iirc... Isn't it the case of if the voltage is starting to drop, it's because the amps are the limiting factor? Once the alternator has reached it's amperage limit, the formula forces the voltage down to compensate. (I'm sure an adult will be along soon to put me right)! I seem to remember, the petrol Focus as an example had an 80A standard or 110A if heated front screen fitted (as the heasted screen draw was approx 30-40A). The voltage should remain constant. If the wrong alternator was fitted, they'd charge at about 11.9v with all consumers on...