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How alternators work
Alternators generate electricity by spinning an electromagnetic rotor inside a stationary cocoon of wire (called the stator). As the rotor spins, pulses of electrical current (AC power) are transmitted from the stator to the diodes. The diodes convert AC pulses into DC current, which is used to charge your batteries.
In order for the alternator to produce power, the rotor must be excited by a controlled source of amperage that’s applied to one of the alternator’s brushes. In automotive type alternators, that current is supplied by an internal regulator, which drives the alternator to a specific voltage value (usually about 14.1 volts). Deep cycle and sealed gel and AGM marine batteries require a more complex program of charging voltages to achieve their optimal charge. Multi-stage external voltage regulators, like the Balmar Max Charge and ARS-5 enable the alternator to vary charging voltages, based on the battery’s temperature and level of discharge, to ensure that batteries are recharged quickly and safely.
What to look for
Safety: Your replacement alternator should meet U.S. Coast Guard Title 33 safety protection standards. If the alternator is being installed on a gasoline engine, the alternator should also comply with international SAE and ISO standards for ignition protection. Balmar 6-Series alternators meet all three standards.
Heavy-Duty Construction: Marine alternators operate at sustained high output for much of their lives. This requires high-temperature grease, large cooling passages, large or multiple fans, heavy-duty bearings, high-amperage diodes, etc. Standard automotive-type alternators rarely offer the construction required to meet the demands of large marine charging systems.
High output: Deep cycle flooded, gel and AGM batteries demand substantially greater charging power than a standard starting battery, and can accept between 25% and 40% of their available capacities. Your alternator should be capable of delivering rated output equal to the maximum acceptance rate of your house battery bank. In other words, the alternator’s rating should be equal to 25% to 40% of your total battery capacity. For flooded batteries, size your alternator toward the bottom of this range. For AGM batteries, aim for the top (they’ll accept up to 40% of the amp hour capacity of these high-performance batteries).
It has to fit: Most marine engines use one of four mounting configurations: 1" single foot, 2" single foot, saddle-type dual foot with a distance of 3.15" between mounting feet, or a dual foot with a 4" gap between front and rear feet. Compatibility depends on engine model, engine year, engine compartment layout and other factors. Inspect your existing alternator mount and compare to the diagrams shown to determine the appropriate high-output replacement. You may need spacers or modifications to brackets, tensioners and wiring on any high-output alternator installation.
Belts & Pullies: High-output alternators use one horsepower for every 25 amps of output. A single 3/8" belt will handle an alternator output up to 70 amps. A single 1/2" belt will handle output to 100 amps. Alternators rated higher than 100 amps require dual drive belts.
Temperature Compensation: The ambient temperature of your batteries and alternator both effect charging efficiency. Balmar Max Charge and ARS-5 regulators both sense battery temperature and can regulate charging voltage accordingly. Alternator temperature sensing can also save your alternator by reducing output if the alternator overheats. Both alternator and battery temperature sensing features require optional temperature sensors (MC-TS-A and MC-TS-B).
Wiring & Fusing: Wiring between the alternator and the batteries must be matched to the amperage rating of the replacement alternator. A wiring chart, based on amperage output and wire length, is included in the Balmar alternator installation booklet.
Glossary
Temperature Ratings are often determined using a military standard of 122°F, measuring the ambient air temperature in the vicinity of the alternator. Sustained operation in hot engine room conditions will reduce alternator output. A "hot" alternator, operating at approximately 180°F –200°F will only supply about 80% of its rated capacity. Large-frame models generally run cooler than their smaller siblings because of their ability to dissipate heat. Small case alternators are not designed for continuous high-output operation.
Alternator Polarity depends on whether field (excite) voltage is applied to the alternator’s positive or negative brush. Balmar alternators and regulators are P-type. An experienced auto or marine electrical shop should be able to convert your N-type alternator for use with a P-type external regulator.
Rotor RPM is the speed at which the alternator turns, not the engine. Most engine pulleys are roughly double the size of alternator pulleys, so alternator speed is typically twice engine speed. Balmar’s 7-Series alternators are designed to spin at speeds to 15,000 rpm, making them suitable for diesel or high-revving gasoline engines. Large frame 94 and 95-Series alternators are designed for diesel engines, and shouldn’t exceed 6,500 rpm.
Dual alternators are an increasingly popular alternative on many engines. If excited by a single multi-stage regulator, both alternators can work together to handle large single battery banks. In addition, many dual alternator applications will use the standard alternator to charge the starting battery, while the second, high-output alternator is dedicated to the house battery.
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