iPhone 6s Charging issue

Try restoring it through iTunes to see if it's a software bug. If that doesn't help make a genius bar appointment at your nearest Apple Store and get it checked out there.

 

Have you let it fully die? Seems like the calibration might be off.

 

I have a lot of experience with battery issues (sadly). What kind of charger are you using? If your charger's legitimacy is questionable, it most likely doesn't know how to float charge which means overcharging and that can easily mess up battery cells.
If the restoration doesn't work out and you're using a safe charger, I might suggest battery diagnostic software. I had the pleasure of seeing just how messed up my iPhone 5 battery was thanks to ibackup bot which is a free OS X app that I downloaded online not off the App Store (not sure if it's on App Store.) It is questionable looking but I have been using it for over a year and had no issues so I think it's legit. Anyway it has an information section that provides battery health information (I am including a picture of what it will tell you.)
If you want to spend 4 dollars for convenience, download the iOS battery health app in the App Store. It will show you cycle count, max charge capacity, current system reported battery charge and actual battery charge (actual and system can vary A LOT) and even detect whether or not a charger is Apple certified.
Don't hesitate to ask me if you have any questions because I know way more about this than I ever would have liked to learn.

 

Glad things are working out now! And just FYI using your iPad Air charger is fine. iPhone batteries can use any charger 10 V or less and I'm not sure what the iPad Air comes with (I think it's 5, I know the original iPad came with a 10, but it certainly would not exceed 10 V. Only charger you might encounter which is more than that is the new iPad Pro charger which is 12 V.

 

Those are wattage ratings, not voltage. All of the chargers are 5V with different maximum currents. You can safely use any of the Apple chargers with any Apple device (the device regulates the current and will never draw more power than it can handle). The iPhone will only draw 5W, even if connected to a 12W charger. You even safely connect your iPhone to the 29W Retina MacBook charger if you have the right cable

 

Good to know. Thanks