I happen to have a factory service manual on my desk for a 1978 Dodge full size van. This would be an electronic voltage regulator system, but should be representative of the older charging system "normal" charging system voltages.
From the book quote:

Test: Start and operate engine at 1250 rpm with all lights and accessories turned off. Check voltmeter. The regulator is working properly if the voltage readings are in accordance with the following chart.

Ambient Temperature
near the voltage
regulator: ------------ Voltage range:
-20 degrees F ------ 14.9 to 15.9
80 degrees F ------- 13.9 to 14.6
140 degrees F ------ 13.3 to 13.9
above 140 F ------- Less than 13.6
It is normal for the car ammeter to show an immediate charge and then gradually return to normal position. The duration the ammeter hand remains to the right will be dependent on the length of the cranking time.

End of book quote.
If the operating voltage is below normal, the entire electrical system will not operate correctly. I believe the OP needs to find out why the charging system is not operating in the "normal" voltage range. Gene