Apple have begun replacing batteries in a limited number of iPhone 6s models after identifying a fault that causes the device to abruptly shut itself off, despite having plenty of charge left.
The fault is believed to affect a small number of phones manufactured in September and October 2015. Any iPhone 6s manufactured within that period is eligible to have a free replacement battery fitted at their local Apple store or Authorised Apple Service Provider.
If you believe your device may be one of those affected, you can check online through the Apple website here. You need to enter your device serial number which can be found by going to Settings / General / About and then scrolling down to serial number.
It is advisable to backup your iPhone data prior to handing your handset over for any work to be carried out.