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.

1 Answer 1


In its simplest and most common form (some cars ECUs may do more than this) -

Initially power to the rotor of the alternator is supplied from the battery through the warning lamp which makes it glow. The rotor needs power to create a magnetic field.

Once the alternator starts producing current, the rotor is powered by the alternator, so no current is required from the battery, hence the warning lamp is extinguished.

There are lots of articles describing this in more detail, for example, this one from my quick search.

The warning light generally only warns of a faulty alternator, not a faulty battery, but as I said before, some cars may use this warning lamp to display more than not charging.

A snapped or loose belt will also light the warning lamp because the alternator will again be drawing power from the battery rather than generating it.

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