Visibility of System Status is a basic design principle that addresses whether or not users can tell what a system is doing.

The application doc on Apple computers, seen below in the screenshot, is an example of effective visibility of system status. Anytime an application is open, the small gray dot appears below it in the doc. For example, in this particular screenshot Safari, iTunes, and Microsoft Word are open. Powerpoint and Excel are not.

In addition, when a user clicks on a closed application, the icon will bounce as it is opened. One stage of this bounce is pictured below (you can see that the P for powerpoint is raised above the level of other applications).

Another good example can be seen in Microsoft Word documents. When a user makes changes to a document and does not save the document, a small dark red circle appears inside the red “quit” button located in the top left of the screen. This indicates that your work is unsaved.

Once a user saves their work, this dark inner circle disappears, indicating that all work has been saved.

A bad example of visibility of system status is the portable charger pictured below. Based on its exterior, there is no way to tell whether or not the portable charger has been charged.

As a solution, I would suggest a small LED light, much like the one on Apple computer chargers, that would shine green if it was fully charged, yellow if it was charged some but not fully, and red if it was very low on battery. With this change, the user would have a better idea of how much charge the device currently has and whether it needs to be plugged in.