I have a 1930's car with a Lucas dynamo. Originally it was a 6 volt system but now 12v. This is done without modifying the dynamo. Unloaded, the dynamo voltage can rise to over 20 volts and of course the working voltage is directly related to RPM. At present the 12 volt battery only begins to charge at 2000rpm engine speed. I've been wondering if a MPPT solar controller could work as a more efficient charge controller. Any thoughts on this heretic scheme ?
My best guess is that a solar MPPT controller is not a viable solution. That said - why not consider using a relatively standard buck - boost DC-DC converter?
There are several different types of buck - boost converter circuits readily available via Google. Start by looking up how a SEPIC converter works.
BTW - I think that using the existing dynamo in this fashion is a great way to go.