iphone xs and battery 85%

So your battery is right where it should be. At about 350 cycles it should be at 85%, so it is meeting its specifications. I’m surprised you had that many charge cycles; you must be a very heavy phone user. One way to reduce the number of cycles is to charge the phone overnight, every night. Once the battery reaches full charge the phone stops charging, and any usage will come from the mains rather than the battery. This includes background app processing and notifications that are received overnight.

If it reaches 80% before your warranty expires Apple will replace the battery for free. If it reaches 80% or less after the warranty expires Apple will replace the battery for $69.

Well, if you haven’t ever charged it overnight that might be a factor. I charge my Xr overnight, every night, and it is still at 100%. Have you also checked battery health at full charge? That is when it is the most accurate.

However, battery life is not measured in calendar or clock time. It is based on full charge cycles, and is rated at 500 full charge cycles to 80%. So at 85% that implies that you have used 350 full charge cycles. Possible, but unlikely in less that a year unless you charge it multiple times a day from 0% to 100%. Apple can check the number of full charge cycles, but that figure is not available on the phone. If you have a Mac you can install the free Coconut Battery app, which does report full charge cycles.