Several times I've wished I had a battery indicator in my battery-powered projects, but it uses so many custom characters in the LCD and sacrifice analog input for voltage indication and components and connections. So we decided to do it the smart way and push the Arduino to its limits "as we always do" and get the maximum out of it.
In this project we use only one custom character from the LCD. So you still have seven free custom characters to use in your project. We also used the trick of reff voltage to get the input voltage from the Arduino without using any analog inputs or components, looks like the complete package.
All you have to is add the function that displays the indicator in the loop function and add the place where you wanted to display it and you are ready.