You could have a high resistance joint somewhere, if it was bad it could cause total volt drop. Is the battery slow to charge?
I've found 1v dropping across the crimp to the battery terminal (The cable had a light coating of a green powder (CuSO4?)).
Such things are mechanical, ie a bit of movement or heat might change the drop. So probe around with a meter. Check the difference between battery voltage and alt voltage and lights voltage , then find where the voltage is being lost. The cables drop very little normally. After repair my starter cable was dropping <0.1V with the lights on.
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