My phone is on drugs

My phone had this little phase when it would turn off when it warned me I had 15% left
I didn't even have a battery saver on or anything

Same for my nook. It'll be like
"Batter critically low. Plug in or your NOOK will turn off" so I'll pug it in, and then it says
"Charging: 38%" And I'm like -_-

It's to keep you from discharging the battery too far, which can destroy a lithium ion battery.