SON 12V-Info
how to install a 12v Audax setup
The Son hubdynamo can also be used to power two 6v lights in series. The dynovoltage will then be around 12 Volts. This is possible because the dynamo can be assumed to be a constant current device. The 6v designation presumes a 3w load. The SON is highly efficient (even more so at 12V)  and will cause at 12v less resistance than a conventional bottle generator at 3 watts. 
Conventional bottle generators can supply 12v as well in theory, but slip will be prohibitive. 
efficiency [rendement] vs roadspeed [snelheid]

The SON dynamo can supply 12V-6w from approx 20 kp/h (12 mph) onwards (see graph) At slow speeds it is to be preferred to revert to 6V, one bright light being superior over two dim yellow ones. 

voltage [=spanning] vs roadspeed [=snelheid]

The wiring schematics for a 12v setup is shown below. Because the SON isn't earthed to the bicycleframe, construction is easy, For other hubdynamo's the front and rearlights have to be kept isolated from the rest of the bike. But with a R&M headlight-mounting this is easily achieved.

For 12v use you need a special headlight fitted with a short-circuit switch: the sophisticated E6-Z. The  E6-Z features a capacitor to raise the dynamo voltage at lower speeds. This lowers the speed where the two lights together have a higher output than a single one from 12 mph to about 10 mph. 
If you are prepared to make your own wiring loom, the Lumotec and E6 headlights are available without switches at a (much) lower price.

This is the original German text