Does Checking Voltage Tell You If Your Alternator's Charging Properly?

In a word, No!

Now lets back up a second and say yes, you can tell if it is 'charging', but the question is, is it charging properly and does it have full capacity.  Checking voltage alone won't give you those answers.  You need to know the current output with and without a load to tell you if it is working properly.  There are even more sophisticated testing procedures that can be done, that tells you condition of various internal parts, but I won't go into that.

Usually I tell my customers, after I've checked or repaired their unit, to charge their battery fully.  Then check the voltage of the fully charged battery across the terminals and it should read about 12.5 volts.  Then start the vehicle and check again and the voltage should read about a volt higher or 13.5 volts.  Once the alternator has re-charged the battery, for what the starter used starting the vehicle, and if all unnecessary electrical loads turned off, voltage should approach the regulators set point of around 14.5 volts.  If you are still only reading battery voltage, then you may have a wiring problem, which is not connecting or energizing the alternator with the system. 

Getting back to our example of why checking voltage won't tell you what the alternators condition is, here is why.  On two separate but identical vehicles with 100 amp alternators, with regulators that have a set point at 14.5 volts, one of them is putting out 13.0 volts and when the current output is checked, it is shown to be putting out 20 amps.  The second vehicle is also putting out 13.0 volts, but the amperage output is 90 amps.  Both of these alternators are charging, but the one only putting out 20 amps is defective.  Why?  Because at a lower voltage, the alternators current output should be much higher to help build the battery closer to the regulators set point.  This lower output would indicate a bad stator or bad diodes in the rectifier and the alternator would need repaired.

I might also mention that if that 100 amp alternator is putting out 90 amps continues output and doesn't regulate down, it indicates either a bad regulator or a weak or bad battery.  If it is the battery, that alternator or it's replacement won't last long, because even though it is a 100 amp alternator, it can not continually run at that high of an amp output very long without burning up!  Alternators are only designed to run continuously at 75 percent of their rated capacity.  So a 100 amp alternator can only produce 75 amps of continuous output.

So just checking the voltage output may tell you that the alternator is working, but won't tell you if it is good!