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Can I use 87W USB-C Power Adapter for MacBook Pro to safely charge a Samsung Galaxy S9 device? I'm curious as on the charger it says the output is 5.2V, while the phone seems to require 5V.

The specs of the Mac's charger can be seen on this image:

enter image description here

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According to Wikipedia the USB specification is:

USB supplies power at 5 V ± 5% to power USB downstream devices. (source)

5% of 5V are 0.25V

Therefore any USB device have to be able to work correctly as long as the "received" power via USB is within the range of 4.75V to 5.25V.

The Apple USB power supply with 5.2 Volt is within this range.

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  • I was about to answer on similar lines but realized that the charger follows USB PD not regular USB. So while your answer is correct that it can charge safely the reasons are different as explained in my answer – beeshyams Mar 3 '19 at 11:59
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Yes, you safely can charge

  • The Mac Charger follows USB Power Delivery (USB PD) as you can see in the picture you pasted.

  • USB PD is a different standard from USB Type C, in that it supports broader charging specs which was initially based on profiles and later evolved to Power Rules. These are well explained in What are USB PD Power Rules?. Quoting relevant extract :

The new rules also introduce a “superset” guarantee. Larger wattage power sources must support all voltage levels below their maximum up to 3A. As the spec says, “Bigger is always better in user’s eyes – don’t want a degradation in performance. Higher power Sources do everything smaller ones do”

As a result, the consumer only needs to know that their device ships with a x watt power supply, and know that any power supply that is rated at > x watts will be at least as good as the one that shipped with the device.

(Emphasis supplied)

  • Also see the questions at the bottom of the 87W charger you linked. It talks of an iPhone charging (Can the 87W USB-C power adapter be used with the NEW iPhone Xs Max? and will it damage the Xs Max?) which supports this (while mechanics of iPhone charging are different, the standard is the same)

Related from SE.Ask Different which confirms what I said Is the Macbook 12 inch's USB-C charger interchangeable with other USB-C chargers or devices? - this is in the context of USB Type C charging and the same logic holds and more strongly for USB PD.

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  • I wonder why there's no USB PD after 5.2V - 2.4A on the charger. – Eugene Yarmash Mar 3 '19 at 12:10
  • Probably space constraints or missed out. But rest assured that it has to be USB PD. It can't be PD for some voltages and not for some other voltage - what purpose would that serve and how would it charge the iPhone (last bullet point)? Besides, that would mean it is NOT following both standards ;) – beeshyams Mar 3 '19 at 12:17
  • See last portion edit – beeshyams Mar 3 '19 at 12:33

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