Quadrotor flying robots, valued for their VTOL capability, agility and compact design, underpin diverse missions such as aerial photography, inspection, delivery and search-and-rescue. Their endurance, however, is fundamentally bounded by the onboard battery, and the mechanical structure of the quadrotor gives it only a poor payload capability, so the flight time cannot be extended simply by enlarging the pack. Two routes are therefore available: raising the capacity and efficiency of the battery, or managing the remaining capacity precisely and in real time. This paper follows the second route and develops a battery monitoring system for a quadrotor flying robot.