With two terminals, large is output and goes to starter solenoid (same terminal as battery connects to). Small terminal needs 12v to start the alternator charging. Normally comes from key switch, a wire that is hot when the key is ON. It should have the alternator light in the circuit. 12v from key switch to alternator light, then to alternator. This light will NOT have a ground. Light will be ON when key is ON, and go OFF when the alternator starts charging. Think of it as battery voltage coming from the key to the light, and once the alternator starts to charge alternator voltage going back towards the light. The voltages cancel and light will go out.
You might want to have the alternator tested.