Last Updated on March 12, 2003
Written and compiled By Bill Darden (freely reproducible public domain)
Re-produced here by www.optimabattery.co.uk for your information, and it will be updated regularly.
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. It's 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 increase the engine speed 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 indicator or idiot light is a comparison between the voltage output of charging system and the voltage output of the battery. The next test requires use of a known-to-be-good, fully charged battery. Attach this battery to the engine and run the engine at 2000 RPM or more for two minutes. Depending on the load and ambient temperature, the voltage should increase to between 13.0 and 15.1 volts. Most cars will measure between 13.8 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 decreased. Other factors affecting the charging voltage are the battery's condition, State-of-Charge, sulfation, 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, the electrical load with the ignition key turned off, tested. (Please see Section 10.) A slipping alternator belt or open diode(s) will significantly reduce the alternator's output capacity. If the output voltage is above 15.1 volts with the ambient temperature above freezing, if the battery's electrolyte level frequently to be low, or if you smell a "rotten egg" odor around the battery, then the battery is possibly being over charged and the charging system should be tested.
5.1 What if you cannot keep your battery recharged and the battery tests OK?
The vehicle's electrical load is first satisfied by the charging system and any remaining power is used to recharge the battery. For example, if the total electrical load is 14 amps and the charging system is producing 30 amps at 2500 RPM, then up to 16 amps will be available for recharging the battery and take approximately six minutes. If the charging system is operating at say a maximum capacity of 80 amps at 5000 RPM, then the battery usually will be recharged within two minutes. Now let us assume that the engine is idling and the charging system is only capable of producing 10 amps. Four amps from the battery are required to make up the difference to satisfy the 14 amp electrical load. The battery is being discharged further. This example is why that during short trips or driving in stop-and-go traffic, the battery may never get recharged and may even become "completely" discharged.
Using the example above, let's assume that you add an after-market, high-power audio system or lights that adds an additional 20 amps of load. With a total electrical load of 34 amps, at RPM below 2500, the battery will never be recharged with an 80-amp system. In this case while the engine is running, the battery must make up the deficit. The solution is to upgrade the charging system to 125% or more of the new worst-case load. In this example, based on your stop-and-go driving habits you would need a high output charging system capable of 105 amps or more. High temperatures can further reduce the maximum output of a charging system. Heat kills alternators so some are water cooled.
5.2 How can I test to determine if charging system large enough?
A simple test to determine if the charging system is large enough is to check the battery's State-of-Charge after the surface charge has been removed. (Please see Section 4.). If the State-of-Charge is consistently above 95%, then the charging system is fully recharging the battery based on your driving habits and electrical load. If is is consistently below 80%, then you will want to consider upgrading your charging system to produce more current. There are several possibilities such as changing the pulley diameters, replacing the voltage regulator, upgrading the alternator, adding a second charging system (for a dual battery set up), etc. Auto electric or alternator rebuilding shops can assist you. If the State-of-Charge is inconsistent, the you might consider using a three stage, microprocessor controlled charger with a quick disconnector, like a Battery Tender, BatteryMinder, etc., to "top off" your battery. If under or over charged, a lead-acid battery will lose capacity or fail prematurely.