Apple has recently announced the free battery-replacement program for all iPhone 6s handsets shipped with faulty batteries from the manufacturing unit. The company has not specified which of its iPhones are eligible for free repairs, even when they are out of warranty.
Nevertheless, Apple has pointed out that only a specific batch of iPhone 6s handsets manufactured between September and October 2015 are eligible for the replacement. So, the catch here is to identify these devices using their manufacturing batch numbers, before taking them to the nearest Apple Store in your locality for repair or battery replacement.
Here's how you find out when your iPhone was made using the batch numbers:
Step 1: Locate your iPhone's serial number, which should be printed on the back of the handset. Alternatively, go to Settings > General > About on your phone to find the product serial number.
Step 2: You can also find the manufacturing code or the batch number in the fourth and fifth position (from left to right) of your iPhone's serial number. Here are a bunch of eligible batch codes for free battery-replacement program from Apple:
All iPhone 6s handsets with serial numbers including the aforementioned characters are eligible for free-battery replacements in and out of the warranty.