Battery health on iPhone 6s

Apple wouldn't consider it defective simply on the basis of the battery health dropping unless it reaches 80%. It could be replaced under warranty (or AppleCare+) if the battery health is 80% or lower during the period of the warranty/AppleCare+, and as long as it's recorded less than 500 charge-discharge cycles. 80% was also the threshold for where Apple would traditionally perform a battery service.

If it bothers you that much, Apple currently allows anyone to have the battery replaced on request even if it's over 80% battery health. The price is $29 plus tax in the United States when performed by Apple. Some Apple certified shops are matching that price.

iPhone Battery & Power - Official Apple Support

When you bought the phone it might have been least a year old, because some models of the 6S were discontinued in September 2016. Only the 32 GB and 128 GB were still in production when you bought it. What model specifically do you have (amount of storage)? Did you buy it new? You can check here: Apple - Support - Check Your Service and Support Coverage The warranty end date is one year from the date it was first sold. If it was really new when you bought it then your battery is not performing to expectations; get it replaced. If it was new, that would be covered by warranty.

If you have a Mac you can get the app Coconut Battery.

MikaylaMay wrote:

I have the 32 GB, I’m on iOS 11.3. When I went on the site it said “Estimated Expiration Date: October 24, 2018”.

So, the warranty is good I guess, so I might get it replaced then.

Unless Apple's official diagnostic determines it's at 80% battery health (and under 500 cycles) it won't be replaced under warranty. Anything over 80% is considered "normal". If it manages to hit 80% before your warranty expires then it can be replaced under warranty as a "defect". These numbers are somewhat arbitrary, but they're round numbers.

Again - if it bothers you that much you can preemptively pay for a new battery under the current promotion.