The 3 brush won't do the job without a battery. The battery controls the system voltage with a 3 brush setup. However, if you convert the 3 brush to a 2 brush, and use a voltage regulator, then there's no problem. If you already have the 3 brush, I'd suggest throwing in a set of 12 volt 65A fields, remove the 3rd brush, add your favorite 12 volt regulator, and you're set. You might want to do a search here for more info on the 12 volt conversion. It should run a halogen headlight no problem.
how about a 2 brush 6v ? panhead maybe ? i'm useing some old 6v lighting i'd rather not convert.