iPhone 6s Turning off at 15%

Hi there,

When my iPhone 6S reaches around 15% battery it dies. It never gets to single figure battery life now.

I tried searching for an answer and saw a few articles about recalibrating the battery, which I tried, but couldn't do properly because I can never fully drain the battery. It reaches 15%, switches off, I charge it enough to turn it back on but then it indicates it's at something like 24%. I begin using it to run the battery down completely and within a minute or so it dies again. And then every attempt is just a repeat of that.

So I'm not sure what else to do. Could anyone offer any ideas?

Thanks!