When should you charge your battery on a iPad Pro 12.9 m1

Lithium Ion (Li-ion) batteries should not be routinely fully discharged - and if you do fully deplete the battery, you should recharge as soon as possible. Li-ion batteries can be damaged if left in a fully discharged state for any period of time.

Your iPad will automatically regulate charging of its battery when connected to its Power Adapter. The battery will not over-charge if the iPad is left connected and charging for extended periods - as this is not uncommon where the device is used in “Kiosk” or “Point of Sale” applications.

Where connected to external power for extended periods, iPad may automatically change its charging strategy - and temporarily reduce the maximum attained charge to appropriately 80% of maximum capacity. Where this occurs, charging to full capacity will be restored upon resuming a normal charge/discharge cycle.

As all rechargeable batteries have a limited lifespan, limited by charge/discharge cycles, there is good argument to operate your iPad from external power where it is convenient to do so, keeping the battery charged and ready for use when working away from a convenient source of power.

If storing your iPad for an extended period, Apple recommend that the battery is not fully charged - but stored with the battery at 50-80% of its maximum capacity.

In conclusion, keeping your iPad connected to external power will do no harm; full discharge should be avoided.

Charge and monitor the iPad battery - Apple Support

Batteries - Maximizing Performance - Apple