The Charging System - Part 2: The Alternator
So far, all we have talked about is voltage. Now we'll discuss the amperage output of the alternator. The state of charge of the battery controls amperage output. When the battery has a full charge, the electro-motive force of the voltage lowers the amperage to almost zero. As the battery charge wears down, the electro-motive force is not enough to stop the amperage, so it flows into the battery and charges it again.
As you use more electrical accessories, such as lights, heater etc., the electro-motive force decreases and more amperage flows into the battery to compensate for the added load. It is extremely important that when alternator efficiency is checked, both voltage and amperage outputs are checked. Each alternator has a rated amperage output depending on the electrical requirements of the vehicle.
In modern alternators, the voltage regulator and diode pack are inside the alternator and are not serviceable by most DIY's and mechanics. If any one of these parts goes bad, the whole alternator assembly needs to be replaced.
Testing the alternator is simple and basic. You need to check the voltage and amperage output. The voltage is easy enough to check with a voltmeter. If the voltage is between 13.5 and 14.5, it's a good bet the alternator is good. To check the amperage output, you'll need some special equipment. You'll need an ammeter and a load tester to check the amperage.
When the alternator is load tested, a simulated load, usually ½ the Cold Cranking Amperage rating of the battery is applied to the charging system. Then the engine is run at 2200 to 2500 rpm and the amperage is read on the ammeter. The reading should be close to or at the rated amperage of the alternator. If it is, then it's good. If it's substantially lower, then it is weak and should be replaced.
There are some signs you can look for that will indicate a problem with the charging system. If the headlights brighten or the blower motor picks up speed when you rev the engine, you may have a weak battery. This indicates the battery may not be holding enough current to run the car and is working off the alternator. If the charge light is on dimly at idle, this could indicate a weak alternator. If all the warning lights come on while driving, this could indicate a bad diode pack in the alternator. It may or may not still be charging, but you don't know unless you have it checked. At any rate, it will need to be replaced.
If you notice a strong sulfur smell, it could indicate a bad voltage regulator. The battery acid boiling inside the battery is causing the smell. This could lead to a battery explosion so be very careful when checking it.
Today's charging systems have come a long way and are very reliable. A new alternator can last seven to ten years and a rebuilt alternator can last five or more years. It's a good idea to have the system checked when you have the car serviced at every 30,000 miles to make sure it is in good shape and it will greatly reduce your chances of getting stuck with a dead battery.
© 2000 Vincent T. Ciulla