Making a circuit isn't a problem, its knowing what the circuit needs to do that is the problem. A partial list of the things I don't know:
1) can you charge series connected lead acid batteries? You can't successfully with LIPOs I don't know about car batteries (my experience is limited to a charger designed to charge the single battery in my car Image may be NSFW.
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2) what constitutes charged? I think (but don't know for sure) the voltage hits around 14.4 volts. Do you damage the battery if you keep charging after that point? I don't know.
3) Do you need to limit the charging current in some way (I doubt, it I expect a car battery will take much more than your generator will provide, but I don't know for sure).
There are probably a bunch more I haven't though of as well. You need someone that is experienced in doing what you want to do (charging series connected car batteries) in order to insure you do this safely. Unfortunatly that isn't me.
