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In an ideal world, all vehicles would have at least one gauge, preferably two, indicating the state of the electrical/charging system. Ideally one measuring the main bus voltage and one indicating either the charging or discharging of the battery.
For reasons of keeping the instrument panel simple, and for economy of manufacturing, many cars don't have either of those, but instead have just a simple light that warns that the charging system has something wrong with it. This light looks like a battery, and in my apparently naive, optimistic little world, this light goes on to indicate that the charging system is having problems. Ideally, the battery light going on indicates battery discharge. In principle, this happens any time the battery is discharging. I assumed it was normally implemented as a voltage sensor; if the charging circuit voltage is above a set value, the light is off; below, and the light is on. Say, if the threshold is 13V, then the normal 14.4V charging circuit value has the light off, but 12V (running on just the battery) it's on.
After having alternator trouble with my 1996 Ford Escort, I found out that apparently this scheme is way to simple for the engineers at Ford that the time this car was designed. What I was told at the Ford dealer the first time I had it fixed (incorrectly) was that the engine computer will turn off electrical components to try to preserve battery life, without ever having turned on the charging light. This defies all logic and reason as far as I can tell. Does anyone know if this is true or not? That the voltage light won't come on, despite the fact that the alternatur clearly isn't working properly, because there are mechanisms that are supposed to be making the battery last longer? If you've had this experience, please drop me a line and confirm this.
The symptoms that happened to me were that the cruise control stopped working, and then the tachometer turned off (engine still running). Not too long after that, it started having trouble staying running, then quit.