Does the OP's generator have the built-in thermostat that alters the charge rate according to temperature? There is also another 3rd. brush circuit that incorporates a resistor built into the light switch to change the charge rate from daylight to dark operation. None of the systems can actually charge a low battery- - - -the best to expect is maintaining whatever state of charge the battery is in at the moment. Back in the 1930's, some battery manufacturers had "battery charging stations" to top off their customers' batteries while they were at work.
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!