There's a nasty iPhone 6s bug that causes the battery life indicator to suddenly deplete. Then the iPhone thinks its battery is dead and shuts down.
I know this because it's happened to me several times. On a recent afternoon, I was walking the cold streets of New York City and my iPhone 6s battery dropped from 41% to nothing. The day before, I was on a bus trying to get home and my iPhone 6s died at 49% while giving me directions.
I'm not alone. More than 68,000 people have viewed one thread discussing the bug on Apple's support forum. The first complaint was posted on September 28, 2015. It wasn't until January 13, 2016 that Apple officially acknowledged the issue with a post saying that it's "aware of this issue and is investigating the cause and a solution."
To fix the issue temporarily, Apple suggests that you manually restart your iPhone, open the Settings app, and go to General > Date & Time. The "Set Automatically" option should be turned on.
If that doesn't work (it didn't for me), then you'll have to wait for Apple to release an official fix.
