From Redmond Pie.
For some iPhone 6S users,you might be entitled to a free battery replacement due to certain models having battery's that have poor life or the phone dies without warning.
So, what do you need to do? Well, you need to find out when your iPhone 6s was made, and here’s how you do it.
Step 1: Find your iPhone’s serial number. It’s located on the back of your iPhone, or if you’d prefer, it’s also found in Settings > General > About.
Step 2: Look out for the following characters, found in the 4th and 5th spot of your iPhone’s serial number:
If you have a serial number including any of those, then you’re good to go to the Apple Store, safe in the knowledge that your iPhone 6s is one of the faulty ones and eligible for free battery replacement even if out of warranty. We bet you have never been so pleased to hear that!https://www.apple.com/uk/support/iphone6s-unexpectedshutdown/