iPads on older iOS versions would sometime have their battery fail after a few years of use.
This was resolved with iOS 11.3 which was specifically done to fix this issue for iPads in kiosk mode (always on).

So we highly recommend that all our customers use iOS 11.3 or later.
For reference here is the article from Apple discussing the topic.
https://support.apple.com/en-us/HT208710

A good practice to prolong the overall life (not just battery) of iPads is to lower the screen brightness to the minimum necessary level.
This will use less power and keep the iPad cooler.