First, some background. Back in November, a consumer protection group in China asked Apple to investigate problems with random shutdowns affecting the iPhone 6 and iPhone 6S. After looking into it, Apple said it had determined that a "very small number of iPhone 6s devices may unexpectedly shut down". This was supposed to just be a small number of phones made in September and October of 2015. The company said that customers with the issue could bring their phones into Apple stores for a battery replacement.
Yesterday, the company offered more details about the problem, claiming that the batteries used in the faulty phones had been "exposed to controlled ambient air" longer than they should have been, which resulted in the batteries dying faster. That's weird, but we'll go with it.
Well, it turns out a lot of users who don't have batteries from those batches are experiencing shutdowns too. Apple won't replace those batteries for you (unless you have Apple Care+), but the company does say it's looking into the issue. Hidden away after the statements about "controlled ambient air" is this takeaway:
A small number of customers outside of the affected range have also reported an unexpected shutdown. Some of these shutdowns can occur under normal conditions in order for the iPhone to protect its electronics. In an effort to gather more information, we are including additional diagnostic capability in an iOS software update which will be available next week. This will allow us to gather information over the coming weeks which may potentially help us improve the algorithms used to manage battery performance and shutdown. If such improvements can be made, they will be delivered in future software updates.
In other words, the next iOS 10 update will include some diagnostic stuff to help determine why iPhone 6S batteries are dying. If Apple can come up with a software solution, it will issue an upgrade.