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I am using Raspberry Pi 3 with latest pixel image:

Linux raspberrypi 4.4.21-v7+ #911 SMP Thu Sep 15 14:22:38 BST 2016 armv7l GNU/Linux

In the right top corner of the connected lcd screen, there is a small yellow thunder icon constantly. What does it stand for and more importantly, how to turn it off? I have connected a 5V, 4A power adapter to the board. When measuring voltage on the adapter, it 5.16V. On the Raspberry Pi gpio header however, the voltage is only 4.5V. Voltage drop of that amount (0.5V) is impossible to be caused by the micro usb cable resistance. There is a little circuit that protects agains usb backcurrents and it definetly adds some voltage drop, but it should be around 0.1-0.2V:

enter image description here

Could someone please explain when does the software add the little yellow thunder in the right up corner? What conditions have to be made? I would appreciate all help.

Łukasz Przeniosło
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    Hello and welcome. Here's a little more about the [flash](http://raspberrypi.stackexchange.com/questions/57963/what-does-the-lightning-bolt-mean) but I guess that is not helping too much. Since you expect the Pi to be responsible and not the wiring (there are really bad micro-USB cables out there - so don't rule that out yet) - try to measure the voltage on PP1, PP35, and PP7. – Ghanima Dec 02 '16 at 09:50
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    Try a different USB cable. Some seem to use tinfoil for the wires. The thunder and lightning symbol is showing the power is under 4.65V. – joan Dec 02 '16 at 09:58
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    It is possible to be caused by cable actually. If the wire inside the cable is as thick as one hair from your head then there is no way the voltage will get to the Pi properly. If bought that cable from chinese bazaar then that wouldnt surprise me. Go and use a cable you got with your smart phone or with some other device you got it bundled. Those work well but are usually only capable of 1A or 2A MAX. But that is OK because the Pi is 2A max. Test the current on the other side of the cable – Piotr Kula Dec 02 '16 at 10:23
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    @ppumkin I believe the Pi 3 is different from the others in that it's 2.5 A max although of course it doesn't need that much for itself. However, it does seem to be distinctly more finicky to me WRT supply. Not that this really solves the problem here. Except I'll add the 4th or 5th observation re, **yes some cables can drop 0.5+ V**. – goldilocks Dec 02 '16 at 11:14

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In the right top corner of the connected lcd screen, there is a small yellow thunder icon constantly. What does it stand for and more importantly, how to turn it off?

Connect to an adequate supply! Full stop! If it lights the voltage is too low (< 4.63±0.07V). See Raspberry Pi Power Limitations which will explain all.

I have connected a 5V, 4A power adapter to the board. When measuring voltage on the adapter, it 5.16V. On the Raspberry Pi gpio header however, the voltage is only 4.5V.

This is not the place to go into the subtleties of measurement, but there are many sources of error, depending on HOW you measured, and what tool you used.

Voltage drop of that amount (0.5V) is impossible to be caused by the micro usb cable resistance.

Definitely untrue - there are some really bad cables out there!

Community
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Milliways
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    Cables! Yes. Now you know why that USB cable cost 0.50 with postage and not 2.50. Those cheap cables are best used for data transfer but they hardly support 0.5A current – Piotr Kula Dec 02 '16 at 10:25
  • Surprisingly you are all right. It is hard to imagine from me from what material the conductor is made of that it has so high resistance... Anyways, thank you. PS: is it knowns what is the voltage threshold at which undervoltage icon occurs? – Łukasz Przeniosło Dec 02 '16 at 11:38
  • @ŁukaszPrzeniosło it may not be the material from which it is made, but rather the gauge of wire used. – Steve Robillard Dec 02 '16 at 12:17
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    @ŁukaszPrzeniosło 4.65V is the undervoltage warning threshold. – Mark Dec 02 '16 at 12:28