Battery health decreased in 20 days

And the Battery Health will continue to decrease little by little with every full cycle battery charge until such time it dips to 80% or below and then it will be time to pay to have a new battery put in your phone.

Battery Health is something which will decrease no matter what you do. You can't stop it. And 1 - 2% per month is common, depending on how much you use your phone and how often you charge it.

Batteries are considered consumable parts, and will degrade and will fail, always, inevitably, inexorably.

The battery health is a current value, and will ~always decrease, and is not a particularly predictive trend to track. The value can drop at different and unpredictable rates, such as when the iPhone is exposed to excessive heat or such. Or when the battery is failing.

Per Apple, iPhone battery health is considered fine until the battery drops below 80%, and then the battery can be replaced.

An iPhone that drops below 80% during the warranty or while AppleCare+ is in place will usually be replaced without charge.

Apple’s reading on this topic:

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

And FWIW…

Take a screenshot on your iPhone - Apple Support

SelvaRagavan wrote:

My battery health dropped 2% in 2 days

One of the various iPhone 12 models, and did you just load iOS 15.1?

If so, Apple indicates they adjusted the battery capacity determination to better represent remaining capacity.

The changes will probably make at least a few of that series go up or down, shortly after loading iOS 15.1.

So long as the health is 80% or up, Apple considers the battery to be healthy.

SelvaRagavan wrote:

My battery health dropped 2% in 2 days

What is the current health value, and how long has it been since the phone was new?