High-Output Alternators

[SailNet]

For most sailors the primary charging source for the DC electrical system will be a high-output alternator driven by the auxiliary engine.

HOW THEY WORK
Alternators, named for the alternating current (AC) they initially produce, actually have internal "rectifying diodes" that change their electrical output from alternating to direct current (DC), making it suitable for battery charging.

STANDARD ALTERNATORS
Standard output alternators are supplied with most inboard marine auxiliary engines.

• These are fine for use on sailboats where time spent aboard is limited or the electrical load is light.
• They are not well suited for the performance battery charging needed by most cruising sailors.
• Standard alternators are inefficient on the order of one to two percent total energy conversion when an auxiliary engine is used just for battery charging. An operating 20 to 45 horsepower auxiliary engine produces an average of about 10 to 15 amps, or about 1/4 horsepower of electricity.
• Voltage regulators used on standard alternators contribute to poor performance. These automotive regulators prevent standard alternators from operating even close to their rated capacity for very long. They are a mediocre compromise between effective charging and battery protection from overcharge during long periods of motoring.

HIGH-OUTPUT ALTERNATORS
High-output alternators can make a dramatic difference in battery charging performance.

• These alternators provide the best power to cost ratio of all charging sources.
• High-output alternators, used in police and emergency vehicles, have high-quality components, close internal tolerances, improved cooling characteristics, higher output at lower engine rpm, and altered voltage regulation. A 100-amp high-output alternator will typically charge batteries four to five times faster than a 35 to 55-amp standard alternator.

PROPERTIES
High-output alternators are classified by rated output and case size.

• Small case models, which are direct replacements for almost all standard alternators, are the least expensive and range from about 75-amp to 150-amp rated output.
• Large-case models, slightly bigger in diameter, are quite a bit more expensive and range from about 160-amp to over 200-amp output.

TIPS

• When using the auxiliary engine solely for battery charging, ensure that time, equipment, and fuel are being used wisely. Charging batteries from an alternator while motoring is a good example of efficient power co-generation using one power source to perform multiple tasks.
• When comparing units, look at the charging curves showing rated output at various engine rpms. Some models give more power at lower rpms, and make a better choice for battery charging at engine idle speeds.
• Small case, 120 to 125-amp units with good output in the low to moderate rpm range are by far the most popular with cruising sailors.
• Most manufacturers rate their alternators' output when the unit is cold; actual output when the unit is hot is typically about 10 to 15 percent less.

ACCESSORIES

• A smart 3-step charge regulator or charge controller.
• Heavier output and ground wiring to handle the additional current.
• A shunt-type analog or digital monitor that displays alternator output.
• A high-voltage spike suppressor to prevent harm to the alternator diodes if the circuit is unintentionally opened while operating.
• If your high-output alternator is being fitted to a Yanmar engine, you'll need the "saddle mount" version, which has a double-footed mounting bracket instead of the more typical single foot.
• Alternators over 150 amps output often require two drive belts.

INSTALLATION
Fundamentals of DC electrical systems and a complete electrical toolbox are required for alternator installations.

Toolbox

• Screwdriver
• Drill and bits
• Bolts, nuts, and washers
• Soldering iron if required for connectors
• Multi-meter
• Wire ties and clips
• Tin-plated wire
• Terminal connectors
• Wire stripper/cutter/crimper

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