Coming from a Samsung Galaxy phone, I took the battery percentage on Android phones for granted. It was a simple checkbox under the Battery settings to indicate the battery’s charge rather than just relying on the tiny graphic. Apparently, it was not available in stock Android until 6.0 Marshmallow. Even now that it is built in, the setting is pretty buried and requires enabling a “secret menu”.
Enable the Android 6.0 System User Interface (UI) Tuner: Swipe down from the notification bar to show the notification panel and then swipe down again to show the Quick Settings menu. Long press the Settings icon (a gear, the wrench is added after you enable the System UI Tuner) in the top-right for about 5 seconds. When you remove your finger, you should be taken to Settings and receive a prompt notifying you that the System UI Tuner has been added to the Settings menu.
Scroll all the way down in Settings to find the System UI Tuner. Tap it to be taken to the now-revealed secret menu.
Upon entering, you will receive a warning that some of the settings are experimental. Tap ‘Got it’ to reveal the settings. (This warning is similar to the about:config warning with Firefox.)
Switch the slider for ‘Show embedded battery percentage’ and you will see the numeric percentage appear within the battery icon on the Notification Bar.
The only time that you will not be able to see the percentage is during charging. The number is replaced with a lightning bolt icon.
If you would prefer to follow the steps along as a video:
To turn off the battery percentage indicator, just head back into Settings, System UI Tuner, and toggle the battery indicator off. You can also remove the System UI Tuner and turn off all settings by long pressing the gear in the Quick Settings again and confirming that you want to remove and stop using all its features when prompted.