A car's charging system is easily the biggest pain in the ass, as far as car audio is concerned. This paper will outline several methods for diagnosing and fixing charging system problems.

    So you're driving along listening to the tunes and suddenly your head unit and your amp start cutting out. You think "that's strange" and try turning it off for a little while. You turn it back on and everything's fine....until you turn the car off. Then you hear the familiar sound of a dead battery trying to turn your car over. Let's try to come up with a list of symptoms related to charging system troubles:

(1) Head unit/amplifier cutting out

This indicates low system voltage which means that either your battery is in the midst of being deep-cycled or that something is draining a lot of power.

(2) Dead battery, even after driving a lot

This probably indicates alternator troubles, although it could also be the battery refusing to accept a charge. It could be as simple as your brushes, or as much of a pain as the windings.

(3) Car not running smoothly

This probably indicates low system voltage which could be depriving your ignition system of power (low spark), or confusing your ECU.

   

    Now that you've decided that you have a charging system problem, here are some methods of isolating your exact problem.

Battery:

This is probably what you should check first. Batteries that often get drained quickly will eventually refuse to accept a charge. First thing to do is pop the covers off of the battery (if it's a wet cell - if it's a dry cell or a gel cell, skip to the next step) and check the water level. If it's low, add some distilled water (it's very important that the water be distilled to avoid deposits forming on the lead in the battery and contaminating the acid). Next, hook the battery up to a charger for however long it takes to charge. If the charger indicates that the battery is not charging, bingo, you've got your problem. If the battery accepts the charge (and you didn't have to add water), you've got charging system troubles. If not, replace your battery, but consider testing your charging system anyway. Most autoparts stores will gladly do a "load test". This is the only good way to determine a batterys condition, and could prevent you from getting rid of a perfectly good battery.

Alternator:

The alternator is the heart of your car's charging system. This ingenious device converts the mechanical motion of your car's engine into electrical current, which in turn charges the battery and helps run the car's electrical system. Diagnosing alternator problems can be a little tough, but here's what you do.

1. Do everything under the Battery header.

2. Use a multimeter to check the system voltage. It should be about 12V. Now turn the car on and check the voltage. It should be near 14.4V. If the voltage rises only slightly above 12V or not at all, you've got alternator troubles. If it rises to 14.4V, your alternator is most likely fine. If you show excessively high voltages, that is, above 15 volts, then the culprit is still your alternator. Another quick test, is to start the car, and then remove your battery + terminal. If the vehicle stalls, or suddenly idles really rough, then it is assuredly your alternator. If your alternator checks out, but you still drain your battery while listening to your stereo, your stereo is drawing much more current than your alternator can put out. Consider replacing your alternator with a high-output (65A+) model.
   

3. Visually inspect the alternator's windings through the case. Are they blackened, or do they still have their insulator? Is the insulator flaking off? If the insulator looks burned or it is flaking off, the windings are shot. This means that you can either replace the alternator or have it re-wound. Re-winding is usually cheaper, but count on not having a car for a week or so.
   

4. If the windings look fine, remove the alternator from your car. For god's sake, turn the thing off first, ok? Now it's time to look at your brushes. There are a couple of different ways to check the brushes on alternators. The first is to remove the (usually orange) rubber stops on the bottom of the alternator and look in. The second is to remove the four bolts holding it together and taking it apart physically. Once you've done whichever you have to (taking the thing apart sometimes involves a hammer ;) ), look at the brushes (these are the things sticking out at the bottom of the alternator and touching the contacts on the part that spins). If you can't see springs and the brushes touch the contacts, they are probably fine. If this is the case, take your alternator to your local auto parts store and have it load tested. The regulator may be shot. If not, then buy a new set of brushes and install them.
   

Well, that's about all I can tell you about your car's charging system. If you have any other questions, feel free to get ahold of your friendly local teamROCS member and grill him/her.