In message 
          Markus Robert Kessler  wrote:

>Hi,
>
>I just bought a Raspberry 3B+ and now I am setting it up.
>
>Nice little box and it's really fast in comparison to my other RPis like
>Zero/W, A, B... It's fun to work with it.
>
>But, besides this, one thing makes me really wonder:
>
>Upper right on the screen there appears a yellow flash all the time and
>one of the boot messages reads something like "undervoltage detected".
>
>The new box is working fine, though, but I connected a USB voltage
>measuring device and saw that the power input on micro USB connector of
>RPi3 never goes under 5.01 Volts, but, the USB power on those 4 USB port
>(I only checked 2 of them with identical results) is only 4,55V (!).
>
>Total input current is at 0.6 A in average, so I doubt that the 5V/2A
>power supply is too weak.
>
>Am I doing something wrong?

Are you actually measuring the voltage at the RPi end of the cable,
or are you measring at the other end and assuming that the same
voltage arrives at the RPi?

Many people have had trouble caused by USB cables that have too
much resistance, and thus a significant voltage drop between the
ends of the cable.  The shorted and fatter the cable, the better!

Dave

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