Charge lamp · alternator · D+

Battery warning light on: what it means

The battery light does not measure the battery. It sits between the ignition and the alternator’s D+ terminal and lights whenever the alternator’s voltage is below the battery’s. Knowing how it is wired tells you where to look.

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How the lamp is wired

Ignition + (terminal 15) → charge lamp → alternator D+. With the engine off, current flows through the lamp into the alternator: the lamp lights, and that current excites the alternator. When the alternator charges, D+ rises to the same voltage, nothing flows through the lamp and it goes out.

What the lamp is telling you

On with the engine running: the alternator is not charging — broken belt, worn brushes, a failed regulator, or a broken B+ cable.
Never lights with the ignition on: a blown bulb or a broken wire to D+ — and many alternators then do not start charging at all.
Glows dimly or flickers: a voltage difference between the two sides, often a bad connection or ground on the alternator.

See the lamp go out

Open the “Starting and charging” demo and turn on the ignition — the lamp lights.
Drag the engine speed up: the alternator charges and the lamp goes out.
In the editor, disconnect the alternator B+ cable and run it again: the battery is no longer charged.

Questions

Can I keep driving with the battery light on?

Only a short way: the car runs on the battery alone and will stop when it is flat, often within half an hour.

Does a new battery fix the warning light?

Rarely. The light means the alternator is not charging; a new battery will go flat the same way.

Why is the lamp needed for charging?

On many alternators the current through the lamp is what excites them at start; with a dead lamp they do not start charging.

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