I have 2004 monte carlo and I have notice that my voltage is never above 14 volts it is always around 13.5-13.9. I run to batteries for my stereo and have all 0 gauge wire for the big three and to the back battery. What could be the reason for this and when i turn off anything that could lower the voltage such as the heater on or playing music from the stereo, it is still around the 13.9 volts.