Checking various sites to solve a sudden quick battery drain problem, I see reference to the battery setting where Peak Performance Capability is listed. But instead of a number, which I’d expect to see, I see a note (that’s been there for several weeks, I think) saying that the phone experienced an unexpected shutdown because the battery was unable to deliver the necessary peak power. And that Performance management has been applied to help prevent it from happening again. And then there’s a link that says “Disable.” But when I tap it, it says disabling may lead to unexpected shutdowns. (!) So do I disable that feature or not?
If you don’t want to see more unexpected shutdowns, don’t disable it.
The power management function wasn’t enabled on your phone before. The unexpected shutdown triggered iOS to enable it upon reboot in order to prevent unexpected shutdowns from happening again. But the “Disable…” link is there just in case you want to turn off the power management and take your chances.
I’m guessing the number you were looking for is percentage of charge your battery is retaining? You’ll find that number after “Maximum Capacity”, which is located just above “Peak Performance Capability.”
Is it a 6 or 6s? There was a run of iPhone 6s with defective batteries which would randomly shut down if they were below 50% charge.
Even a 6s battery will be near its end of life at this point. I had my iPhone 6 battery replaced just over a year ago, and it was barely hanging on. A defective battery would have shown up much sooner.