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If you should ever look at the un-loaded output of an Automotive alternator, you would be astonished at what shows up on the scope. Huge spikes upwards of 1,000 volts, RF noise, Inductive kicks, etc. The alternator is (electrically speaking) a very noisy device that will destroy just about every electrical device on your car if it's output is not regulated, filtered, and damped.
The capacitor on the alternator helps to suppress some of the electrical kicks that your alternator produces. While it doesn't stop them, it helps to soften them and weed-out the larger ones and some that start passing into the RF spectrum and generate radio interference. Your car battery does the rest... The battery functions as a HUGE capacitor and absorbs the rest of the spikes that the alternator produces and helps to 'float' the voltage at a nice constant value. If it were not for the capacitor on the Alternator, your battery cable (between the alternator and the battery) would be radiating all sorts of RF noise at large amplitudes in a very broad-banded way.
During the late '90s while I was in college and interning at Donnelly Corporation, I had to perform several "Full-Field" tests on the electronic parts we developed before we shipped them for production. In the lab, we had test equipment that would simulate the Full-Field output of an alternator without a car battery hooked to it. All of the automotive manufacturers (Ford, GM, Mitsubishi, -all of them) did not expect the parts to survive this test, they simply wanted to make sure that they would not catch fire if these circumstances should arise. Let me say that again: If you should disconnect the battery of your car while it is running, none of the automakers expect the electronics within the car to survive. While in the design stages, much of the EMC/EMI issues can be simulated and addressed (before actually building a prototype) with advanced automotive electrical modeling. Software packages are developed for these applications and Saber Modeling is one of the "top-dog" computer electrical modeling simulators in the industry. While the benefits are great, the price will set you back about $1 Million US Dollars (said without putting my pinky finger to the corner of my mouth).
Lastly, regarding the battery light in your instrument cluster. When you initially turn your key to the ON position (but not to START), this sends power through the Battery Light to the alternator and energizes the electromagnetic field. As the alternator starts turning, the magnets are spinning with an pre-established field and thus it produces power right away. If the coils are not energized before starting the car, they can develop the fields on their own, but you have to get the RPMs up there a bit before they'll "kick-in." It's takes a moment since you're working with a ramp-up voltage (usually comes up pretty fast) rather than starting at full potential. You can watch this with a multi-meter hooked to the posts of your battery if you've disconnected the Battery Light.
Sorry for the extra info. -Fond memories of college and the cool toys (err, lab equipment) I got to play with. When we got bored, sometimes we'd blow holes through our co-workers styrofoam coffee cups with the ESD guns. Those were BIG electric shockers that could punch holes through objects the size of a few water molecules using only a few PicoFarads (thus generating a very slow and visually undetectable leak). Hmmm.....now lets go play with the sonic welder.
God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 236K
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