Battery Health Decreasing Fastly

Third party apps are notoriously inaccurate. Please review the Apple Support links at the bottom of the post.

Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical makeup, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently.

Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and other factors mentioned above. Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

Please review the Apple Support articles on battery optimization. It contains the best  current practices for battery charging.

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

About the battery and performance of iPhone 11 and later - Apple Support

First off, stop using 3U Tools. It is a very poorly written hacking app and the results it displays are always off.

You bought a used device, who knows how much the battery was used/abused by the previous owner. When the health reaches 80%, have the battery replaced.

Read my post and make screenshots as requested. If you don’t tap Show More you’re not seeing the whole post.

Please follow this path,

iPhone > Settings > Battery > now screenshot showing usage for last 24 hours and another for 10 days. Here’s my 10 day screenshot.