BatOn did not work for me either. Use AirBtts instead. It's 1,99 unfortunately and I never buy apps normally but the reviews were positive and so I decided to download it. Fully works, displaying both AirPod's battery level and also the battery level of the case. Only downside is it shows the battery level in increments of 10%, but I tested it and it's accurate as can be. It works, so definitely worth the money for me.