Many iPhone users like to charge their devices to 100%, likely because it is the satisfying number that ensures your device will last as long as possible but it is not something you should do on regular basis.
The type of battery used in most modern electronics including the iPhone is Lithium-Ion and its capacity is only good for a certain number of chrges or cycles before it begins to degrade.
To check how much your battery has degraded by going to Settings, battery Settings and tapping Battery Health
Your battery is at 98 % of its full capacity means 2% of your battery cells has worn out from being charged and discharged over the years used.
We want to prevent our batteries from wearing out as much as possible which is why charging all the way to 100% and allowing the battery level to drop down to 10% or less is considered a bad practice.
These deep discharges exhaust the lithium battery cells faster than charging to 80 or 90% and then getting it back on its charger by the time it reaches 30 or 40%.
This not only ensures you get as much battery life out of your iPhones for as long as possible for as days when you really needed but it also prevemts performances issues caused by deeply integrated batteries.
And Apple tries to alleviate these issues with software features like Optimize battery Charging where the iPhone will wait to charge past 80% until it knows your rate to use it based on your charging routine so it won’t set it 100% for hours at a time but if that does happen, you actually receive a notification that says your iPhone should be removed from power and discharge to improve battery health.
But deep discharges also negatively affect the battery which is where low power mode comes in. At 20% your iPhone will automatically begin drawing performance and background tasks to prevent the battery from draining even further and all that means more battery life for the user but it also means a higher likelihood that the device will be connected to power before reaching an extremely low battery percentage.
So it’s best to keep your battery level between 40 and 80% when possible so on those busy days you need every minute of battery life, your device will be there to deliver.