iPad shuts down when fully charged

My 3 year old iPad Air 2 started shutting down even when fully charged. When I plug in the charger, the charging screen shows. Most times within 30 seconds the white Apple logo screens comes on as if it was restarted. Usually I can sign in and it works fine, however, it has to stay plugged in, otherwise after a few minutes it shuts down again. Is it the battery or is something else going on.