iphone battery degrading rapidly

Hello,

I purchased a used iPhone 11 Pro Max 256 GB. It was in 91% battery health at the time of purchase (around three weeks ago). Now, its battery health has fallen to 88%.

I am extremely worried. Please help me.

I have switched off the background refresh & also set the ‘reduce white point’ feature to 60% (which I have heard reduces battery consumption). I’m also on the latest iOS.

I have no idea what is going on with my battery health — less than a month & 3 incriments of fall in battery health.

I will share a photo of the charger I use.

Also, the fact that I left my iPhone on charge overnight more than a few times during the days after purchasing it, does worry me. But, apparently that cannot be the sole cause of this downfall in battery health, correct me if I am wrong.