Borrowing a trick from the Air Force, whose planes can complete long-range missions thanks to in-flight refueling from tanker aircraft, researchers at UC Berkeley showed off a novel way to keep small drones in the air almost indefinitely: flying replacement batteries that can be swapped without landing.
The use of four propellers not only helps improve the stability of quadcopter drones while hovering in position, it also makes them incredibly maneuverable and fast. Drone racing has become a dizzying spectacle, but races rarely last longer than a few minutes because four propellers also require four electric motors which can quickly drain a battery. Adding more batteries adds weight, which requires stronger motors, and eventually you reach the point where the length of drone flights reaches a limit and fresh batteries are needed.
For drone racers, swapping in a new power pack between races isn’t a problem. But for other drone applications, like long-range reconnaissance missions or delivering parcels across a large city, having to land to recharge or grab a new battery can add unwanted complications and time delays. Seeking a different solution, researchers at UC Berkeley’s HiPeR Lab (High Performance Robotics Lab) came up with replacement batteries that can not only fly, but also connect to larger drones mid-flight.
To streamline the battery swapping process, which usually requires a cable to be unplugged and plugged back in, the researchers created a quadcopter with a landing pad featuring electrical contact pads on top. The replacement batteries have their own sets of propellers and motors and are designed to fly up and meet the larger drone and then touch down on the landing pad after the craft has stabilized itself. Power is then transferred through the battery’s landing legs, which feature electrical contacts.
The prototype created at UC Berkeley’s lab is powered by a 2.2 Ah lithium-polymer battery that can keep it aloft for about 12 minutes. The flying replacement batteries each have a capacity of 1.5 Ah (in addition to a smaller battery used to power the smaller drone as it flies to and from the larger craft) that extend the prototype’s flight time by roughly five minutes. The flying replacement batteries power all of the main drone’s electronics and motors once they land, but don’t recharge its primary battery. So during times when the drone’s landing pad isn’t being used, it slowly drains its primary battery. But in testing, the concept has shown to boost the prototype’s overall flight times from 12 minutes to well over an hour.
There are other logistical problems to overcome to make this approach feasible. Fleets of flying replacement batteries would have to be set up along a drone’s flight path to boost its range. That’s not a terribly difficult proposition for larger cities and delivery drones, but it adds complications for autonomous military drones which are often flying in areas where installing bases for additional flying batteries would be problematic.