There will be anywhere from 13.6-14.4 volts at the battery terminals with a healthy battery and the alternator charging. Think of the battery like a bank account. You take money out and you put money in. As long as you put in more than you take out, no problems. The Alternator is like your salary. Your radio, fridge and phones are like your bills. However in the case of your charging circuit, the currency isn't dollars, it's amperes.
So say your alternator is putting about 30 amps into the battery and you are only using 20, everything is great.
Alternators work by sensing the voltage of the battery. If this voltage falls below a certain point, the alternator turns on and charges. When the voltage gets high enough it turns off. Now the alternator is subject to engine speed as well. At low engine speeds, the alternator isn't making many amps. Even if it is charging, it is a very low rate so if the load is more than what is being put back, you are discharging the battery.
Two ways around that. Bigger battery bank or bigger alternator.
I won't even get into the advantages of a deep cycle and a starting battery and a split charging system.
1966 109 5 door wagon 300Tdi "spermaceti fueled"
1994 RRC LeWiB "ruining the air behind me"
All my troubles are Rover