[Source: Vintage Triumph Register]
5.1. What If I Cannot Keep My Battery Charged and the Battery Tests OK?
5.2. How Can I Test To Determine If Charging System Large Enough?
CHARGING SYSTEM FUNCTIONAL DIAGRAM
[Source: Vintage Triumph Register]
Referring to Dan Masters' diagram above, 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 vehicle's battery. Its output capacity is directly proportional to the RPM of the engine and alternator temperature. Charging systems are normally sized by the car manufacturers to provide at least 125% of the worst-case OEM (Original Equipment Manufacturer) electrical load, so that the Car battery can be recharged.
ALTERNATOR OUTPUT
[Source: Balmar]
When the charging system fails, usually an indicator or "warning" 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 (as known as an "idiot") light is a direct 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 2500 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 during this period. 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 below 77° F (25° C), the charging voltage is increased to overcome the higher battery resistance. Conversely, as the ambient temperature increases above 77° F (25° C), the charging voltage is decreased. Other factors affecting the charging voltage are the alternator temperature, battery's condition, State-of-Charge (SoC), sulfation, electrical load and electrolyte purity.
VEHICLE CHARGING VOLTAGE
[Source: Bosch]
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 Car 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 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 is frequently low, or if you smell a "rotten egg" odor around the battery, then the battery is being over charged and the charging system should be tested.
5.1. What If I Cannot Keep My Battery Charged and the Battery Tests OK?
The vehicle's electrical load is normally satisfied first 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 35 amps at 2500 RPM, then up to 11 amps will be available for recharging the battery, which will take approximately six minutes. If the charging system is operating at say a maximum capacity of 90 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 Car battery are required to make up the difference to satisfy the 14 amp electrical load and the battery is being discharged further. This is why making short trips, driving in stop-and-go traffic, or during bad weather, the starting 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, electric wench 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 90 amp system. While the engine is running in this case, 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 alternator temperatures can further reduce the maximum output of a charging system, so cooling and sizing based on the continuous load matters. Heat kills alternators so Bosch, for example, has water cooled models available.
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. If the State-of-Charge is consistently above 95%, then the charging system is fully recharging the Car 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 to increase the capacity of your charging system to include changing the pulley diameters, replacing the voltage regulator, upgrading the alternator, adding a second charging system (for a dual battery set up), etc. An auto electric or alternator rebuilding shop can assist you. If the SoC is inconsistent, the you might consider using a temperature compensated, "smart" charger with a quick disconnector to "top off" your battery. If consistently under or overcharged, a lead-acid battery will lose capacity and prematurely fail.