  This paper proposes an Emergency Battery Service (EBS) for drones in which an
EBS drone flies to a drone in the field with a depleted battery and transfers a
fresh battery to the exhausted drone. The authors present a unique battery
transfer mechanism and drone localization that uses the Cross Marker Position
(CMP) method. The main challenges include a stable and balanced transfer that
precisely localizes the receiver drone. The proposed EBS drone mitigates the
effects of downwash due to the vertical proximity between the drones by
implementing diagonal alignment with the receiver, reducing the distance to 0.5
m between the two drones. CFD analysis shows that diagonal instead of
perpendicular alignment minimizes turbulence, and the authors verify the actual
system for change in output airflow and thrust measurements. The CMP
marker-based localization method enables position lock for the EBS drone with
up to 0.9 cm accuracy. The performance of the transfer mechanism is validated
experimentally by successful mid-air transfer in 5 seconds, where the EBS drone
is within 0.5 m vertical distance from the receiver drone, wherein 4m/s
turbulence does not affect the transfer process.
