Right, dropping to 80% battery capacity is the "something wrong" that will get you the free battery during your AC+ coverage period.
Well, the unimaginable happened, or Apple changed their policies, because they just replaced my son's 88% capacity iPhone 7+ battery for free under AppleCare+ tonight, with about 3 months remaining on his AC+ warranty.
Coconut battery was reporting 85%, and he's been able to drain the battery in 4-5 hours while just using it normally. It would not drain too fast if he just stuck it in his pocket and used it for nothing but taking phone calls. Today, his charge dropped from 82% to 68% in the Apple store in 25 minutes, after doing the 12.1.2 update there. And doing similar activities could cause it to shut down in about 4 hours.
We totally expected to fight with Apple to be allowed to pay the $29 for a new battery, and not only did they not argue with us, but they didn't charge us. It makes no sense. Every time in the past when we've had a phone draining faster than normal, we've been forced to wipe and restore, and if that didn't fix it then they made us wipe and set up as new, and only then would they consider a replacement. And even that was not enough for them when he had issues with his iPhone 6 in Feb 2017.
Even when we were willing to pay for it, they would always refuse to replace the battery if it was reporting over 80% capacity. My son went through that whole process in Feb 2017 with his iPhone 6 shutting down at 20% charge with battery wear down to 83% capacity, and Apple had refused to replace the battery. It was under AC+ and they wouldn't even let us pay for one. Not only that, but the phone seemed to be running in slow motion.
At the time, I bought the 6 from him to use as a car iPod, and helped him buy his current 7 Plus. But 3 months later I had Batteries+Bulbs replace the iPhone 6 battery and it fixed all of it's problems. Now Apple won't touch that iPhone 6 which no longer sees LTE, because they forced us to take it to a 3rd party for a battery 18 months ago.