An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles - whereas an iPhone is rated to have 80% of initial capacity after 500 full cycles. These figures are typical - not absolute. Individual battery performance will vary, some achieving better a better performance lifespan than others.
The maximum level charge can be affected by many factors - as can the overall lifespan of the battery.
Do you leave your iPad connected to its Power Adapter for extended periods? If so, this is not necessarily an issue - as the iPad will optimally manage battery charging. When connected to a Power Adapter for an extended period of time, such as when used in Kiosk or PoS (Point if Sale) applications, iPad will change its charge management strategy - and reduce the maximum battery charge level to about 80% of maximum capacity.
This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. As soon as a “normal” charge/discharge pattern is resumed, charge to full capacity is restored. If your iPad has paused charging at ~80%, simply disconnecting the Power Adapter from the iPad for several seconds may trigger completion of the charge.
Temperature is also a contributory factor in charging being paused; the iPad is designed to reduce the charging rate if the temperature is too high for optimal charging. Summer high temperatures may cause this protective measure to be triggered. Try moving yout iPad to a cooler location whilst charging.
More information about iPad’s battery management can be found here:
About iPad and iPhone Charge Management Feature – Apple Support
