@scottinpollock told you that you need to create a customised Device Type handler (rather than a SmartApp) to add a tile to show the battery status. When you’re looking at a device in the SmartThings application, you’re looking at the interface to a device, not a SmartApp. That interface is implemented in a Device Type handler.
@Sticks18 told you how to create a customised Device Type handler based upon the existing Device Type handler code.
As an analogy - in Windows you have applications like Word and you have device drivers such as one that lets Windows (and applications) talk to your printer. Word doesn’t understand your printer, the device driver does.
When you go into Devices and Printers to manage your printer, it’s getting and setting information via the printer driver.
When you want to print from Word, the Word application is getting information about the printer and sending information to the printer via the printer driver.
If you wanted to manage details about how much toner is in your printer, that code would be in the printer driver, not in Word.
So - The toner level is your battery level, Word is a SmartApp and the printer driver is a Device Type handler.