Over the past couple of iPhone versions users have complained of “unexpected” shutdowns of their devices. Some iPhone 6, 6s, 6 Plus and 6s Plus devices could basically go dark unexpectedly, forcing a user to have to plug them into an outlet to get them to power back on.
Apple has been working on this very annoying bug and it says it has come up with a fix of sorts that should mitigate the problem on a majority of iPhone 6 and iPhone 6s devices. The fix is actually already on your iPhone if you have installed iOS 10.2.1 — something that around 50 percent of iOS users have already done. After letting the fix simmer on customer devices, Apple now has statistics to share on how it has improved the issue, citing 80 percent reduction on iPhone 6s and 70 percent reduction on iPhone 6 devices.
Currently, iPhone 7 devices are not impacted by this issue. It's also important to note that there was a similar but different issue that had the same unexpected shutdown symptoms late last year that resulted in a battery recall of some devices. These are not the same issue or fix.
Brand new batteries would not be affected, but as most phone batteries run through charge cycles they get less effective and more susceptible to these kinds of triggering spikes.
Whatever tweaks Apple made to its power management system have enabled them to reduce the shutdowns heavily — but not eliminate them entirely, this should provide some sort of relief for users who were annoyed by the problem. 3uTools has written an iTips for 6S users, you may check here : How to Fix iPhone 6S Auto Shutdown When it Still Has Battery?
Source: techcrunch