You might want to check the battery level in from within your app. This is easy to do with [[sys:battery]]. [[sys:battery]] returns a number from 0 to 100. You can display it as a number, or you might want to have it correspond to some images.
Let's say you have a few images for battery levels, "level0.png", "level1.png", "level2.png", "level3.png", "level4.png" and , "level5.png". If you've uploaded images with these names to the media folder you can use this expression to get the right battery level image to show: "level[[sys:battery | div 20]].png". Remember to check the "Access by Name" box after upload the images to the media folder.