Bypass a voltage regulator so your 6 volt dynamo charges a 12 volt battery. The voltage regulator is wired in line with the dynamo and regulates the output to 6 volts, ensuring that any electrical equipment powered by the dynamo isn't overloaded. Voltage produced from a dynamo is controlled by the speed the rotor turns -- the faster the speed, the higher the voltage, although there is a limit. Put simply, voltage is the pressure of electrical force flowing through wires. It can be compared to water pressure in a pipe -- the greater the pressure, the faster the water flows.
Disconnect the voltage regulator from the 6 volt dynamo so you can increase voltage. The voltage regulator is fitted in line between the 6 volt dynamo and the 12 volt battery so you need to bypass it.
Remove the wires that connect to the voltage regulator; the opposite ends of the wires connect directly to the 6 volt dynamo. Remove the wires that link from the voltage regulator to the 12 volt battery. Any other wires connected to the voltage regulator can be left in place as the regulator is now inactive.
Attach the wires from the dynamo that you removed from the voltage regulator to the wires that go to the battery. The wires are coloured the same -- red and black -- so match them up. Use some insulating tape to hold the sets of wires together.
Power up your dynamo. Faster speeds generate more voltage than slower speeds. Let your dynamo charge your battery for a couple of hours.
If other electrical equipment connects to your dynamo it needs to be turned off while you are charging your battery as the voltage may be too high and may damage the equipment.