Shutko Nov 03 '16 at 03:18
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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 03 '16 at 06:07
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    The hardware can't just "draw 100W" from an adapter that's pushing more power than the components can handle. If the adapter is supplying 200W, the internal hardware that limits the power input to 100W must be able to handle 200W as it's the one that's taking the full hit. – JJJ Nov 03 '16 at 09:43
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    Adapters don't "push" the power to the device. – dunni Nov 03 '16 at 09:49
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    @JJJ If that were true, every appliance in your house would fry as the outlet "pushes" 1800W. – Random832 Nov 03 '16 at 22:51
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    I suspect that the 100W "limit" is more business that technical - the top apple adaptor is 87W and they don't want people using third-party adaptors. – zelanix Nov 04 '16 at 13:34
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    or to put it another way, saying that 'you should not connect any power supply that exceeds 100W, as it might damage your Mac' is the same as the old statement that 'you should not connect any non-apple power supply, as it might damage your Mac'. Claiming that wattage has anything to do with it just makes it sound more convincing. – zelanix Nov 04 '16 at 13:47
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    If you look carefully at various MacBook power supplies, you'll note that the higher wattage supplies often have a higher voltage. I suspect that the >100W power supplies may output more than the (up to) 20V that 85W supplies provide. While the USB-C PD specification dictates power negotiation, Apple may have this "exceeds 100W" warning as a disclaimer due to cheaply-made power supplies that may not negotiate correctly and just slam >20V. – Doktor J Nov 04 '16 at 14:45
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    But what happens if you plug 4 identical chargers? Which one is used? And then if you unplug that one that is used, then what? – MicroMachine Nov 04 '16 at 15:44
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    @fabriced it probably polls them when they connect, and if subsequently-connected supplies are the same power as the first (which it would be using at that point) it would continue using the first. If that first one is disconnected, I imagine it'd poll the connected ones again (or possibly have "cached" the capabilities of them and simply refer to that) and select another, probably based on port order or some similarly opaque mechanism. This is all conjecture, though :) – Doktor J Nov 04 '16 at 16:01
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    Pushing >100W probably just damage the battery or shorten its lifespan. Also voltage has nothing to do with it AFAIK, motherboard contains a buck-boost converter circuit that'll convert voltage to what it needs. – Sarim Nov 04 '16 at 16:17
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    The USB Power Delivery Specification only goes up to 100W. Anything that can provide (it doesn't push) more power is not USB-compliant. Therefore, all bets are off on what happens. – penguin359 Nov 04 '16 at 18:29
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    @Sarim as mentioned above, the vast majority of power supplies do not "push" power, but rather "make it available" (that would be Very Bad in most circumstances, e.g. if your genuine Apple adapter shoved ~80 W into your computer with a fully charged battery, problems would ensue). – Nick T Nov 04 '16 at 19:58
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    @Sarim Pushing power would be the same thing as raising the voltage in order for more current to flow (watt=volt*ampere). The computer will have a certain capacity to raise its resistance to meet various voltages without a crazy amount of current flowing through it but once the resistance is at its limit the voltage will cause too much current to flow, making conductors heat up and break. Saying power is pushed and voltage has nothing to do with it seems very dangerous to me. – Daniel Schlaug Jan 21 '17 at 14:11
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    @DanielSchlaug i suppose you are right. If you provide 1000V to an 10-15V device, it'll definitely constitute as pushing power by raising voltage. But my point was more like between 5V and 20V, it doesn't matter that much. Also apple's power circuits are more complex than just being able to "raise its resistance". As far i can remember, they had specialised ICs for buck-boost, voltage-current detection, and MC(s) controlling all those. So i guess if you continue to increase voltage, the circuit will try to adjust the duty cycle and keep the volt,amp to what it needs. – Sarim Jan 21 '17 at 17:12
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    Is Apple saying its safe to connect multiple power adapters? I currently have two USB C cables at my desk for two monitors. One carries power but I have other usb devices and id like them both to carry power so when my laptop isn't there I can charge them. If I use the included apple USB charger for the 13 inch and a 30watt USB C charger will it fry my Macbook Pro? After watching this video I am especially cooncerned https://www.youtube.com/watch?v=-rvpq8LopLU&t=345s –  Sep 26 '17 at 02:55
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    Apple is saying it is safe to connect multiple power adapters, and it will charge from the one that offers the most power. I did not look at the video much but I noticed they were using a USB Killer, which is certainly unsafe. – Alan Shutko Sep 27 '17 at 13:32
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    Does this mean you could plug in multiple portable chargers and it will go through them sequentially - switching from one to the next most powerful one once the more powerful one runs out of juice? – Mark Jan 13 '19 at 07:49
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    @ChristianJuth Yes. I regularly use two LG MD5KA monitors which both do USB-C charging, so I can use devices plugged into both of their built-in USB-C hubs. – deed02392 Apr 03 '19 at 10:58
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    The linked support article seems to have had the verbiage you quote removed sometime prior to August, 2017. Now it's just talking about identifying ports and adapters. The only references to power or charging make no mention of using multiple chargers. It does link to another support article about power adapters, but that doesn't say anything about multiple power supplies either. – 8bittree Jul 24 '19 at 17:52
  • Where is this quote from? – Max Coplan Feb 26 '21 at 19:23
  • Actually it draws power from both. I am using Soopii USB-C cables to measure it, and currently my 2019 MBP is drawing 32W from my 65W supply and 7.5W from my 30W supply, with the battery 100% charged. – David M W Powers Mar 23 '23 at 05:27
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    Note that I routinely have two charging connections, as with one there is a tendency for the port to overheat (when the battery is low and it is charging) and then to shut down the port (particularly if using left-side USB-C ports to charge) and eventually the MBP (without warning). The left rear is the hottest part of the MBP and the most susceptible. It can sometimes take a resetting of the system management controller or the NVRAM to get the port going again (viz. port shutdown can survive normal boot). – David M W Powers Mar 23 '23 at 05:32
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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.

Craig W
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    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: https://www.youtube.com/watch?v=mvuHsu8S6v8 – Joshua Richards jr2 0x6A7232 Nov 24 '17 at 23:12
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    This is fear mongering. A USB charger will not "provide too high voltage" unless Urs broken. – Michał Leon May 07 '23 at 12:01
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This person tried it and it lost its ability to charge. Given this Apple support page that Alan Shutko linked in his answer, however, my guess is that this damage would be covered by the warranty.

Sparkette
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    Unfortunately, the linked support page doesn't actually say anything about using multiple chargers, so you might not have much luck basing your warranty claims on it. – 8bittree Jul 24 '19 at 17:41