See >>> Batteries - Maximising Performance - Apple (UK)
All batteries have a finite service life - with a limited number of charge/discharge cycles. Every battery charge cycle diminishes the available battery capacity by a tiny amount; this effect is cumulative - and causes the battery to slowly lose capacity over its usable life. It therefore follows that minimising battery use by using external power, where available, will prolong the overall battery lifespan.
Apple quote expected iPad battery capacity to be 80% of the initial total battery capacity (when new) after 1000 full charging cycles.
A full charge cycle is any combination of charging that totals 100%. Unlike older battery chemistries, fully discharging and recharging is not beneficial - but instead increases the risk of the Li-ion battery being left in a discharged state - leading to unrecoverable battery damage (a phenomenon known as “deep discharge”).
The iPad is designed to manage and maintain the battery during charging. Leaving the iPad connected to the Power Adapter for extended periods (such as Kiosk applications) will not adversely affect the battery.
However, many resources also correctly state that maintaining a Li-ion battery at maximum charge accelerates the natural chemical ageing process - this effect being more apparent with small capacity batteries (such as used in iPhone). Apple devices are similarly designed to minimise the chemical ageing process by optimising the charging strategy.
Taken together, chemical ageing (through maintaining full battery charge) is significantly less than the cumulative effect of charging cycles. Whilst there is a trade off, maximum battery lifespan will be achieved through using external power whenever it is available.
I hope this information and clarification prove to be helpful in addressing your questions.
