Failure of one or more diodes can cause the maximum charging current to be
low. This can cause a low battery condition when lights or air conditioning
are being used but adequate charging when loads are low and trips are long
enough for the battery to become fully charged.

Since I do not know your skill level or the mechanics of your alternator I
would suggest having it tested. In our area most of the auto parts stores
will test it for free.

Don Young

A number things come to mind in order of most likely occurance.

1- worn sticky brushes
2- blown triode
3- dirty connections (this could be #1)
4- regulator (maybe)

The voltage regulation on just about any modern car is set about 14.5
volts -- or more -- not 13.5.  No.
With a healthy charging system and battery, your voltage should come up
to 14.5 pretty soon after starting the car (as measured with a decent
digital meter, not the dash gage).

Some reasons the voltage might be low:

Battery going bad (probably not the case here).

Battery that has been deeply discharged.  A battery that's low enough
can suck up all the alternator power, holding voltage down.  But
eventually, as the battery charges, the voltage will come up.

Bad voltage regulator.  But they are either good or fail altogether.  A
voltage regulator causing mere inaccuracy or weak charging is unusual.
I wouldn't think first of a bad voltage regulator based your
description of the symptoms.

A bad diode in the three phase six-diode pack.  Note that in floating
field alternators such as the Delcos used on GM vehicles, there is
another set of diodes besides the heavy rectifiers:  the exciter
diodes.  The exciter diodes are much smaller than the output power
recitifiers because the exciters only have to carry the field current.
But if you open the alternator to check the heavy rectifiers you should
also check the exciter diode pack.  It has three diodes in it.  I don't
know if they still look like this, but on the older alts I've worked on
the exciter diode pack is a little orange rectangular plastic thing
with three tabs on one side and a single tab opposite it.  The three
tabs are connected to the wires on the stator.

Corroded connections, either in the wiring harness or inside the
alternator.  When I bought my used 86 Burb, it would hardly charge at
all.  Opening up the alternator, I found heavy rust and cobwebs.
Unscrewed the connections and scraped everything clean, now it charges
like a champ.  Apparently it sat for years in heavy humidity.  You
should also check the wiring harness connections that carry any
substantial current.  Like the battery connections and ground strap,
for example.

Pulley on the alternator too big.  This results in an alternator that
turns too slow at idle to charge.  I had that problem on an old car I
owned once.

There's more, but I'd need to know more about your particular car to go
much further.

A few of these have been mentioned already, but...

Get the battery load tested (NAPA etc can do this).  Even if it's holding a
~13V charge, if you can put a 1 amp load on it and have it drop to 9 it's
not going to do you any favors.  ;)

IF you go to the trouble of pulling the alternator out on your auto to just
have it tested - put a remanufactured one in and be done with it!  Trust
me - after you go through it once - you won't want to do it again.  On a
standard it's not a big deal.  Auto's are a PITA.

Don't overtighten the belt or you'll be pulling it again to replace
bearings.  Look into the kit from Rodney (www.rodneydickman.com) that lets
you use a larger belt, with a MUCH better wrap to stop it from
slipping/squealing.

Within ~20 sec's after start, you should see 14.2-15V (as usual, don't take
the dash gauge as gospel).

...

Thanks to everyone for the answers - I believe my next step is to take it
to the shop. I used to work on my own car, when I was 20 years old and
you could climb into the engine compartment, but I fear those days are
long past. ;-)

Thanks!
Rich