Alternator Basics From Powermaster
While modern electrical systems are generally referred to as 12-volt systems, in operation with the engine running voltage will be in the area between 13.5 and 14.4 volts. If you have a one-wire alternator on your truck try this. Start the engine and let it idle and note the voltmeter reading. Now turn the lights on and rev the engine to the point the alternator begins to charge and two things should happen, the voltage will increase and the lights will get brighter. What's happening is the increased voltage is pushing more current through the electrical system, consequently the lights become brighter.
Normally all the power for the electrical system is delivered by the alternator when the engine is running. The only time that this isn't the case is when the current capacity of the alternator is exceeded, which may happen at very low idle speeds with all the electrical accessories turned on, or if the alternator isn't "excited." Many one-wire alternators have to turn a certain rpm (generally around 1,400 rpm) before the internal voltage regulator will turn on (or excite) the charging system. A "blip" of the throttle may be necessary to rev the engine high enough to "turn on" the alternator. Interestingly, a similar problem can occur with too much gear. Combine tall rear gears, large-diameter rear tires, and an overdrive transmission and engine speeds when cruising on the highway may be low enough that the alternator can't keep up with the electrical system's demands. There are a few simple strategies to prevent the preceding problems. First, don't use underdrive pulleys that slow down the alternator. Second, make sure the alternator exceeds the maximum current requirements. Finally, use an alternator that is capable of delivering its maximum output at an appropriate rpm.
Alternators are usually described by their output-65-amp, 100-amp, etc. When replacing the alternator on the family car, this is probably the only information that is necessary to get a replacement that matches the original. But when building a custom truck from the ground up a deeper understanding of the power curve of an alternator is required. When selecting an alternator the first thing you must know how is many amps the electrical system consumes.
This, of course, varies from truck to truck and can be figured out two different ways. One way is to add the total amp requirements of all the components together. Generally, all electrical items will have their supply demands included with the instructions. Another way is to use an inductive ammeter with a "peak hold" function clamped around the battery cable while the car is running. While the latter method would be more accurate since it would be testing the electrical system as a whole in real world conditions, it's not always the most practical. The point is that when everything is turned on at the same time, the amperage demands on the electrical system can be surprisingly high, so select an alternator accordingly.