We believe the alert sound of the low battery of 20% and 10%, drives a lot of people crazy specially, especially at evening time.
Now there a way of turn it off, this working on any iPhone that runs the iOS 12.
Visit this link on your safari browser on iPhone:
https://www.icloud.com/shortcuts/497bf35cce234870815f06aa40cea82b
Tap Get Shortcut > Turn off and on your iPhone
Then your iPhone is low battery alert sound Free!!!