Firstly, both observed and reported behaviour of iPadOS 13.x battery utilisation indicates that battery consumption appears higher than iOS12.x; this translates to shorter run-times for an indicated “full charge”.
Battery utilisation of 15-18% per hour is now not uncommon in some multi-tasking scenarios, with 12-15% consumption per hour being typical. This obviously translates to reduced run-time.
Batteries also lose capacity over their lifespan. For iPad, Apple quote 80% retained capacity after 1000 “full” charge cycles.
You indicate that you habitually leave your iPad connected to power for long periods. Whilst not publicly documented by Apple, considerable evidence exists (and multiple reports seen here) that iPad now utilises a battery management strategy intended to reduce “stress” to (and premature ageing of) the internal battery when connected to external power for extended periods - such as when the iPad is used in Desktop, Kiosk and Point of Sale applications. When triggered, this battery preservation strategy limits maximum charge to a lower value, typically 90-95% of maximum. When a more normal charge/discharge regime is resumed, charging to maximum battery capacity is restored.
I hope this proves to be helpful in determining potential causal factors for the behaviour you see.
