ARGUMENT CHANGED INTO ” DOES FAILING ALTERNATOR AND BATTERY POWERS THE VEHICLE SIMULTANEOUSLY OR ALTERNATOR HAS TO FAIL COMPLETELY BEFORE BATTERY WILL KICK INN”
We can agree and disagree on certain things such as maybe multiple power sources can fed same load. As far as Vehicles are concerned, Output Voltage across battery should be 13.8 Volts and 14.2 Volts once alternator is charging the battery( Roughly). I understand, once alternator gets week then maybe it may not be providing 13.8 Volts to 14.2 Volts to the battery . Maybe it is providing 12.4 Volts and definitely a week current of maybe 0.1 A once headlights are on during Night, even then polarity has been reversed by Load Management Unit. Battery is still being charged but with a week current which may not be sufficient to restore the loss of charge during starting the vehicle or current maybe in few milli Amperes because Alternator is even facing hard time powering the headlight as well. Battery will only kick inn once Alternator fails completely.
This is my understanding because I read all this somewhere and I believe it to be true as other explanation cannot be true and I will be surprised if it is. Recently problem was developed in my vehicles alternator. I had to diagnose it completely, once I saw a battery indicator sign on my speedometer. I was able to take the vehicle to my home and later once I diagnosed there was no current coming out from Alternator and entire load was on Vehicle Battery. Problem was fixed later as one of the rectifiers within Alternator was found faulty.
Maybe, we can present references from site or let other members also comment on this.