One of the advantages of running a alternator is once the motor is running the correct size, or to big, alternator will provide all the current needed to run the car and charge the batteries at the same time
I have seen my voltmeter show 13.? volts on start up after sitting a long time between slow charging and having the carb needles stick open and flood the motor at the races, it showed less than 14 volts for around one minute and then showed 14+ volts for the rest of the day
My 15.to 1 comp ratio E85 motor starts fine on two group 24 R.V. Marine deep cycle 12 volt batteries