Fast charging is an excellent feature, particularly when you’re in a hurry. But with it comes the dangers of extreme wear and tear, especially if the charger isn’t certified. The heat produced when you’re charging quickly can shorten your smartphone battery’s life span. To extend the life of its batteries further, Sony has introduced Soft Charging in the beta version of the Marshmallow AOSP update for the “classics” Sony Xperia Z2 and Z3. What it does is it “slows down” charging to keep the temperature of the battery as cool as possible. You can activate soft charging with its own on/off toggle.
Source: Android Community