Measured and photographed by Snilard (Matěj). Thanks!
Tiny, rim-driven generator. According to manufacturer, it weighs about 52 g (without bracket) and advertises over 60 % efficiency. There are three variants: weaker Sport (red), medium Trekking (silver) and stronger Special (blue).
Friction between the rim and dynamo rotor is maintained by an O-ring made of some special UV-resistant rubber, expected lifetime is about 5000..7000 km. Noise levels are about the same as with tyre-driven generators. The ring slipped under big loads during the experiment, even more after wetting the rim. The owner reported slippage in wet conditions in normal use and eventually replaced the generator by accumulators. I don't know if it could have been remedied by better adjustment, or if it's a design flaw of the dynamo.
The output is more or less independent on wheel size, the rim travels at the speed of road. The output was rectified by a diode bridge and smoothed out by a capacitor for these measurements. The bridge was made of DB2141300L Schottky diodes with a voltage drop of cca 0.3 V. All other articles on this website use 1N400x diodes with voltage drop about 0.8 V - if you want to compare different generators, count with it.
Only voltage and speed was measured, current is calculated from the measured voltage and known loading resistance (I=U/R). Considering temperature dependence of the resistor and accuracy achievable by a standard multimeter, the results may not be perfectly accurate.
We get standard 6 volts at about 20 km/h and 12 Ω.
The generator is designed for low inductance, so it doesn't have the usual current limit. You just get more heat generated by ohmic losses as the current goes up.
Standard 3 W at 20 km/h, then the plot skyrockets. 16..20 Ω seems to be an optimal load.
Power doesn't differ much from other dynamos, but consists of higher current and lower voltage.
The manufacturer provides a thermistor for current regulation. It's basically a classic halogen bulb whose resistance grows with temperature. It brings the generator's characteristics close to the usual high-inductance types, so you can use it to power standard light sets. Of course, it would be more efficient to use the superfluous high-speed current to light up more and more lights, but that would need more sophisticated control circuit (not included).
Besides a passive diode bridge, we also tested an "ideal" bridge made of MOSFET unipolar transistors driven by a special integrated circuit. Its advantage is practically zero voltage drop and therefore better efficiency. A disadvantage is slightly more complex circuit board and a limited range of acceptable voltage and frequency. The control circuit was LT4320-1 with specified voltage range of 9..72 V and frequency up to 600 Hz, but it worked well from 2.5 V up to about 750 Hz (50 km/h).
How the Velogical stands against other high-end generator, the SON 28? Up to around 30 km/h SON is more powerful, Velogical takes over at higher speeds. If you need more power, you can "overclock" the SON by lacing it in a smaller wheel; with Velogical, you can buy the Special version. The Velogical is smaller, lighter, can be easily (without wheelbuilding) mounted to any bike, and its rolling resistance drops to absolute zero if disengaged. The SON doesn't heat up or wear and doesn't mind getting knocked a bit. But you can't wring more than half amper from the SON, so if you need to power some current-hungry electronics, the Velogical is an obvious choice.
More data and comparisons can be found for example in this German article.