Guys, that light is a very, very simple system. Ingeniously simple. It works like this.
Every light bulb has two ends: an end that has voltage, and an end that has no voltage. This is how 99% of light bulbs are wired: Power source, and ground. As long as the source has voltage, electrons flow, and the light lights up.
The battery light is different. It has two sources - one end is the alternator output and the other end is the battery output. If the alternator is doing its job, it is charging the battery to 12-14 volts. This means the alternator output is ~13 volts, and the battery output (since it's being charged by the alternator) is also ~13 volts. Since the two power sources are putting out equal voltages, they cancel each other out (think sound canceling headphones), and no electrons flow, and the light doesn't light up.
Now, the charging system is very simple... there are only a very few things that can go wrong. Either a.)the alternator isn't putting out enough (or nothing at all), b.)the alternator is putting out too much, or c.)the battery is not accepting the voltage that the alternator is producing. I will map out these three situations for you.
a.) There are a few reasons the alternator isn't putting out voltage. Some of the more common reasons are that the belt broke or is slipping (it happens more often than you'd think!), the voltage regulator ate the dust, the diode pack broke, or the brushes have worn out. In any case, keeping in mind the two-power-sources scenario I talked about earlier, that means that now one of the power sources isn't supplying as much power as the (12 volt) battery. Think arm-wrestling people - if both people are equally strong, the arms will stay up, but if one is stronger, then... well, anyway, you get the picture. That light now has one power source (the battery) and one ground (the dead alt). Electrons flow, and the bulb lights up.
b.) Now this failure is less common than A is, but I've seen it happen. The regulator can short out internally, and allow unregulated DC voltage into the system. I've seen alternators put as much as 18-20 volts into the system. Again, we'll recall the arm-wrestling scenario - since one "arm" is stronger than the other, voltage will flow, and the light bulb will light up. And your battery will blow up if left uncorrected for long! Man, if there is one thing you do NOT want happening, it's your battery blowing up...
c.) I'm sure you can spot the trend by now. A dead (or dying) battery means that it's not accepting the voltage that the alternator's putting out. If the battery is only taking 8 or 9 volts, then you will get 3 or 4 volts' differential through the bulb, and it will light up.
SO! Basically, what that all means is, if your battery light is lighting up, then I'd say there's about a 60% chance that your alternator is dead or dying, a 30% chance your battery is dead or dying, and a 10% chance that your alternator is putting out too much voltage.
Now, if your light is on at idle, but very, very dim, then that most likely means that your alternator is functioning within spec, but you have a very strong battery! If you have UDPs you will run into this situation a lot. I have UDPs, and at idle and a few loads my alt is only capable of putting out ~11.5 volts. Since I have a cheap battery, it's discharging equally so I don't get a light. But, when I had my red top in there, the light would glow constantly at idle... it would go away as soon as I tipped the throttle in. Now that I have a tweecer that isn't a problem any more.
I hope this helps some of you understand the charging system! It's really quite simple once you take a hard look at it.
