Your car's charging system is composed of an alternator (or DC generator), voltage regulator, battery and indicator light or gauge. While your engine is running, the charging system's primary purpose is to provide power for the car's electrical load, for example, ignition, lighting, audio system, accessories, etc., and to recharge your car's battery. Its output capacity is directly proportional to the RPM of the engine. Charging systems are normally sized by the car manufacturers to provide approximately 125% of the worst-case OEM electrical load, so that the battery can be recharged.

When the charging system fails, usually an indicator light will come on or the voltage (or amp) gauge will not register "good." The most common charging system failure is a loose, worn or broken alternator belt, so check it first. If you rev up the engine and the alternator light becomes brighter, then the battery needs to be fully recharged and tested. If the light becomes dimmer then the problem is most likely in the charging system. The next test requires use of a known-to-be-good battery. Attach this battery to the engine and run the engine at 2000 or more RPM for two minutes. Depending on the load and ambient temperature, the voltage should increase to between 13.0 and 15.1 volts for a fully charged battery. Most cars will measure between 14.4 and 14.8 volts on a warm day, depending on the battery type that the charging system was designed for.

Most voltage regulators are temperature compensated to properly charge the battery under different environmental conditions. As the ambient temperature decreases, the charging voltage is increased to overcome the higher battery resistance. Conversely, as the ambient temperature increases, the charging voltage is reduced. Other factors affecting the charging voltage are the battery's condition, state-of-charge, electrical load and electrolyte purity.

If a battery terminal's voltage is below 13.0 volts and the battery tests good after being recharged, or if you are still having problems keeping the battery charged, then have the charging system's output voltage and load tested. Also, have the car's parasitic load tested (the load with the ignition key turned off). A loose alternator belt or open diode will significantly reduce the alternator's current output. If output voltage is above 15.1 volts with the ambient temperature above freezing, and the battery's electrolyte level frequently will be found to be low or if you smell "rotten eggs" around the battery, then you are probably overcharging the battery and the charging system should be tested.

What if you cannot keep your battery recharged and the battery tests OK? The vehicle's electrical load is satisfied first by the charging system, and then any remaining power, if any, is used to recharge the battery. For example, if the total electrical load is 64 amps and the charging system is producing 80 amps. Up to 16 amps will be available for recharging the battery. If the charging system is operating at maximum capacity of 80 amps, then the battery usually will be recharged within five minutes. Now let us assume that the engine is idling and the charging system is only capable of producing 20 amps. Forty-four amps from the battery are required to make up the difference to satisfy the total electrical load. The battery is being discharged further. This example is why that during short trips or while in stop-and-go traffic, the battery may never get recharged and may even "completely" discharge.

Using the example above, let's assume that you add an after-market, high-power audio system or lights that adds 20 amps of load. With a total electrical load of 84 amps, even at maximum output, the battery will never be recharged with an 80-amp system. During operation, the battery must make up the 4-amp deficit. The solution is to upgrade the charging system to 125% or more of the NEW worst-case load. In this example, you would need a charging system capable of 105 amps or more of electrical output.

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