When my iPhone 6s started shutting down earlier this month even though it still had 20% or 30% battery life showing, I suspected maybe it was just getting too cold. After all, I've seen my iPhones be temperature sensitive in the past.
But it turns out that the problem really stemmed from a bug in version 10.1 or 10.1.1 of Apple iOS, and now Apple is offering free battery replacement by those affected. The weird thing that those of us affected by this have experienced is that after charging the phone back in, it almost immediately turns back on, back at its 20% or 30% battery level.
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This iPhone 6s battery issue differs from an earlier, overblown iPhone 6s battery problem story. It also is separate from an issue earlier this month in which Apple began offering to repair, for $149, iPhone 6 Plus smartphones with display flickering or multitouch issues (i.e., "touch disease").
To determine whether your iPhone 6s is eligible for a free battery, hop over to the "iPhone 6s Program for Unexpected Shutdown Issues" page on Apple's website and plug in your phone's serial number, which can be found by going to Settings>General on your device and scrolling down. Apple emphasizes that the battery issue is not a safety concern, and that the problem affects only devices within a limited serial number range made between September and October last year.
I plugged in my iPhone 6s info and I'm a winner! (Or loser, depending on how you look at it.)
Note that you'll need to get your replacement battery from Apple if you qualify, as carriers aren't doing the deed. Also, if you do wind up replacing the battery, you should first back up your data to iTunes or iCloud, turn off Find my iPhone, and Erase data and settings.
That all sounds like a pain, but "NOTE" that it could have been much worse (See: "Samsung Note7: A phenomenal phablet flame-out timeline").