How to increase battery health

Why?

The laws of thermodynamics, in the most general terms.

No battery made and no storage device and no computer can or will last forever.

All will degrade. All will eventually and inevitably fail.

As do we ourselves. We too get old and fail.

Battery replacement is an option Apple offers, though it’s common to be replacing the iPhone or iPad before or by the time battery replacement is needed.

More reading; four links to additional details...

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

Why Lithium Ion Batteries?

Battery Service and Recycling

Batteries are consumables; they start to lose capacity the day the leave the assembly line. Your battery is expected to maintain at least 80% capacity for 500 full charge cycles. Doing the math, that means on average it will lose about 1% of capacity for each 25 fill charge cycles.

The best way to reduce the rate at which the battery loses capacity is to enable Optimized Battery Charging and charge the phone overnight, every night.

It will only go down. It can never go higher.

Charge it when not in use. Day and night. You can not overcharge it.