Battery maximum capacity 100%

Batteries - ALL BATTERIES, with every full cycle charge, lose their ability to hold the same charge it did when new. A decline of from 1 -2% per month in Battery Health is normal and expected. As muguy correctly suggests, when battery health dips to 80% or less, it will be time to pay Apple to replace the battery in your phone.

Sure, they can request a battery, as long you also let the OP know it won't be under warranty and they will have to pay for a new battery. And Apple may not even agree to replace a battery which is not failing and is in fact operating as designed.

Understood. I don't know know for sure what's available in other countries, but can only take an educated guess as to whether or not the OP can get a battery service on demand without being under 80%.

lobsterghost1 wrote:

I would hope they would discourage or even refuse such a service. A battery at 91% should operate normally for some time to come.

I understand concerns about electronic waste, but batteries are just one thing that can be replaced proactively. I'm pretty sure that after the iPhone battery replacement promotion in 2018, (depending on the location) Apple has replaced batteries on demand, like I got for $29 during that promotion when my battery was still at 88% battery health.

lobsterghost1 wrote:

That applied to specific models of iPhone, which were clearly noted in advance. It did not apply to just any iPhone model. Different time, different circumstance.

I haven't tried since then, but there are some discussions that indicate Apple (or an AASP) will perform a proactive battery service if the customer pays for it, but certainly not under warranty or AppleCare.

I totally understand other Apple devices where it's under 80% or no go on a new battery.

No help is necessary. Your battery is fine. Should it fall below 80 percent have it replaced.

If your want a battery with 100% battery health, you should be able to request a battery service to install a new one. However, there’s no specific need unless it’s really bothering you.

I would hope they would discourage or even refuse such a service. A battery at 91% should operate normally for some time to come.

That applied to specific models of iPhone, which were clearly noted in advance. It did not apply to just any iPhone model. Different time, different circumstance.