Put a continuous-duty solenoid switch like the ones used as a coach battery isolator on motorhomes between the alternator and the battery. The battery can't self-discharge if the circuit is physically disconnected from the rectifier. Energize the relay with a wire from the hot side of the ignition coil.
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!