Sudden Drop in Battery Health

Your iPhone’s battery health is fine and is undergoing normal usage . Apple recommends that the iPhone battery should be replaced once its battery health has reached to 80% or lower. On average, this will will take about 2 years. Based on this math, the battery health of 99% - 97% on your 9 month old iPhone is fantastic and is performing better than average.

Why, for 9 months, did your iPhone 13 Pro Max maintain its battery health percentage to 99%? Batteries are a consumable product and will degrade over time. The battery health measurement process is not an exact science and its rather a process of approximation. This means that there is no set amount of percentage that the battery will degrade every month, however, it will degrade and eventually will get reported.

Axel F.

Your battery had higher capacity than the design spec initially; probably around 108%, but the phone never reports higher than 100% (you can see the actual capacity and design capacity with some 3rd party apps, such as Coconut Battery for Mac). Once it dropped to 100% it started declining on a typical usage schedule, of about 1% per month.

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.

Once the capacity drops below 80%, or if there is a message in Battery Health that the battery is not meeting peak performance expectations, it’s time to change the battery→iPhone Battery Replacement - Official Apple Support

The absolute best way to get maximum use on a charge, as well as slow the decline of battery capacity 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 nighttime pause 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. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates. Here's more information→About Optimized Battery Charging on your iPhone - Apple Support