It is very important to monitor charging system performance during the course of vehicle operation. Vehicles are equipped with an ammeter, voltmeter, or indicator light. These allow the driver to monitor the charging system.
The ammeter is placed in series with the circuit to monitor current flow into or out of the battery.
Ammeter connected in series with the charging system to monitor its operation.
When the AC generator is delivering current to the battery, the ammeter shows a positive (+) indication. When not enough current (or none at all) is being supplied, the result is a negative ( - ) indication. This is because the current flow is in the opposite direction, from the battery to the electrical system.
This is the simplest and most common method of monitoring AC generator performance. When the charging system fails to supply sufficient current, the light turns on. However, when the ignition switch is first activated, the light also comes on. This is due to the fact that the AC generator is not providing power to the battery and other electrical circuits. Thus, the only current path is through the ignition switch, indicator light, voltage regulator, part of the AC generator, and ground, then back through the battery.
Electronic regulator with an indicator light “on” due to no AC generator output.
Only the battery, regulator, and alternator are in the circuit. With no current flowing through the indicator light, it goes out.
With the engine running, the indicator light comes on again if the electrical load is more than the AC generator can supply. This occasionally happens when the engine is idling. If there are no problems, the light should go out as the engine speed is increased. If it does not, either the AC generator or regulator is not working properly.
On late-model cars, the indicator light can be combined with the oil pressure warning light; it usually is labeled "engine." If this light turns on while the engine is running, either the AC generator is not charging or the oil pressure is low - or both.
Voltmeters indicate the amount of voltage across an electrical circuit. The voltmeter shown below measures battery voltage when the ignition switch is in the run position.
Voltmeter connected to the charging circuit to monitor its operation.
A fully charged battery indicates just over 12 volts. As energy is taken from the battery, the voltmeter reading drops. At the same time, the voltage regulator senses this and raises the charging voltage output to 14 volts or more. When the battery has been recharged, the AC generator cuts back. The voltmeter now shows the fully charged battery voltage of just over 12 volts again. This on again, off again action of the AC generator is constantly monitored by the voltmeter.