Alternator regulators were traditionally built into or attached to the back of the alternator and were functionally either producing power or not producing power. This approach is fine for many applications, particularly with power boats that run their engine, and therefore the alternator, for long periods of time. For other vessel operators, the engine run times may be shorter. This is prevalent with sailboats that will operate their engine typically for a fraction of their cruising time, or for power boaters doing only a short cruising day. To maximize charging efficiency, and thereby shorten engine run times to fully charge the battery bank from the alternator charging source, an external regulator has several advantages.
Most external regulators take advantage of the digital processing technology to create charging regimes that maximize charging efficiency. The power output will be tailored to the battery type and follow the three step charging protocol (a bulk phase, an absorption phase, and a float phase). This will charge the battery bank quickly and completely without over charging the batteries. The battery bank voltage can give the regulator a voltage reading that is more accurate, as it eliminates any error that may be introduced due to voltage drop on associated cabling.
There are other small advantages that are gained with external regulators. A soft start-up protocol allows the engine to be well started and belts to be well seated in before output power is ramped up. In addition is the ability to limit the output from the alternator in cases where full output is unnecessary or undesirable. However, the main reason for choosing external regulation over internal regulation is efficiency.
– Lyle, PYS Marine Electrical Technician