How to Know When Your Alternator Is Defective
You can test batteries with a voltmeter or multimeter. Depending on how it reads with and without a load, you can determine whether your alternator is failing to charge the battery when the engine is running, or if the battery is the problem.
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Instructions
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Run your engine for half an hour to fully charge the battery.
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Set your voltmeter to DC, since car batteries are sources of direct current.
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Set the meter to "Volts" if you are using a multimeter. Otherwise, in an ammeter setting, the resistance could be too low and you could seriously damage your battery. A voltmeter, on the other hand, has a high resistance, which is why you won't short the battery.
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Turn off the car ignition and all electrical devices, like the radio, headlights and air conditioner. Remove the ignition key to prevent any circuit from completing.
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Touch the voltmeter's metal probes to the battery: the black probe to the negative (-) terminal, the red probe to the positive (+). It doesn't matter much which way you do it, but this way you'll get a positive reading.
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Adjust the voltmeter's display to read between 8 and 14 volts. If the display reads between 12.5 and 12.8 volts with the transmission off, this is a good sign that the battery gets sufficiently charged. Below 12 is not satisfactory and suggests a charging problem. (Below 10 is almost dead.)
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Insert the key and turn on a turn signal or the hazard lights. Don't turn on the engine. Test the voltage again. The battery should still manage around 12 volts under this load. This tests the internals of the battery, so if the test of the alternator fails you know it's not the battery's internals that are the cause. The battery should stay above 9.6 volts under load. If not, the solution is to restore it with a three-step recharger and Epsom salts to reverse any sulfation.
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Turn on the engine. This is when the alternator starts working, converting gasoline energy into stored battery energy. Test the battery's voltage again. A reading of 13 to 14 volts indicates the alternator is working. Less than this indicates the alternator has a problem--if you haven't found other problems with the battery up to this point. Voltage above 15.5 means the battery isn't accepting the charge. This is a fault of the battery, not the alternator.
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Check voltage drops along connections as well. The voltage difference between alternator and battery should be less than 0.1 volts. More than 0.4 volts means a problem like dirt, corrosion or a loose connection. Check the voltage drop between the battery terminal and the clamp that goes on the battery terminal as well. If the voltage drop is high, you may want to clean the terminal.
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Tips & Warnings
Don't disconnect the clamp from the battery terminal while the ignition is running. The resulting sparks can ignite battery gas and cause blindness.
References
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