Hello Adam1V,
Thanks for asking the Apple Support Communities about your iPhone battery, and Find My. You mentioned that you removed the AirTag and added it back, and the same behavior occurred. Just to confirm, if you remove the AirTag from the device for 24 hours (if possible), does Find My continue to use most of your battery? This can help us to fully isolate to the AirTag, and not the device itself.
Additionally, if you place the AirTag beside the iPhone overnight, does the same behavior occur?
90% is still a good battery and should give you no issues, but if there is an issue found later, the warranty would cover it as mentioned here: iPhone Battery and Performance - Apple Support
“A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge.”
Battery percentage is a direct result of use and charging, please check your settings, as some simple changes can really help extend the life of your battery.
Follow this for more tips: Batteries - Maximizing Performance - Apple
It may also be beneficial to disable Background App Refresh for the apps that you do not normally use. This article talks about Background App refresh: Switch apps on your iPhone, iPad, or iPod touch - Apple Support
Also, the testing for iPhone batteries are described here: iPhone - Battery Test Information - Apple
Where the settings are all default, any deviation from these settings on your device will reduce the battery life: “All settings were default except: Bluetooth was paired with headphones; Wi-Fi was associated with a network; the Wi-Fi feature Ask to Join Networks, Auto-Brightness, and True Tone were turned off.”
Now that you are adjusting your settings, you may get longer battery charge, and also longer battery life.
Thanks.
