  We present a novel approach to increase the flight time of a multirotor via
mid-air docking and in-flight battery switching. A main quadcopter flying using
a primary battery has a docking platform attached to it. A 'flying battery' - a
small quadcopter carrying a secondary battery - is equipped with docking legs
that can mate with the main quadcopter's platform. Connectors between the legs
and the platform establish electrical contact on docking, and enable power
transfer from the secondary battery to the main quadcopter. A custom-designed
circuit allows arbitrary switching between the primary battery and secondary
battery. We demonstrate the concept in a flight experiment involving repeated
docking, battery switching, and undocking. The experiment increases the flight
time of the main quadcopter by a factor of 4.7x compared to solo flight, and
2.2x a theoretical limit for that given multirotor. Importantly, this increase
in flight time is not associated with a large increase in overall vehicle mass
or size, leaving the main quadcopter in fundamentally the same safety class.
