Abstract: A band system for a wearable device, or alternately, a flexible smart strap, worn as an article of apparel, for providing reserve power and enhanced functionality to a portable digital device. The band system or smart strap provides both a stylish fashion accessory and a functional rechargeable secondary power source to address an increasing demand for additional battery life due to greater digital demands and functionality of digital smart devices and optionally includes an interchangeable detachable strap with integrated rechargeable batteries, a charging circuit that integrates with a digital device, an interface for recharging the smart strap batteries or a connected portable digital device battery in combination with the smart strap batteries. An additional interface is also described, capable of providing further functionality for various types of data transfer.