High Power marine alternators and regulators

Just about everybody interested in improved electrical system performance ends up upgrading their alternator sooner or later. The stock alternators that come with standard engines typically have an output of 55Amps. When the alternator gets hot the typical performance would be in the region of 25-30 Amps. Alternator upgrades can boost this to anywhere from 75 Amps to 150Amps and when combined with a high performance regulator they can make a dramatic difference in energy production.

Factors to consider when choosing alternators include:

Engine size
Mounting configuration
Mounting bolt size
Pulley configuration
Intended use.

The regulator is the device that drives the alternator and determines how much power it produces. Modern high performance regulators give a three stage charging cycle for maximum output, are adjustable to suit different battery types and offer temperature compensation. Some offer temperature protection for the alternator itself.

Most high power alternators use external regulators so that they can be adjusted to suit battery type and size. For details on how to modify an existing alternator you might want to contact your local alternator shop. Beware though that many alternators might not be able to stand the hard work they are called on to give by an external regulator and this might lead to premature failure.

All other things being equal, bigger is better. However for more practical purposes you need to look at two aspects.

1. Mechanical : How big an alternator can your engine comfortably support.
2. Electrical : How big an alternator do you really need.

1. Mechanical
Look at belt size, alternator size and the size of the alternator mounting bolt. A 3/8"belt is maxed out at something between 80 - 100 amps. A 1/2" belt gets maxed out at about 120 amps. After that you need double belts.
Look at mounting bolt size. 5/16 mounting bolts are good up to 100 Amps. For bigger alternators it is possible to use this bolt but you are probably over doing what your engine was designed for.
Engine size. For smaller engines it is best to limit alternator size, so for a 3 cylinder engine 90-100 amps would be max. for an alternator.

2. Electrical
The Rule of thumb is that the alternator output in amps should equal 25% of the battery capacity in amp hours. For a 400 amp hour bank, a 100 amp alternator would be a wise choice. You can figure 1 hp draw per 25 amps (12 volt) at maximum output.

Look at your engine to determine the mounting configuration of your existing alternator (use the mounting diagram above as a guideline) and then turn to the Balmar pages to select a suitable size for your application.

Yanmar power takeoff pulleys from Mack Boring

Suggested reading:
Balmar's guide to selecting an alternator and regulator. Includes recommendations for different engine types. 8 pages in *.pdf format