It's possible to crank a 6V generator/regulator up to 12 volts for a short time, but it's not a good idea. More likely, the battery just hasn't run down enough to fail to start the engine yet, and the generator is doing nothing. Unless the regulator has been readjusted to take the higher voltage battery into account, I can't imagine the battery getting charged much, if any.
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!