CharlieMander
To get your pop up banner:
In settings, search the word "saver"
Go to battery saver. Temporarily set it to a % higher than what you are now to make the notification appear.
View notification, hold finger on it for 5 sec, press the gear icon that is now on it, expand the collapsed option called "other", it highlights " routine mode info", tap that, change what you want:
Pop up banner across the screen
Vibrate
Sound
Now change battery save to 20%. Now you have your pop up over your anime video :)
(I set mine to 25% because having any battery below 20% is bad for it. Your 100% to 0 or 50%->0% won't last as long) 20% isnt a magic number. Just having it close to 100% or 0% lowers lifespan.
But you do you