Apple did announce some time ago that they will replace your iphone 6S battery if it is faulty.
Actually, this news was received as a big welcome because many iphone 6S owners found that their phone was shutting down void of any warning that this was supposed to happen.
This replacement service is offered worldwide by Apple for the iphone 6S. So if the battery is faulty, well, just have it swapped for free.
You need to check though if your iphone 6S is suitable for the free replacement.
Apple says only iphone 6S made between September 2015 and October 2015 are eligible.
This can be checked by looking up the serial number of the iphone 6S and should be done in the Settings>General>About.
This number should be entered on the Apple support page.
Not all iphone 6S are covered as we found out.
In case your 6S isn’t covered by Apple replacement service and yet you are receiving battery issues, well then it may be correlated to the iOS10. This version has had its fair share of battery issues if I may say so.
Many consumers complained about the iOS 10.1.1 update. This update was turning off phones after reaching 30% of the capacity. And others complained about shut down at about 50% level which even happened within an hour.