Battery Health sucks

I bought an iPhone 11 pro around about November during Black Friday, when I got it battery life was around 89%, I thought wow this is great, I quickly noticed that every 3 days it would drop by a percentage and then I thought maybe I'm not charging it right so i began looking for ways to preserve the battery health, the situation kept getting worse and worse and 3 months later I'm sitting at a battery health of 75%. I would suggest just going to istore to buy your phone instead of these second hands.