[twoodfin]

I think there’s more to this story than mere aging batteries.

I had the exact symptoms of this issue with my one year old iPhone 6 Plus in fall of 2015. Add in some very strange battery % readings: Even when it failed to shut down, the phone would drop rapidly from 20% to 1% but then remain there for ten minutes or more.

Bizarrely, this behavior (albeit less frequent or severe) jumped to my brand new iPhone 6S Plus in November of that year when I restored it from a backup of the 6 Plus. Both phones had Geniuses run battery diagnostics without finding any unusual degradation.

My guess: Apple’s battery “farming” code designed to maximize its life, level drain and charge across the cells, manage load, estimate charge % remaining... is incredibly sophisticated, even so far as to profile your typical use of a device to inform these decisions and estimates. There have been user-visible bugs in this code related to manual clock updates (something I did a fair bit while traveling), and I bet some fraction of these downclocked users have been the victim of that or related bugs. If the code is “farming” the battery incorrectly, some cells may be charged or drained too much for too long, and greatly increase a phone’s susceptibility to these voltage drops, even while the global status of the battery reads as fine.