Alternator Theory and Voltage Regulation

All alternators are very similar in design and construction, providing rectified AC for 12 volt and 24 volt systems. The voltage regulator is a simple , robust, low cost device designed essentially for the motor car, where the demands on the battery are minimal. Engine starting is the basic function of the battery, after which the alternator supplies all the essential equipment on the vehicle.

The role of the regulator is essentially to 'clamp' the alternator output voltage at approximately 14.2 volts ±0.4 volts in order to protect electrical equipment. Conventional regulators perform this function well and, because voltage losses are usually very small on motor cars, it is extremely unlikely that sophisticated battery management systems will find their way into average-sized family saloons.

Also, because the alternator voltage-regulator provides little or no compensation for changes in ambient temperature, car batteries tend to be overcharged in the summer and undercharged in the winter causing premature battery failure, usually during a cold spell. However, the cost of battery replacement is not prohibitive.

In the case of the following:

• Marine craft with long cable lengths, considerable electrical equipment and reduced engine running time;
• Executive cars with engine management-systems, telephone and on-board computers;
• Large commercial vehicles with electric tail-lifts, cab-heaters, telephones or radios;
• Fire Engines, Ambulances and Social Services Vehicles, Mobile Library Vehicles;

the requirements are totally different and the demands on the battery system substantial. The same type of voltage regulator is employed and this is inadequate for the circumstances.