What does the 'battery' light on the dashboard actually mean?

Question

I know that when the battery symbol lights up on the dashboard it indicates a problem with the electrical system, which is commonly either an ageing battery or a broken alternator, but for the purposes of diagnosis, what does it actually mean in a circuitry sense? Does it come on when the alternator charging voltage is too low, or when the voltage supplied by the battery is too low?
I've had it come on in two different cars, for different reasons - first one the alternator was fine and the battery needed replacing, and second one, the alternator was worn out.

Solar Mike · Accepted Answer

The "classic" warning light was connected between battery positive and the alternator output.
This would then exhibit various states of illumination, which with experience, would tell one what was wrong without having to do all the tests stated in the workshop manual.
So, if the light was on solid bright then the alternator was not charging and that could be the alternator or a broken wire between alt and bat.
If the light had the merest glimmer then it was a failed diode - there were 3 for the warning light and 6 for rectification.
Some would have the light flickering and that would be brushes, but all of these would vary between the different makes of alternator - as what was one fault on one would be a different fault on another. Fine if you worked on several though.

What does the 'battery' light on the dashboard actually mean?

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