Power a Raspberry Pi 3 from an Arduino Uno I would like to power my Raspberry Pi 3B using an Arduino Uno.

I have been able to do this by connecting the 5v on the Arduino to the 5&nbsp;V on the Pi and connecting the GND to GND, and the Pi turns on. But I get a constant under voltage sign.

How can I get the Arduino to output the right amount of power (5&nbsp;V and 2.5&nbsp;A)?
You can't.

The on-board regulator of the Uno, the NCP1117, is only able to supply up to 1A before starting to dropout voltage (hence your undervoltage warning).

Even worse, because the NCP1117 lacks a proper heatsink, it will probably start dropping out voltage before reaching 1A, due to its internal thermal shutdown protection.