low charge rate

Just finished mounting my battery to the rear of my el camino, and have noticed at night, with every thing on that the volt guage is showing only 11 1/2 volts. I have a 100 amp single wire voltage altenator, and its always showed 12 volts or more on the guage. I ran the single wire off the altenator, back to the battery with a 10 guage wire, to the + post, am i running to heavy a wire or is it something else. I've cut out my night driving for fear the battery will go dead! Any ideas?