The "Battery" light is run by the voltage regulator. It means the charging system is not working, but on some models the light also gets turned on for low and for high charging voltage.
Also, on some models, once the generator is up and running, a tiny amount of output current goes right back in to run that field winding. GM's very nice 1986 and older system is a perfect example. Those are self-energizing, but that current to run them has to be "rectified" through three diodes, called the "diode trio". Those are quite different than the other six large diodes used in all AC generators. ("Alternator" is the same thing, but they were developed by Chrysler for 1960 models, and they copyrighted the term). When one of those diodes in the diode trio fails, which was very common, in effect, the voltage regulator turned off one-third of the time. That resulted in the dash warning light turning on dimly. Often there were no other symptoms or problems because those cars did not use much electrical power. It is possible to have a similar problem today, but a computer circuit detects the problems, then turns the warning light on full brightness. We do not see dimly-lit lights any more.
When one of those six output diodes fails, all you will be able to get is exactly one third of the generator's maximum rated capacity. 30 amps from the common 90 amp generator is not sufficient to run the entire electrical system under all conditions. The battery may have to make up the difference until it slowly runs down over days or weeks. Also, it varies by brand and model how the voltage regulator will respond. The output is three-phase alternating current that gets rectified and turned into direct current by the diodes so it can be stored in the battery. When a diode has failed, one of those phases is missing, and when that one is supposed to be occurring, output voltage will drop quite a bit. Some voltage regulators respond to that by bumping up charging voltage. Some systems continue to operate normally, but the drop in voltage results in less current through the field winding, a weaker magnetic field, reduced output current, and therefore reduced voltage.
The point is, with a failed diode, it is quite possible for charging voltage to go up a little. The clue is since the regulator still has control of the system, voltage will not increase when engine speed is increased. I should clarify that three things are always needed to generate current mechanically. That is a wire, (coil of wire works better), a magnet, (an easily-controlled electromagnet, in this case), and most importantly, movement between the two. That's where the drive belt comes in, and is why speed is a factor.
What you should consider now is having the charging system tested at a shop. You did the first step already by measuring charging voltage, but all that means is it's okay to do the rest of the tests. You need a professional load tester to measure "full-load output current" and "ripple voltage". If one of the diodes has failed, as I already mentioned, you will only be able to get one third of the rated current. Ripple voltage is the difference between the highest voltage developed, and the lowest it drops to during the missing phase. Most testers simply show it between "low" and "high" on a relative bar graph. A few that can make printouts will show it as an actual voltage.
Sunday, February 19th, 2017 AT 10:47 PM