Iphone 14 plus battery health dropping way too fast

I bought it from refurbed.se, a usually credible site to buy refurbished electronics from. It also had 100% battery health when i got it

I've seen other complaint posts here regarding iPhone battery issues involving such refurb sites. The most likely cause is that they did not use a genuine Apple battery when they or their refurb partner replaced it.

Apple only sells batteries they install themselves, or to a real Apple Authorized Service Provider who installs it. I emphasize "real" because there are a lot of people outside the US who claim to be Apple Authorized Service Providers but are not.

Non-Apple batteries, whether for mobile devices or notebook computers, have a terrible track record for service life and reliability. There are also scads of counterfeit batteries in circulation

The unfortunate thing is, now that someone else has opened the phone, Apple will not longer service it. Return the phone for a refund.

If you what a refurbed phone, buy only from Apple or a major carrier.

The measure of battery health deterioration/drainage is subjective and can vary depending on individual usage patterns. There is no universal standard to determine whether the battery health deterioration/drainage is normal or abnormal because it heavily relies on how the battery is used and maintained.

Protect & Prolong the Life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance

Try to monitor Battery usage by iPhone Activity as shown below

What is shown under battery usage? Checking that would give you an indication of the usage behavior.

1. Background Activity
2. Notifications
3. Connected to Charger
4. Or any Other

See the pic below (iPhone 12 Pro iOS 17.3.1)

"Show Activity" feature will show you more info see the pic below

Try some of these options too.

To prolong the life of your iPhone battery, you can follow these tips and best practices:

To prolong the life of your iPhone battery, you can follow these tips and best practices:

1. Optimize Battery Charging: Enable the "Optimized Battery Charging" feature on your iPhone. It helps reduce the wear on the battery by slowing down the battery aging process. You can find this option in Settings > Battery > Battery Health. About Optimized Battery Charging on your iPhone - Apple Support (IN)
2. Avoid Extreme Temperatures: Extreme temperatures, both hot and cold, can negatively impact your battery life. Avoid exposing your iPhone to excessive heat or cold, such as leaving it in direct sunlight or in a freezing car.If your iPhone or iPad gets too hot or too cold – Apple Support (UK)
3. Use Original Chargers and Cables: Always use Apple's original chargers and cables or certified third-party accessories. Low-quality chargers or cables can damage the battery and reduce its lifespan. Identify counterfeit or uncertified Lightning connector accessories - Apple Support (IN)
4. Avoid Draining the Battery to Zero: Charge it whenever the charge becomes lower than 20%. The rest is up to you when to charge. how long to charge, how often to charge, which adapter to use starting from 5W to 140W USB-C Power Adapter etc.
5. Optimize Display Settings: Adjust your display settings to optimize battery life. Lower the screen brightness, enable auto-brightness, and use shorter screen timeout durations. Adjust the display and text size on iPhone - Apple Support (IN)
6. Minimize Background App Refresh: Limit the number of apps allowed to refresh in the background. Go to Settings > General > Background App Refresh and disable it for apps that don't require real-time updates. Switch apps on your iPhone, iPad, or iPod touch - Apple Support (IN)
7. Disable Push Email: Push email notifications can drain your battery quickly. Instead, set your email accounts to fetch data manually or at longer intervals. You can change this setting in Settings > Mail > Accounts > Fetch New Data.

I agree with Allan Jones and it was where I was headed too. The refurbished iPhone has a defective battery. It could be an Apple brand or third party, but it doesn’t matter, it’s defective.

What is the warranty from the place you bought your iPhone?

Who did you purchase the iPhone from? Did they tell you the battery was new?