Replace that old, dead or dying battery with a brand new one!
For optimal performance, calibrate your newly installed battery: Drain battery below 10%, then charge uninterrupted to 100%.
The Fix Kit includes most of the tools needed to change your iPhone 6s Plus battery.
Note: the new “Battery Health” feature of most batteries in the current iOS Version is not compatible with this aftermarket battery. The battery will perform properly but iOS will not report on its health. The battery bar that displays how much charge you have left still works properly as well.