We are not electricians. We collect clever work around solutions from automotive enthusiasts to help automotive enthusiasts.
A customer told us his recipe for using a 12V fan in his 6V system.
A ballast resistor allows an ignition system to operate at a lower voltage until the engine starts. The ballast resistor then functions to regulate the voltage (4-6V) going to the ignition system to avoid additional wear on the system e.g. burning up points.
That's it. That's the recipe. All of it.