Optimized Battery Charging protects your iPhone's battery by limiting or preventing the kind of aging that occurs when a lithium-ion battery is held at 100 percent charge. Essentially, it uses an algorithm to predict your schedule based on your use patterns. It then delays charging as needed to allow your phone to reach full charge right when you need it to. As Apple puts it, "Optimized Battery Charging is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged." That way your iPhone doesn't sit in a high-tension state known as trickle charging, which damages the health of your device battery!