Cheap noname "bottle" generator from a big department store. Nominal output 6 V / 3 W.
The generator has two spring-loaded terminals instead of usual one, so you can choose between the second pole being grounded to the bike frame, or going through a second cable. There is also a built-in overvoltage protection:
This device is called transil and it works like two antiserial Zener diodes: opens when voltage exceeds a predefined value (8.2 V in this case), regardless of polarity. So it cuts off peaks of the generated alternating voltage waveform:
The purpose is to prevent blowing the light bulbs when riding fast. But it is not suitable for my direct current LED lights because the peaks are smoothed out by a capacitor and a Zener diode is behind it anyway. So I removed the transil and all the following measurements were taken without it.
I usually equip my bottle dynamos with speed reducers (see photo) which significantly reduces their rolling resistance at higher speeds. In this case it would increase the cog diameter from 19 to 28 mm and reduce speed by cca 68 %. Both speeds with and without the reducer are shown in the plots.
The output is independent on wheel size, the tyre always travels at the speed of the road. Measured without overvoltage protection, rectified by a diode bridge and smoothed out by a capacitor.
Open circuit voltage rises steadily, I haven't reached any limit. At 35 (52) km/h it is 45 V.
Maximum short circuit current doesn't exceed the 590 mA even at higher speeds.
3 W at 20 km/h - standard. Optimum load is somewhere between 30 and 40 Ω.
590 mA is the largest current I have ever got from1 a dynamo. That could mean less ohmic losses and rather good efficiency. If only there wasn't that inefficient friction transmission from the tyre...
Source data to download (XLS, Excel 97).