Battery life is the amount of time a portable device can operate before its battery needs recharging. It depends on battery capacity and how much power the device uses. Displays, wireless radios, processors, and frequent user input can all increase consumption.
Battery life matters because a device that stops working too soon may be unreliable or unusable in the field. In wearable hardware, designers estimate runtime and test the complete device under realistic conditions, then reduce power use or adjust the battery and charging design as needed. Security devices also depend on adequate battery life: a depleted sensor, badge, or authentication token may be unable to perform its intended function.