Charging Indicators

There are three basic methods of informing the driver of the charging system’s condition:

  • indicator lamps
  • ammeter
  • voltmeter

Indicator Light Operation

As discussed earlier, most indicator lamps operate on the basis of opposing voltages. If the AC generator output is less than battery voltage, there is an electrical potential difference in the lamp circuit and the lamp will light. If there is no stator output through the diode trio, then the lamp circuit is completed to ground through the rotor field and TR1.

Electronic regulator with an indicator light on due to no AC generator output

FIGURE. Electronic regulator with an indicator light on due to no AC generator output.

On most systems, the warning lamp will be “proofed” when the ignition switch is in the RUN position before the engine starts. This indicates that the bulb and indicator circuit are operating properly. Proofing the bulb is accomplished because there is no stator output without the rotor turning.

Ammeter Operation

Ammeter connected in series to indicate charging system operation

FIGURE. Ammeter connected in series to indicate charging system operation.

In place of the indicator light, some manufacturers install an ammeter. The ammeter is wired in series between the AC generator and the battery. Most ammeters work on the principle of d’Arsonval movement.

Ammeter needle movement indicates charging conditions

FIGURE. Ammeter needle movement indicates charging conditions.

The movement of the ammeter needle under different charging conditions is illustrated. If the charging system is operating properly, the ammeter needle will remain within the normal range. If the charging system is not generating sufficient current, the needle will swing toward the discharge side of the gauge. When the charging system is recharging the battery, or is called on to supply high amounts of current, the needle deflects toward the charge side of the gauge.

It is normal for the gauge to read a high amount of current after initial engine startup. As the battery is recharged, the needle should move more toward the normal range.

Voltmeter Operation

Voltmeter connected to the charging circuit to monitor operation

FIGURE. Voltmeter connected to the charging circuit to monitor operation.

Because the ammeter is a complicated gauge for most people to understand, many manufacturers use a voltmeter to indicate charging system operation. In early systems, the voltmeter is connected between the battery positive and negative terminals.

When the engine is started, it is normal for the voltmeter to indicate a reading between 13.2 and 15.2 volts. If the voltmeter indicates a voltage level that is below 13.2, it may mean that the battery is discharging. If the voltmeter indicates a voltage reading that is above 15.2 volts, the charging system is overcharging the battery. The battery and electrical circuits can be damaged as a result of higher-than-normal charging system output.

In most modern systems, the voltmeter is controlled either directly by the PCM or by information sent to the instrument cluster by the PCM. A dedicated circuit from the battery to the PCM allows the PCM to constantly monitor the battery voltage.