You only need about 7.5 volts to charge a 6V battery. Some of the 3-brush systems had an auxiliary circuit through the light switch to boost the charge output with the lights on. Unless you're really stuck on an original-equipment system, a later model 3-unit regulator and a 2-brush generator will keep the battery charged much better. I'm really surprised the 3-brush setup was able to develop 9 volts!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!