That is normal and to be expected.
Even rechargeable batteries get worn down and 1-2% health loss per month is perfectly normal. Referring to the "Your battery's maximum capacity" section of the support article: 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." Apple apparently only considers it unusual (and covered by warranty) if a battery drops below 80% maximum capacity in less than a year after you receive it. Ergo, some storage loss as you use it is anticipated and acceptable. A possible 20% drop over the first 12 months equates to 1-2% drop per month being within the range of what Apple finds acceptable. "The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if maximum capacity is below 80%, the battery is below normal performance and worth replacing. A notification will appear about this. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80% battery health.
What this basically means:
- If battery health drops a few percentage per month that is normal.
- If battery health drops below 80% before the end of the first year, contact Apple about a warranty replacement. (Apple will test it to determine its actual eligibility.)
- If battery health drops below 80% (you will see a warning message) after the first year, consider getting the battery replaced but anticipate paying for it.
