since installing iOS 15, my battery needs to be constantly recharged. How can i improve the battery drain

1. Determine what or which app is using up the battery charge. Open Settings ➔ Battery: Here you will see a list of activity/apps that were active and using up battery. Confirm which apps are the top 3 or four using up the battery charge. See whether that makes sense to you or not.
2. What is the Battery Health of your iPhone? Open Settings ➔ Battery ➔ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.

Axel F.

First check to see your battery’s max capacity to see if you need to service your battery. Next check peak performance to ensure you have normal performance and the see what drains your battery. If you have over 7 hours of screen time on avg then you might need to just limit yourself