Yes, all models of the Apple Watch utilize an internal lithium-ion (Li-ion) rechargeable battery. Apple chose this technology because lithium-ion batteries are lighter, charge faster, and have a higher power density than traditional batteries, which is essential for a small wearable device that needs to last through a full day of activity. These batteries work through the movement of lithium ions between the anode and cathode during charge and discharge cycles. Because they are "consumable" components, they will naturally chemically age over time, which reduces the amount of charge they can hold (Maximum Capacity). In 2026, Apple Watch software includes "Optimized Battery Charging" to help slow this aging process by learning your daily charging routine and waiting to finish charging past 80% until you need it. Because it contains a lithium battery, you are generally required to keep the watch in your carry-on luggage rather than checked bags when flying, as lithium batteries are a fire safety concern in the cargo hold, though a watch on your wrist is always permitted.