Use a simple self-holding relay circuit to isolate power to your buzzer silliness until the engine starts. You can use oil pressure to activate the self-holder, which will continue to provide power until the engine is shut down again. A freshman electrical engineering student knows how to wire a self-holding relay.

The only time a 100 amp alternator would even approach supplying its full output to the battery would be during a deliberate load test using a carbon-pile battery load tester to confirm the capacity of the alternator. If you had an ounce of practical experience to go with your theoretical pipe dreams, you would already know that.
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!