This is a discussion on Alternator Not Charging Battery within the Chassis and Body forums, part of the CJ Forums category; After two months of ownership and picking away at some punch list repairs, I was anxious to take my 74 ...
When an alternator does not charge, the charge light will come on. Since the alternator needs a small amount of electrical power to make more power, many systems will run a circuit where current has to travel through the charge light to power the rotor. Once the alternator starts producing voltage (usually within a second), the alternator can use its own current to run itself. At this point, current stops flowing through the charge light because there is equal voltage on both sides of the light and the light goes out. If the alternator does not produce voltage, the light will remain illuminated. Many newer systems will simply have the PCM control the charge light.
When the alternator is not charging, (eg. engine not running) all the electrical power for the vehicle comes from the battery. Electricity will flow (conventional theory) from the positive side of the battery, through the circuit and any switches/control devices to the load. From there it will travel to a frame ground which is connected to the battery negative terminal. Once electricity has flowed back into the battery, the battery’s job is to take that energy, and put it back into the positive side of the battery. This process consumes an amount of the battery’s electrical potential by changing the battery’s internal chemistry which means that this process cannot continue indefinitely.
This 1998 Ford Windstar with the 3.8 liter engine came in with the complaint that the alternator would not charge. The alternator had been replaced twice and still nothing. Imagine that? I confirmed that the alternator was not charging and then moved onto circuit tests.