Battery charge indicator

The indicator in the instrument cluster has two positive feeds: one from alternator terminal D+ (blue wire) and the other from terminal 15 through the blue multi-pin connector that attaches to the rear of the instrument cluster from the ignition switch (black-blue wire).

When the ignition is switched on, voltage is applied to terminal 15. The alternator, however, is not yet running, so the de-energized D+ terminal acts as "negative". The indicator lights up because a potential difference exists between the vehicle electrical system, supplied by the battery, and the non-running alternator.

When the engine is running and the alternator has reached the speed required for charging, the voltage regulator connects it to the vehicle electrical system. Power is now supplied from terminal 15 and additionally from terminal D+. When there is no potential difference, the charge indicator goes out.

When the ignition is switched on, the illuminated charge indicator must pre-excite the alternator. Only in this case can the alternator produce current at low engine speeds. However, pre-excitation is only required when the alternator is first started.

Charging does not always occur
Even if the charge indicator is off, this does not necessarily mean that the battery is actually being charged. It only means that there is no longer a potential difference between the battery and the alternator. If, for example, all electrical consumers are switched on at engine idle, the charge indicator remains off, although more current is drawn from the battery than the low-output alternator can supply; nevertheless, there is no voltage difference with the battery.