user57673: I got an Arduino project that need a constant source of power. Rather than dealing with batteries, I want to use a power bank like this:

Better yet, I want to keep the power bank plugged in to the wall outlet using a wall charger like this:

The USB wire coming out of the power bank, I can modify it and connect it to the 5V on the Arduino (Pro Mini 5V version). Is it safe to do so? The application uses a stepper motor that draws around 200mA and only runs intermittently. 
Thanks

user58224: Yes, that is my standard setup for Arduino and Raspberry Pi.   

user42539: Yes you can.  A promini will run very well on 5V from a wallwart.  Connect 5V to VCC (not RAW or VIN).  You can also find wallwarts with 2.1mm/5.5mm plugs, and a mating barrel jack adapter that ends in screw terminals so you don't have to try hacking up a USB cable.
Power bank, you will have to play games to convince it that something is connected and drawing sufficient current to keep it turned on.
Wallwart example  https://www.mpja.com/5-Volt-DC-Plug-Power-Supply-4A-Regulated/productinfo/18520+PS/
Barrel jack to screw terminal adapter example  https://www.mpja.com/55_21mm-Power-Jack-Screw-Terminals/productinfo/19454+PL/

57673: <p>I got an Arduino project that need a constant source of power. Rather than dealing with batteries, I want to use a power bank like this:</p>

<p><a href="https://i.stack.imgur.com/eFJYV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eFJYV.jpg" alt="enter image description here"></a></p>

<p>Better yet, I want to keep the power bank plugged in to the wall outlet using a wall charger like this:</p>

<p><a href="https://i.stack.imgur.com/cVfbv.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cVfbv.jpg" alt="enter image description here"></a></p>

<p>The USB wire coming out of the power bank, I can modify it and connect it to the 5V on the Arduino (Pro Mini 5V version). Is it safe to do so? The application uses a stepper motor that draws around 200mA and only runs intermittently. </p>

<p>Thanks</p>
 58224: <p>Yes, that is my standard setup for Arduino and Raspberry Pi.   </p>
 42539: <p>Yes you can.  A promini will run very well on 5V from a wallwart.  Connect 5V to VCC (not RAW or VIN).  You can also find wallwarts with 2.1mm/5.5mm plugs, and a mating barrel jack adapter that ends in screw terminals so you don't have to try hacking up a USB cable.</p>

<p>Power bank, you will have to play games to convince it that something is connected and drawing sufficient current to keep it turned on.</p>

<p>Wallwart example  <a href="https://www.mpja.com/5-Volt-DC-Plug-Power-Supply-4A-Regulated/productinfo/18520+PS/" rel="nofollow noreferrer">https://www.mpja.com/5-Volt-DC-Plug-Power-Supply-4A-Regulated/productinfo/18520+PS/</a></p>

<p>Barrel jack to screw terminal adapter example  <a href="https://www.mpja.com/55_21mm-Power-Jack-Screw-Terminals/productinfo/19454+PL/" rel="nofollow noreferrer">https://www.mpja.com/55_21mm-Power-Jack-Screw-Terminals/productinfo/19454+PL/</a></p>
