Today, Apple is finally giving us a little bit more insight into what is actually causing the problem for customers. Since Chinese customers have complained the loudest about the issue, the statement was issued via Apple’s Chinese-language website:
We found that a small number of iPhone 6s devices made in September and October 2015 contained a battery component that was exposed to controlled ambient air longer than it should have been before being assembled into battery packs. As a result, these batteries degrade faster than a normal battery and cause unexpected shutdowns to occur. It's important to note, this is not a safety issue.
That last line is no doubt a reference to the Galaxy Note 7, which was taken off the market for explosions that caused injures to customers and property damage. Apple also went on to indicate that if your iPhone shuts down without warning, this action was likely taken in order to save your device from significant damage:
We also want our customers to know that an iPhone is actually designed to shut down automatically under certain conditions, such as extremely cold temperature. To an iPhone user, some of those shutdowns might seem unexpected, but they are designed to protect the device’s electronics from low voltage.
If you want to see if your early iPhone 6s is covered under Apple’s free battery replacement program, insert your serial number at this support page. If your iPhone 6s is identified as being eligible for replacement, you can then schedule an appointment to visit a local Apple Genius Bar. If you’re having unexpected shutdowns, but your iPhone 6s doesn’t qualify for replacement, then it seems as though there’s no recourse at this time.