On iPod Touch devices running iOS 7, there's a section of the Settings app called "Usage." In this section there's an easily overlooked switch labeled "Battery Percentage." If you turn this switch on, the current battery charge percentage displays alongside the battery icon on your iPod's status bar.
Display Battery Percentage
To display the battery charge percentage on your iTouch's main display, first tap the "Settings" icon on the home screen, which resembles a small gear or cog. Tap "General" and then tap "Usage" to display the Usage screen. Scroll down and toggle the virtual "Battery Percentage" switch to the "On" position -- the switch is enabled when it turns green. Press the "Home" button to save your changes and return to the home screen.
Battery Maintenance Tips
When that battery percentage gets way down, press the "Sleep/Wake" button to put the iPod's screen to sleep. When an iPod's screen is asleep, the device uses considerably less battery power than when it is awake, but you can still listen to music and manipulate the physical volume buttons on the side of the iPod.
Only connect your iPod touch to a computer for charging purposes if the computer is powered on. If your computer is turned off while your iPod is connected to it, the computer will actually drain the battery on the iPod rather than charge it.
Instructions in this article are written for iOS 7 and can be applied to an iPad or iPhone as well; other versions or models may differ slightly or significantly.