Why does my iPod battery appear to charge and lose charge quickly?

my ipod touch 6th gen. loses power quickly but as soon as it is plugged in, it registers at 72 percent charged

After charging to 100 percent, once I start using the ipod, it drains to red in about 30 min. As soon as it is plugged back in, it immediately registers at 72 percent. Why?

[Edited by Moderator]