You can use Tasker

the instruction goes something like this:
if battery full, play ringtone

In tasker,
create a profile with the condition of:
Event>power>battery full

then add task to:
media>play ringtone>(choose your ringtone that you wana play)

done.

alternatively, you can add it to vibrate, create a popup, or even make it speak a phrase like "charging completed, please remove charger"

the possibility with tasker is endless