My question is not “why does the red battery light come on”. I know it’s a battery or alternator problem. What I would like to understand is the physical mechanism that makes it turn on. Is it a voltmeter and when the voltage between the battery terminals is lower than the 14.5-14.7V expected in a charging electrical system it triggers ? Where is typically this voltmeter ? Is the dashboard ECU taking care of this ?
To summarize, I am asking about the electronic implementation, not about the meaning of the light. I didn’t find anything helpful googling as the web is flooded with article explaining that it’s to indicate an alternator or battery problem.