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I have a 15" 2019 MBP and want to be able to charge it at work without carrying my adapter and cable back and forth. At work I have a Dell OptiPlex 7050 and two Dell monitors (one is HDMI, the other is VGA/DVI). Unfortunately, the way the office is set up I don't have easy access to an actual plug into a wall....

So far I've tried plugging my USB-C cable from my MBP to the Dell desktop. With that, the computer recognizes it's plugged into a power source (gets the little charging icon and says "Power source: Power adapter") but also states that the "Battery is not charging" and the percentage continues to drop. So that didn't work.

Are there any other suggestions?

Would be helpful to get input on a few things:

  • Is there a way to get the USB-C from the Dell desktop to work?

  • Would USB-A from the Dell desktop with a USB-A to USB-C cable work any better?

  • Do all computer monitors allow pass through charging? What would be the best way to go about trying this (HDMI? VGA? DVI? Or USB-A?)

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Assuming that the monitors use an IEC plug on the backside, you can use an IEC cable splitter, like this

https://www.amazon.com/Server-Splitter-Power-Extension-Cable/dp/B01G1JMFZQ/

to grab the mains power from the monitor power cable and gain an extra power plug.

Then use an adapter like this

https://www.amazon.com/SF-Cable-Prong-Adapter-60320-C14/dp/B004OC579E/

to convert the extra IEC plug to the power plug of your choice (this is a US NEMA plug, but these adapters are available for any power plug standard – UK, Euro, Schuko and whatnot).

Keep in mind that in many countries you have to ask you employer first if you want to charge external devices at work, otherwise it w/c/ould technically be stealing.

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    I don't think most employers would be concerned about 'stealing electricity' but they might well be concerned about the security threat of connecting a personal device to a business computer via a data-capable connection such as USB.
    – nekomatic
    Aug 23 '19 at 15:30
  • This is not a question of concern but of legal requirements. I can only speak for Europe: Most employers obviously tolerate but you technically have to have permission. Btw. an IEC cable does not carry data, only power.
    – Gummibando
    Aug 23 '19 at 15:37
  • This gains you mains power, it still means you have to have a power supply & its attendant cable.
    – Tetsujin
    Aug 23 '19 at 17:38
  • And then what? The data from the computer transcends in a magical way via the USB-C PD cable and the power supply to the A/C line?
    – Gummibando
    Aug 23 '19 at 17:49
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The USB ports in the Desktop and monitors don't provide enough power to charge your MacBook. These ports tend to be build "to spec", meaning they output only about 500mA (USB 2.0) or 900mA (USB 3.0). Sometimes PCs have dedicated charging ports (maybe colored yellow) which provide more than 1A (they exceed the official USB specs), but that's still not nearly enough to charge a laptop. [1]

So you will need to get a charger (or a hub/dock[2]) that supports the "power delivery" (PD) protocol.

You can get them from anywhere between 18 and 100 Watts, and the power output makes a big difference in how fast your MacBook will charge. If you have patience, especially if you're not using the computer while it's charging, you can get away with a lower power (and smaller!) adapter (I use a 56W one, I would not recommend going lower than about 45W). Companies like Anker, Aukey, RavPower make some that are much cheaper and more compact than the original Apple charger.

[1]: The power delivery protocol allows the power supply to step up the voltage in order to provide more power at a reasonable current. The Apple 61W charger goes up to about 3A at 21V (the equivalent of 12.2A at 5V, or 24 times as much current as a USB 2.0 port can provide).

[2]: Note that some of the USB PD compatible hubs need a USB-C PD input in order to provide power on the output, so your best bet is to get a USB-C charger with Power Delivery and skip the hub. The ones that don't need a PD input tend to be (expensive) docks with lots of different ports.

Here are Apple's recommendations for how much power your charger should have: https://support.apple.com/en-us/HT201700

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  • So this would be a USB-A or USB-C device that I plug into the desktop computer but then also needs a separate power source (a wall plug) to increase the power output to make it enough for the MBP? Is that right?
    – Jon
    Aug 23 '19 at 15:18
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    @Jon No, you would only plug it into the wall and the MacBook. Here's the one I use. Aug 23 '19 at 18:58
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Is there a way to get the USB-C from the Dell desktop to work?

