Battery light circuit is the ON/OFF switch for the alternator
When you turn on the key the battery light in the dash gets 12volts on one wire
The other wire from that bulb runs to alternators voltage regulator, the Light Green wire.
Alternator is not spinning so is technically a Ground
With 12v on one wire and 0volt(ground) on the other wire the Battery Light comes on
And this power flowing thru the light bulb gives the voltage regulator "startup voltage", it applies this voltage to the alternators brushes.
You can spin an alternator all day long and as fast as you want and it won't generate any power at all, not 1 volt.
But if you give it Start Up voltage, then it can start generating power and will be self sufficient, as long as it is kept spinning.
So Battery Light circuit is the ON/OFF for alternator.
After alternator starts generating power, say 14volts, then Battery Light has 14volts on one wire and 14volts on the other wire, voltage regulator wire, light green wire
If a light bulb has the same voltage on both wires, no power can flow so bulb is OFF
Light bulbs work by heating up when voltage flows thru them, no flow is no heat up and no light up.
If Battery light starts to flicker or come on when alternator is spinning then voltage on one wire is BELOW voltage on the other wire, power is flowing thru the bulb.
Since the battery(12volts) is part of the system then Light Green wire's voltage would have to drop below 12volts(battery voltage) for the battery light to come on
So get volt meter out and test battery voltage when battery light is flickering.
Alternator would be drawing power not generating power at that time, so 12volts at battery not 13.5volt or higher with working alternator.
Yes, could be voltage regulator, or if it only happens at lower RPMs it could be alternators Fields are/have failed
One wire alternators
They only have the one B+ wire connected to battery
These use an RPM switch inside
When alternator starts to spin the RPM switch closes and internal voltage regulator is connected to the B+ wire, and that's the startup voltage
Why do alternators even need an OFF switch?
Because they can act as an electric motor, if you leave them connected to power, the rotor part, they will try to spin the fan belt, and of course they can't, lol, and this will drain the battery pretty fast.
So there needs to be a way to disconnect the rotor from power, since voltage regulator powers the rotor, it is shut off, either with Battery Light circuit or RPM switch