USB offers great opportunities as a power source for all types of low power electronics, many of which are battery operated. The widespread availability of USB presents unique opportunities as well as challenges for battery-charging designs. This article describes how to interface a simple battery charger to a USB power source. This review of USB power bus characteristics include an overview of NiMH and Li+ battery technologies, charging methods, and charge termination techniques, and a complete circuit example for smart-charging NiMH cells from a USB port.