ah yes I see now, my sincere apologies, in the old days the 'excitation ' current was always sent to the rotor via a dedicated cable
from the ignition switch. that circuit has changed now taking the initial 12v from the alternator output cable, which would receive it via the battery, but continue to receive after output is established , thus it is possible to disconnect the battery +, and run ok.
get out of touch in my old years 55