Many of iPhone 6S users report that they find problem about their phone battery. They find that their iPhone 6S battery life is poor. This case is reported by iPhone 6S users quite often.
There is a fuss often discussed online that there are two possible variants of the A9 chip. Those chips are from TSMC or from Samsung production. Some users give opinion that the A9 chip from Samsung could result for an hour less battery.
If you get the same problem where your iPhone battery life is poor, you can try this several potential solution to overcome the problem.
You can check your chip type by Lirium Device Info Lite app if you curious which one do you have in your phone. You can get the application by downloading it for free.
You can check the model number of your chip at the bottom and you will see N66AP or N71AP if you have Samsung-designed chip in your phone. And you will see N71mAP code if you have the TSMC chip.
If you have Samsung-designed chip in your phone, may you want to change it with the TSMC chip to get the better battery performance, but there is no guarantee that it will totally overcome your phone poor battery life.
Please try to check whether there are apps that make your phone power drain easily. Check those apps by taping the Settings, then tap General, and choose Usage to see which apps are problematic. Please change or uninstall the problematic apps if you find some, in order to save your battery from draining quickly.
You also can use Low Power Mode that available in your phone by tapping the Settings and scroll down until you see Battery, then you are able to turn on Low Power Mode by tapping on the white circle toggling to green. You can optimize the battery usage by turning on that Low Power Mode. For more options to overcome poor battery performance, you can check this general battery saving tips article.