Battery health

The only way to increase it is to replace the battery. Battery health is primarily dependent on the number of full charge cycles, not days, weeks, months or years. It should remain above 80% for 500 full charge cycles. But it decreases gradually, it doesn't suddenly drop from 100% to 80% on the 500th charge.

Sort of, but not quite. It is the number of full charge cycles, meaning 0 to 100% or any combination that adds up to 100%. For example, charging from 50% to 100% twice is one full charge cycle. Or from 80% to 100% 5 times.

Here is my essay on the subject-->When to charge your iPhone or iPad - Apple Community