Battery Indicator shows the remaining battery time and percentage in the menu bar. This is especially useful on macOS Sierra 10.12.2 or later where clicking the battery icon no longer shows the remaining time.
You can hide the system battery icon by dragging the icon out of the menu bar while pressing the Command key. You can also reorder menu bar icons by dragging them around while pressing the Command key (Sierra only). That means you can place Battery Indicator exactly where the system battery icon used to live.