Greg Mayman wrote in a message to Jay Emrie:

 GM> In general when alternators are on test in the workshop, there are
 GM> three things that are looked for:

 GM> (1) that there is output voltage as the speed is increased past    
 GM> the nominated minimum speed;

 GM> (2) that the regulator will stabilise the output voltage at the    
 GM> required value; and

 GM> (3) that the alternator can continue to produce this voltage    
 GM> while it is delivering increasing current into a load.

 GM> These factors are tested by watching the output voltage as you
 GM> gradually increase the speed, and then as you switch in resistive
 GM> loads.

 GM> But when you're charging a battery, the battery can maintain a
 GM> voltage even when the alternator is putting out zero current. 

 GM> Any dumbo should know that an ammeter reading zero means no
 GM> current flowing into the battery.

 GM> But a voltmeter that reads 12 volts... It's a 12 volt battery, so
 GM> everything must be alright, mustn't it?

Actually,  a fully charged battery (after surface charge goes away,  which
happens with a bit of a load in a fairly short time) sits at 12.6 volts. 
If it's being drained and not recharged,  the voltage tends to go somewhat
lower. If it's being charged,  the voltage applied _as measured at the
battery terminals_ is somewhat higher.

When I was selling and installing batteries for a living,  the first thing
I'd tell people who came in was not to turn it off,  and I'd put a
voltmeter across the terminals and see what was there.  If what I saw
looked like just battery voltage,  then I'd tell them they had a problem
with their charging system that should be fixed before a new battery was
installed.  Typically we'd expect to see from 13.5 to 14.5 volts.  Some
vehicles were somewhat higher,  but not too often.

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