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I have a Retina Macbook Pro 2014, in clamshell mode attached to a full HD monitor.

I see that battery lasts much less when connected to an external monitor. Is there any explanation to why this happens? External monitor has its own power source, and Full HD is less than retina resolution.

Is there any overhead associated the mini Display Port?

  • I'm curious if you see the same results with the laptop open, but the internal display still off. On a related note, do the fans seem to run more often/faster when in clamshell mode vs open? – 8bittree May 16 '16 at 14:32
  • @8bittree I can only turn off display via dimming it when its open. I didn't test this. there is no difference in fan noise, its quiet on both. – user15322 May 16 '16 at 14:48
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The reason is because when you connect an external display, even if you close the laptop and use it in clamshell mode, the laptop automatically switches from an integrated video card (this uses much less power) to a more powerful, discrete video card (this uses a lot of power).

This is also true regarding Intel Iris Graphics, which has different power profiles (see the "Power Management" section). Also read on Embedded DisplayPort (eDP), and it's possibilities in terms of saving system power and further extend battery life in portable PC systems. I'm not the specialist in GPU-chips architecture, but my guess is, that the difference is exactly in using of Embedded DisplayPort on a mobile CPU, against of DisplayPort connections when on external monitor.

So the battery is drained more on when external monitors are connected, it needs more GPU power. Monitor itself does not drain the battery, as it's connected to external power.

| improve this answer | |
  • MBPr 2014 13 inches doesn't have dedicated video card. – user15322 May 16 '16 at 12:31
  • @SandroDzneladze, please find out my updated answer with facts provided by links. – Farside May 16 '16 at 13:29
  • I also found this article embedded-computing.com/articles/… useful – user15322 May 16 '16 at 21:16

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