If you are an iPhone 6S owner and have been having issues with your battery suddenly running flat or your device suddenly shutting down the problem may not just be you. It may in fact be due to a faulty battery and Apple may owe you a free replacement.
According to Apple's support page the problem affects "a very small number" of iPhone 6S handsets manufactured in September and October of 2015. The phone affected are limited to specific serial run so finding out if you are one of the afflicted is pretty easy! All you need is the serial number of your iPhone 6S and you can check it on the linked support page.
To find your iPhone 6S's serial number, go to Settings > General > About.
Once you have your serial number check it here. If you qualify for a free battery replacement, you can take your iPhone 6S to an Apple retail store or an Apple authorized service provider, or contact Apple Support about sending in your phone for repair.
It should be noted that if you have any other issues with your phone such as a cracked screen Apple will require that it be repaired prior to replacing your battery. In either case Apple suggest that you make sure to prep your phone before you take it in, or send it off. To do so start by Back up your data to iTunes or iCloud. Then make sure you turn off Find my iPhone. Finally erase all your personal information and data by going to data and settings in Settings > General > Reset > Erase all Content and Settings.
If you believe your iPhone 6s was affected by this issue, and you paid to replace your battery, you can contact Apple about a refund.