voltage gauge reading

[Kent McLean]

At the start of my most recent trip from NC to NH (850 miles,
one way), the voltage gauge on my '89 200 TQ was reading
less than 12 volts.  Bravely, I pressed on.  It worked
its way back up to just over 12 volts.  It made for an
interesting trip. You may recall I lost the alternator belt
during a snow storm on the previous trip, and the new belt
isn't a Continental.

My question is, "What is the normal reading for the voltage
gauge?"

Here's my car's "normal": When the car has been sitting for
a week or two and I start it, the gauge will hit 13+ volts
(or so it seems; the markings are poor). After 10 minutes or
so, it will drop down to just over 12 volts (~12.2).  On
occasion (usually when it's wet, but lately when taking off
from a standstill), I'll hear the belt squeal a little.
Sometimes when I start the car, and it is idling, the gauge
reads below 12V, then works its way up. So far, I haven't
been stranded by a dead battery.

Does the voltage gauge read battery voltage? Alternator
output?  Voltage regulator output?  Some door handle
engineer's idea of voltage? I'd try and decipher the
Bentley wiring diagram, but the electrical experts on
this list are so much better at it. :-)

Thanks,
Kent and "Bad Puppy" back in NC