The world of microdrones—tiny flying robots barely larger than insects—has been limited by a fundamental constraint: battery life. These miniature marvels have struggled to stay airborne for more than a few minutes, weighed down by the bulk and power limitations of their energy sources. But a breakthrough from engineers at the University of California, San Diego could change that, potentially keeping microdrones aloft for hours on end.
Solid-State Solution
The key innovation lies in a new “flying batteries” topology that combines ultra-thin solid-state batteries with a dynamic circuit capable of stacking and unstacking their voltages. Traditional lithium-ion batteries can provide around 4 volts, but the piezoelectric actuators that power microdrone wings require tens or even hundreds of volts to operate effectively. Patrick Mercier, an electrical and computer engineering professor at UC San Diego, explains, “The best way to [keep microdrones airborne] is to use lithium-ion batteries, because they have the best energy density. But there’s this fundamental problem, where the actuators need much higher voltage than what the battery is capable of providing.”
Enter solid-state batteries developed by the French national electronics laboratory CEA-Leti. These thin-film stacks of material, including lithium cobalt oxide and lithium phosphorus oxynitride, can be diced into tiny cells just 0.33 cubic millimeters in size and 0.8 milligrams in weight. Yet each minuscule cell can store 20 microampere-hours of charge, an energy density comparable to larger lithium-ion batteries.
Dynamic Voltage Stacking
The UC San Diego team, led by Mercier’s student Zixiao Lin, developed a circuit that can dynamically rearrange the connections among these tiny solid-state batteries, moving them from parallel to serial and back again in a matter of hundredths of a second. By starting with the batteries in parallel and then serially connecting them one by one, the circuit can rapidly build up the high voltage needed to trigger the microdrone’s piezoelectric actuator, causing its wings to flap.
But the true innovation lies in what happens next. Instead of simply discharging the batteries, the circuit reverses the process, unstacking the batteries one by one and allowing the residual energy stored in the actuator to flow back into the batteries in a process known as adiabatic charging. “Just like you have regenerative breaking in a car, we can recover some of the energy that we stored in this actuator,” says Mercier.
Impressive Power Density
The results are promising. With just 1.6 grams of solid-state batteries from TDK, the UC San Diego circuit reached 56.1 volts and delivered a power density of 79 milliwatts per gram. But with only 0.014 grams of custom batteries from CEA-Leti, it maxed out at 68 volts and demonstrated an astonishing power density of 4,500 milliwatts per gram—enough to potentially keep lightweight microdrones airborne for hours on a single charge.
While challenges remain, such as reducing the internal resistance of the microbatteries to allow for higher operating frequencies, Mercier is optimistic about the potential of flying batteries. “Adiabatic charging with charge recovery and no passives: Those are two wins that help increase flight time,” he says. With applications ranging from search and rescue to environmental monitoring, the ability to keep microdrones aloft for extended periods could open up a world of new possibilities.
Source: IEEE Spectrum