Your alternator is the primary part of your car or truck's charging system, which includes the battery, alternator, and associated wiring and controls. Those controls may include a voltage regulator, separate from the alternator, or on many newer vehicles, an on-board computer.
The alternator's job is to generate electrical current to replace what was used from the battery to start the vehicle, and to run all of the electrical accessories that are in use as you drive. That includes lights, A/C fan, radiator fan, electric seats, windows, radio, and whatever other gadgets you have.
The alternator was NEVER intended to be a battery charger. If you forget and leave your lights on, never jump-start the vehicle and "drive it around" to build the battery back up. I know grandpa said you could do that, and in grandpa's day of 35 amp generators you could easily get away with it. Today, most alternators are capable of making over 100 amps of current, but only for short periods of time.
If you drive on a discharged battery, chances are, you will overheat your alternator. If it is not destroyed right then and there, it will surely be damaged and it's life greatly reduced. Jump-starts should be for dire emergencies only, with full knowledge of the consequences. As the amperage of the alternator has gone up, so has the price. That jump-start could cost a pretty penny.
Also, NEVER EVER disconnect the battery with the engine running. Grandpa could do it when there were no electronics in the car to be damaged. Some vehicle computers cost over $1,000 to replace. Do you really want to take the risk? Of course, if the alternator is not working, this will rarely damage a thing. But isn't that what you are trying to find out? If it is charging, you will likely send a huge voltage spike through every electronic part on the car, including on-board computers and electronic control modules which operate the engine, air-conditioning, cruise control, anti-lock brakes, transmission shifting, and oh yes, the radio! It's your money!
There is a simple way to check the alternator. All you need is an inexpensive voltmeter.
First, read the voltage across the battery terminals. It should read about 12.6 volts. If it is lower than 12.2, you must charge the battery before you continue.
Second, start the engine and observe the voltage. It should INCREASE to 13.5 to 15 volts (depending on the vehicle, temperature, and several other factors).
Third, turn on the lights and A/C-heater fan on high. These are the two biggest electrical loads you can add. Now raise the engine speed to a little above idle and observe the voltage. If the alternator is working, it should maintain a voltage ABOVE 13. Some vehicles will read higher.
If you suspect the alternator is not charging after this test, you should have the system checked further. The alternator could have failed or another part of the system could be preventing it from doing the job.
The battery, its state-of-charge (or discharge), its condition (internal resistance), and the condition of the cable ends are the most important part of your vehicle's electrical system. Taking care of the battery and cables will add many miles to the life of your alternator. Always inspect them when you check your oil. You do check your oil, don't you?
Information Compliments of B&H Electric, Orange Park, FL