Battery Health Draining Fast.

I bought my Iphone 11 pro max almost 1.5 yrs ago and till last june battery health drained from 100 to 97. But since June, it has dropped to 88. I usually try to keep my battery life at 40 to 80 and use fast charger that came with the device. But Battery has drained like 9% in the past 3 months which is quite worrisome for me.