Wow – where will technology take us next? The Master Lock No. 4400D Bluetooth Padlock features a 1-29/32in (47mm) wide metal body for durability. The 9/32in (7mm) diameter shackle offers 7/8in (22mm) vertical clearance and is made of boron alloy, offering maximum resistance to cutting and sawing. The smartphone/app interface offers speed, ease of use, & greater control with the ability to share access; with keyless convenience. The locking mechanism features anti-shim technology, offering maximum security. And don't worry about the battery running out. There is backup access using directional code on lock keypad; low battery notification & reminder on padlock and in app; and a battery jump that allows for temporary access if battery dies while locked. It comes with an easy to replace CR2450 battery.