Why This Alternator Test Works

Testing A BAD Alternator: Symptoms And Diagnosis

Here's a little simple theory (and I stress simple) as to the how and why of this test. After all, we didn't test the amperage output of the alternator, only voltage. So why is this test valid?

First, we must remember the definition of Volts and Amperage.

  1. Volts: a measurement of potential energy. Can also be referred to as an electrical pressure or force.
  2. Amperage: a measurement of the flow of electricity.

The battery is designed to maintain a voltage on its own (when not in use- potential energy) of about 12.6 Volts. When the battery is made to work, for example, to start the vehicle. A chemical process takes place that releases electrons from the mixture of acid and metal plates within. As this process continues (the releasing of electricity) the stored voltage (potential energy) starts to decrease AND the plates within the battery start to sulfate. This causes the voltage to drop even more.

If after the car has started and lets say for the sake of this conversation that the headlights are turned on, the exit of electrons will increase (amperage) and voltage will further decrease (if the alternator is not there to supply the load's needs).

In a nutshell, the alternator must charge above 12.6 Volts to force the plates inside the battery to desulfate, thus restoring the acid's chemical and the plates' physical properties. And to supply the electrical needs of the vehicle. All this just to repeat the process of starting up your vehicle again.

So if the alternator is working properly, measuring voltage should and will suffice to test its performance. My rule of thumb has been that a steady 13 ( to 14.5) Volts is an indication that Amperage is present to meet the electrical loads of the vehicle. Since one does not exist without the other. You can think of it this way: Voltage is the force or pressure behind the ‘flow’ (amperage) and that without this force or pressure (voltage) there wouldn't be any flow.

Put this test to practice, It works! Y'all come back, now... store hours are from 8 to 6, Monday thru' Saturday, Closed Holidays.

As promised in the first part of this article, this is the link to the second article on testing the alternator that you may find helpful:

  1. Easiest way to Test for a bad ALTERNATOR on the Vehicle.
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