iPhone battery health dropping fast

Do something about exactly what?

Anything above 80% health is fine.

Batteries can and do degrade, inevitably, always, and die.

Battery health numbers can change based on various factors, too.

If (when) the iPhone battery health gets to be 80% or less, or when you find you're having to re-charge during your normal usage, then replace the battery or replace the iPhone.

Click or tap on the following blue-text link, and learn more about the battery and battery health:

iPhone Battery and Performance - Apple Support

Irfan1092 wrote:

But my phone only 3 months old how it happens with in three months

All batteries degrade. Inevitably. Inexorably. Different batteries have different degradation profiles. Different batteries have different environmental exposures; some have exposures to excess heat, for instance. And as Apple states in that same health display, the “Actual Maximum Battery capacity may be different.”; that displayed number can and does vary from the reality of how long you can use your iPhone before it needs to be recharged. Somebody here was concerned about and posted that their number increased after some updates, for instance.

Set a calendar alarm to check again in ~8 months from now (~11 months from purchase) or check in a couple of years if you bought AppleCare+, and check then if the battery health value is at or below 80%. If you’re that low within the warranty, Apple can or will swap the battery. Past the end of the warranty, watch for when you need a second re-charge during your normal usage cycle. That’s when you’re headed to a battery replacement.

Click or tap on the following blue-colored text link for more info: iPhone Battery and Performance - Apple Support

MrHoffman wrote:

Closing apps and reducing background activity will have negligible or no effects on the overall battery health.

According to a number of sources closing apps unnecessarily actually increases battery drain and slows down the phone→Do not close "background" apps | Communities

Blade0817 wrote:

Also make sure any apps that you're not using are closed, apps that are running even in the background use power, especially if they're constantly communicating with the internet.

As this can get confusing to some folks reading here, closing background apps and deferring or disabling app refresh operations and other steps can potentially be useful for reducing power consumption and for extending today's battery charge.

Total battery capacity drops over time as the battery inevitably degrades, and it is the current total capacity as a percentage of the original total capacity of the battery that is shown with the battery health value.

Closing apps and reducing background activity will have negligible or no effects on the overall battery health.

Irfan1092 wrote:

But my phone only 3 months old how it happens with in three months

Batteries are consumables; they lose a little capacity every time they are discharged, then recharged. On average this works out to about a 1% loss for every 25 “full charge cycles”. As one example, if you charge the phone overnight, every night (and that is what you should do; it is a best practice), it starts the day at 100%. If it drops to 20% by the end of the day before you charge it again overnight that counts as 0.8 full charge cycles (20% to 100%), or about 24 full charge cycles per month of use. For this example your battery capacity will lose about 1% per month. Of course, if the end-of-day level is higher than 20% the capacity loss will be a little less, and if it is lower than 20%, or you charge it during the day, the capacity loss will be higher.

The absolute best way to improve the life of your battery long term is to enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the night the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern.

I think this is the forum you need, this is phone support: https://discussions.apple.com/community/iphone

This is a forum for Mac computers not for phones. Please find the correct forum for phones.

Also make sure any apps that you're not using are closed, apps that are running even in the background use power, especially if they're constantly communicating with the internet.