Richard,
From Brian's post, I gather that your ped has two searate alternator coils, like my Motobecane.
If one end of each of these coils is grounded, as mine are, then you can only use a half wave rectifier and depending on the phasing of the two coils, perhaps only one of them. Given these restrictions, Brian's idea of charging with the brake lamp coil and switching to battery at night makes some sense. Ask him for the schematic and check it out.
Ron