When Low Power Mode Is Turned OFF Automatically…
Answer: That’s the way iOS 9 works. After your iPhone / iPad is charged 80% or above… iOS 9 automatically disables the Low Power Mode feature.
Is there any other way to make sure that Low Power Mode is never turned off?
Answer: Unfortunately, no. You can always turn ON Low Power Mode from the lock screen by swiping right, and it shows you “Enable Again.”
You can also go to the Settings menu. From there, tap on Battery and enable Low Power Mode by toggling the switch (green indicates that the feature is enabled).
New to Low Power Mode? Read this post to learn more.