Battery power is a limiting factor for robots around the world, but it is particularly problematic for drones, which must weigh the number of batteries they carry, the number of other more useful items they carry, and how long they can last. The air. The consumer drone appears to have solved about a third of its total battery mass, resulting in a flight time of 20 to 25 minutes at most, after which you must bring the drone back to replace the battery. If everything a drone should do depends on it staying in the air, you're out of luck.
When this happens to larger aircraft, especially military aircraft that sometimes need to stay on the ground for long periods of time, the solution is in-flight refueling. Why take the plane all the way back to the fuel source when possible and bring the fuel source to the plane? Sure, it's easier to run on a liquid fuel than a battery, but researchers at the University of California, Berkeley, have come up with a clever solution: simply charge the battery. Or, in this case, add a shaft.
The large 820-gram quadrotor aircraft carries its own 2.2 Ah lithium polymer battery, which has a flight time of about 12 minutes. Each small quad-rotor aircraft weighs 320 grams and includes its own 0.8 Ah battery and 1.5 Ah battery as cargo. Small rotors can't stay aloft all the time, but that's ok, because when flying batteries, their only job is to turn from the ground to the large quadrotors and back again.
How a flight battery works
As each flight battery approaches the main quadrotor, the smaller quadrotor will be about 30cm above the passive docking tray mounted on top of the larger uav. It then drops slowly to about 3cm above, waits for it to align properly, then drops and lands on a tray, which helps to align its feet with the electrical contacts. Once the connection is established, the main quad-rotor aircraft will be able to fully power itself from the smaller UAV's battery payloads. Each flight battery can power the main quad-rotor aircraft for about six minutes, then fly away and get a new flight battery. If all goes well, the main quadrotor will use its main battery only during the disassembly and docking phase, increasing its flight time from 12 minutes to nearly an hour in tests.
