iPhone 15 battery health rapidly declining: Is this normal?

Yes, I see "normal" as well. However, this concerns me:

I got my iPhone 15 mid-October 2025 second hand...

Many iPhones sold as used or "refurbed" ended up with aftermarket batteries that cannot hold a candle to Apple's batteries in terms of either runtime performance or service life. Typically, only phones refurbed by Apple have new Apple batteries. This Apple article shows how to check for non-Apple service parts:

About iPhone Parts and Service History - Apple Support

If the test show Terms of Use have non-Apple parts installed, I recommend returning the phone from=to the seller and looking at Apple refurb phones:

Refurbished iPhone Deals - Apple

Refurbs from major carriers are usually good, but use the service check test.

You can use Settings to check for a factory refurb. Tap Settings > General > About and look a the first block of info for "Model Number," like this:

The first letter in the model number is the key:

— If the model number starts with M, it was purchased new from Apple or a major carrier.

— If the model number starts with F, it was refurbished by Apple or a carrier.

— If the model number starts with P, it was sold as a personalized device with Apple engraving.

— If the model number starts with N, Apple provided it as a replacement device for a device returned for service

Non-Apple refurbs will not have model numbers starting with "F."

Right now my battery health sits at 92% with 233 cycles. Is this decline normal or is there any potential problem I should pay attention to?

The battery in the iPhone 15 is designed to go 1,000 Charge Cycles before it reaches a Maximum Capacity of 80%. That works out to an average 1% decline for every 50 Charge Cycles.

To make the math simpler, let's say that your battery has gone through 250 Charge Cycles. So, 250 divided by 50 = 5%. Ideally, the battery would be at 100% less 5% = 95%. But 92% is not that far off, so I would not be concerned about the battery. The decline is not constant. It will vary. This is normal.

For best performance, set up the battery for Optimized Battery Charging and charge the battery all night.....every night. It might be another discussion, but my advice would be to Ignore any Charge Limit settings, since they are more of a gimmick than anything else.

Your battery health seems normal. Battery chemistry does not age in a linear fashion so it’s normal for the battery health to speed up and slow down. But if we look at averages, over a period of time such as a year and a half (October 2024 to January 2026) a decline of ½ to 1 percent a month is normal. Your iPhone is doing better than average at about .4% a month. Keep doing what you’re currently doing because you’re doing better than average.