The Ultion Nuki takes 4x AA batteries, which typically last several months. When the battery charge level falls below 20%, you will be informed via the Nuki app and the Smart Lock through the LED lights. You can also set email notifications. Even if you overlook these, the Ultion Nuki is a regular mechanical lock, too. You can use a key from the outside and the dial on the interior until the batteries are replaced. The Ultion Nuki is also compatible with rechargeable batteries. However, we suggest more extensive usage of the rechargeable batteries Panasonic Eneloop Pro. These batteries last about double the life of alkaline batteries.