Fitness and sleep tracker, ADI military-grade sensor Monitors heart rate in real-time Alert for incoming calls Ultra-thin 8mm battery Battery capacity: 41 mAh Battery type: lithium polymer Input current: 25 mA(TYP) Input voltage: DC 5.0 V Length: adjustable between 157mm and 205mm Bracelet material: Dow Corning TPSiV Requires Bluetooth® 4.0. Compatible with Mi 3, Redmi Note 4G, or any device with Android 4.4 and above. Compatible with iPhone models 4s and above that run on iOS 7.0 and above. Note: Mi Band's phone unlocking feature requires smartphones with MIUI or Android 5.0 and above.