iPhone 6S (iOS 9.3.2) starts switching off whenever battery is low So I have an iPhone 6S running iOS 9.3.2 and recently, I have observed this very annoying trend.

The phone switches off automatically when the battery level is around 15-25% [It always happens in this range]. 

I might not be using a very power consuming app, maybe writing a mail or checking my messages and it turns off. When I switch on the phone again, it automatically turns off within a minute.

Only when I leave it as it is for 15 minutes maybe, it turns on again.
This is not regular but does happen once every 4-5 days.

Is there anything I can do to investigate and maybe get rid of this issue?

Thanks.
This happened to me when my iPhone 5 battery died. If it's less than a year old, take it to an Apple Store or Authorized Reseller for replacement.
If it's older and you don't have an AppleCare plan, you can buy a compatible battery from eBay; I did and has worked great for over a year. 

It may even happen that Apple replaces it for free if it's a batch of defective batteries (mine was, but I replaced it before the announcement.

You can check the health of your battery with coconutBattery if you have a Mac.