The alternator constantly senses the output voltage and when a preset voltage is reached – usually 14.2 volts – will progressively shut itself down to supply just enough current to maintain that voltage.
It is possible to buy devices which sense battery condition and allow the alternator to push through a higher voltage, thereby ensuring faster charging. These are useful when the batteries are used a lot, but not the engine. Consult a specialist when considering this type of equipment.
Some alternators can be offloaded electrically at start up to reduce the load on the starter motor. The lubricating oil pressure switch is usually the agent for bringing the alternator onto load. When the oil pressure rises, the warning light goes off, simultaneously switching in the alternator.