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While charging my phone (a Samsung Galaxy S5) with a power bank that displays the power output with 0.1 W resolution, I turned the screen on. To my surprise, the power pulled by the phone went from 7.1 W to 5.0 W instantly.

I repeated the experiment many times. Screen on, 5.0 W. Screen off, 7.0 W.

I would have expected the phone to either pull more power to both charge at full speed and power the sceeen, or to pull the same amount, and simply charge the battery at a lower rate.

But I find the drop in power when turning on the screen counter-intuitive.

Does anyone know why this would happen?

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closed as off-topic by Elliot Alderson, Colin, RoyC, Dave Tweed May 17 at 21:40

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Elliot Alderson, Colin, RoyC, Dave Tweed
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ My guess would be that the screen raise temperature enough that manufacture consider it too high to charge at same rate as with screen off. \$\endgroup\$ – Rokta May 13 at 7:51
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    \$\begingroup\$ It could be that while the screen is on, additional heat is generated. Charging also generates heat. So the firmware decides to reduce charging current, so that the total amount of heat is kept in check. \$\endgroup\$ – anrieff May 13 at 7:51
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    \$\begingroup\$ "0.1 W accuracy ...". That should probably be "0.1 W display resolution". The accuracy could be a lot less. \$\endgroup\$ – Transistor May 13 at 8:55
  • \$\begingroup\$ @Transistor: thanks, that's correct. I edited my question \$\endgroup\$ – raggot May 13 at 11:50
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Maybe your power bank or the power management system of the phone itself is not able to supply enough power for charging and power for normal phone operation.

I.e. you may only charge while the phone is in stand-by or operate the phone normally but without charging. If e.g. charging (+ stand-by) consumes 7W and nomal operation consumes 5W , the power system is, however, only able to provide 10W (but not 12W) you get the behaviour you have described.

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  • \$\begingroup\$ Do I understand correctly if I say that the phone basically says "hey, I need 7 W in standby for charging" and the power source provides that. Then the phone says "hey, I need 12W" and the power supply gives 5 W instead? I'd expect the phone to take the maximum possible from the source. Can it have to do also with heat as suggested in the comments by Rokta and anrieff? \$\endgroup\$ – raggot May 13 at 15:12
  • \$\begingroup\$ I don't think so: (1) if the power bank is the limiting factor it probably will drop it's voltage if the phone tries to get more than it is able to provide, which will cause the phone to stop the activity of lower priority (charging). (2) if the power management system of the phone itself is the limiting factor: it probably "knows" that it can't do both and turns of the activity of lower priority (charging). \$\endgroup\$ – Curd May 13 at 17:21
  • \$\begingroup\$ Concerning heat: yes, it could have to do with heat, but heat generation is nothing else but a limiting factor of the max. power a power bank/power management system can provide. \$\endgroup\$ – Curd May 13 at 17:22
  • \$\begingroup\$ Thanks for the answer, and sorry for me taking so long to notice it. I went back to my power bank and it can provide 65W, so despite yours being a possible theory, it's not viable in this case. \$\endgroup\$ – raggot May 21 at 9:57
  • \$\begingroup\$ @raggot: did you realize that I offered two explanations? If not the (external) power bank is the bottle neck, still the (internal) power management system of the phone itself might not be able to provide enough power to do both. \$\endgroup\$ – Curd May 21 at 16:59

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