Chevrolet Repair: 1993 Lumina- Charging of Battery when idle, rear window defroster, voltage indicator

Question
Dear Sir;
When I am stuck in traffic, just idling, with lights on and rear window
defroster on, the battery voltage indicator starts to get low. (So have to put in
neutral and idle engine faster.)  And this occurred both before and after I
replaced the alternator and battery.  Is it just an underrated alternator that is  
standard?
Thanks-Joe

Answer
It might be the gauge itself.  Check battery voltage with a known good gauge.  There is nothing wrong with a vehicle that dips down to 13V.  MOst GM alternators charge at about 14.4-14.7 Volts at 1500rpm.  As long as more energy is being put into the battey than being removed don't worry about it.  Drive vehicle for a while and check electrolyte with a hydrometer.  If the specific gravity is good there is nothing to worry about.

Wayne