86 2.3 Electrical/Charging system questions

Hi;

My 97 cobra is parked and my 86 2.3 LX winter beater is out!

Background;

With headlights, turn signal and heater/rear defrost on, the rate at
which the turn signal flashes goes down significantly than when there
are no accessories running. My assumption is that the electrical system
is not producing enough juice at idle to power everything properly.

With the car off, I am reading 12.1 volts at the battery with my Fluke
multimeter. With the car at idle, and no accessories turned on, my
multimeter reads 14.09 volts at the battery. HOWEVER, with all
accessories on, and the car in drive (yes Jim W, parking brake on and
someone in car with foot on brake ), my multimeter only shows around
12.5 volts at the battery.

Question:

Is 12.5 volts at the battery a normal voltage reading given all
accesories running and the car in drive? The car seems to run fine
enough, but I want to make sure I dont have electrical/charging problems
over the winter.

TIA,

Chris