The short answer, no. It would be rare for any desktop to provide more than 15 watts to it's USB-C ports. Why this is comes with the long answer that follows.

USB-C ports are required by the USB standards body to supply 7.5 watts minimum. Most computers supply more, either 12 or 15 watts, so don't expect more unless the computer lists USB-PD support from it's USB-C ports.

Supplying more power from USB-C out of a desktop computer is likely quite trivial. Inside most any desktop computer today is a power supply capable of providing many hundreds of watts but there's no real demand for more than a few watts of power from USB-C ports on a desktop. Any large peripheral, such as a printer or display, will simply plug in to the wall for power. Any small peripheral, such as a mouse or keyboard, will happily operate on a few watts of power.

Inside a common desktop computer will be a number of devices built to run on a standard 12 volts, such as cooling fans and hard drive motors. It would be nearly trivial for a computer maker to run 12 volts to a USB-C port but, as it turns out, this would violate the USB-C spec. USB-C allows for 4 voltages to be output to the USB-C port, 5V, 9V, 15V, and 20V. I don't know why the USB people disallowed 12 volts output to a USB-C port, I can only speculate.

If 12 volts were allowed on a USB-C port then there is a 3 amp limit, that's 36 watts. USB-C only allows more than 3 amps when supplying 20 volts, again I can only speculate why but I consider this a relatively wise choice. Even if 5 amps were allowed at 12 volts then that's 60 watts. If you look at the power supplies for laptops of the last decade or three you will find that the power supplies are often rated for output of between 15 and 20 volts, and about 3 or 4 amps. This is likely why the people that designed USB-C chose 15 and 20 volts as standards, and considered 3 amps minimum for a passive USB-C cable and 5 amps for active cables. There's simply a ready market for this and all they needed to do was package this into their already popular data and power standard called USB.

Putting more than 12 volts on a USB-C port means needing a power supply in the computer built to provide the 15 or 20 volts that USB-C allows, and currently there's little demand for this voltage but for providing more power to USB-C, and there's little demand for desktop computers to provide more than 12 watts from a USB-C port.

Just to show how popular 20 volt power supplies are in computing look at any Lenovo laptop, desktop display, and many numbers of peripherals, that they produced in the last decade or three. They've been using a standard 20 volt supply for many of their products, first with a yellow barrel connector, then with yellow "SlimPort" connector that shares a lot of features with USB-A and USB-C. Other manufacturers did much the same though perhaps with not the vigor to consolidate their hardware to a single power supply standard that Lenovo or Apple had. The Apple MagSafe connector was popular and provided 20 volts like their USB-C chargers today. Apple abandoned MagSafe for USB-C fairly quickly, other computer makers will allow charging with their old power supplies in addition to the option of using USB-C.

Would USB-A from the Dell desktop with a USB-A to USB-C cable work any better?

No. This is likely going to get you the same 2.4 amps that USB-C provides. Some USB-C ports will provide 3 amps, as will some USB-A ports. To indicate the higher power they may have a yellow or red color to the plastic in the port. When comparing 7.5 watts, to 12 watts, to 15 watts input to a laptop that wants to see 30, 45, or 60 watts, there's likely not much to lose or gain when picking a port. They will all be slow charging.

Do all computer monitors allow pass through charging?

No. This is a rare feature, and often comes with a high price tag since such displays act as a laptop dock as much as a display. Along with a display and power will come things like a web cam, speakers, Ethernet port, USB-A ports, and perhaps other useful items like a display output for dual display action, or a flash drive slot.

Perhaps just as common as a display passing power to the computer is a computer passing power to the display. I've seen Lenovo computers fit in a slot in the back of a display with the display taking power from the computer, and the computer taking power from a "SlimPort" 20 volt power supply.

What would be the best way to go about trying this (HDMI? VGA? DVI? Or USB-A?)

There is no good answer. For most computers there's no output port that is going to supply more than 7.5 to 15 watts from a USB port. Even so called "Rapid Charging" USB expansion PCIe cards I've seen supply only 15 watts to a USB port.

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