Is 88% battery health after 18 months normal for iPhone 15 Pro?

I’d say it’s normal. It’s nothing I’d be worried about.

In general, you can a good idea about how your battery is doing by noting the information in this reply.

On average, Battery Health will normally decline about 1% for every 50 Charge Cycles. So, if your battery has completed 500 Charge Cycles, it is easy to compute that about 10% off the battery capacity has been used.

These are not exact numbers, as the decline is not linear or constant, so there is some leeway up and down for normal performance. For example, the battery might go a few months with no decline at all, then drop 2% the next month after that.

So, the Maximum Capacity would be in the 90% range at this point. Heavier use of the battery might display numbers in the 85-86% range while very light use might display 92-93%.

I would say that your battery is doing fine.....nothing to be concerned about. At or about 1,000 Charge Cycles in another 18 months or so, it will be time to replace the battery.

The "months" are not nearly as important as the number of Charge Cycles that the battery has completed.

Why? Someone who has used the phone very heavily for only a few months might have much less Battery Capacity left compared to someone who has not used the phone much at all each day and the phone is 2 years old.

How many Charge Cycles on your iPhone when you check the Battery Health & Charging settings?