So-called supercapacitors, AKA ultracapacitors, are amazing devices. While they don’t store as much total energy as a comparable battery, they can discharge this energy extremely quickly. They can also charge rapidly, as demonstrated by Mike Rigsby’s “Little Flash.” As described in his project write-up, the mini rover “runs for twenty minutes, charges in ten seconds.”
This energy storage capability is due to a trio of 350 farad supercaps wired in series, and he’s even experimented with charging them using another bank similar charged caps. Rigsby, probably wisely, decided to do this experiment from a distance, which transferred current at a staggering peak rate of 45 amps. While the charges in this experiment stayed under 10 volts, with that kind of power transfer, using caution with this kind of setup is certainly a good idea.
As far as what the robot actually does, an Arduino Uno board along with a motor shield cause it to wander around, powered by a pair of continuous-rotation modded former servos in a differential drive configuration. Motor over-current detection, a bump switch on the front, and random path adjustments keep it from getting stuck. The little bot doesn’t really go anywhere in particular, but it’s reportedly “somewhat relaxing to let everything else go and just watch the movement,” as seen in the video below.