Ipad mini 6: How can I check the health of my battery?

Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. 

When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.

Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

Charging you iPad every night will help to ensure that your iPad is always ready for use. What you shouldn't do is deliberately fully discharge the battery - as, if the battery is permitted to remain in this state, the battery may enter a state of deep-discharge, this being a condition from which the battery may not recover or may be permanently damaged.

Modern Li-ion battery chemistry does not benefit from full charge/discharge cycles. Regular partial charging will not harm the battery in any way - and may actually extend the usable life of the battery.

In summary, don't try to micro-manage your battery charging. Instead, charge when you can, not necessarily when you need to do so.

It’s not possible to define a range of performance, because it is dependent on what apps and features you are using. 4 days is actually very good. Ideally, you will charge overnight, every night, and enable automatic nightly backup.

The iPad is never doing nothing, and thus is always using energy. The practice you are following means that all of the energy to run the iPad comes from the battery. However, if you charge overnight the battery fully charges quickly, then the iPad’s built in charger stops charging, and instead powers the iPad. Thus, most of the night that the battery is unused while the external power source provides all of the energy the iPad needs. This will reduce the need to charge, and reduce the number of charge cycles. There is the added advantage that the iPad can back up automatically overnight, so when the device is damaged or lost there will be minimal loss of content.

Here’s the long answer→When to charge your iPhone or iPad | Communities