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Alternator Selection

How Do I Choose The Proper Alternator For My Vessel?
Once battery capacity and battery type have been determined, the size of the alternator can be calculated. If the batteries used are standard or deep cycle flooded, the alternator output should be sized at 25 percent of the combined capacity of all house batteries. If your house battery capacity is 400 amp hours, for example, the alternator’s rated output should be 100 amps. If AGM or gel technology is used, the alternator output may need to be increased to 35-50% of the combined battery capacity, as those battery technologies will demand greater output.

Will A High Output Alternator Work With My Engine?
Depending on your application, the size of the engine and drive belt may limit ability to support a high-output alternator. In most cases, an engine with a 3/8″ wide drive belt will support an alternator rated at 70 to 80 amps. A greater load may result in belt dusting and premature belt failure. A single 1/2″ belt will typically support a 100-amp to 110-amp alternator. It may be necessary to consider converting to a dual pulley system if a 120-amp or larger alternator is used. Some engine applications use serpentine (flat) belts to drive the alternator and auxilliary loads. In most cases, a 5-groove or 6-groove serpentine belt will be compatible with alternators rated to 150 amps.

What About Mountings?
Nearly all marine engine manufacturers rely on one of four specific alternator mounting styles:

  • 3.15″ I.D. Dual Foot (Hitachi style) – Mounting arm at engine pivot point features two mounting feet, one in front, one in rear. Distance between mounting feet is 3.15″. (See Image 3 at left)

  • 4″ I.D. Dual Foot (J-180 style) – Mounting arm at engine pivot point features two mounting feet, one in front, one in rear. Distance between mounting feet is 4″. (See Image 4 at left)

Our Alternator Application Guide provides a general guideline for various engine applications, matching engine manufacturer and model to the corresponding alternator series and model. Unfortunately, actual mounting styles can vary based on model year, marinizer, and other factors. We strongly recommend comparing your existing alternator to the different mounting configurations shown at left to ensure a proper fit and an easier installation.

How Does My Alternator Size Up?
Making sure your new alternator fits into the same location as your old alternator can make all the difference in avoiding installation irritations. Balmar alternators are broken down into three categories: small case, large case, and extra-large case.

Small case 6-, 7- and 8-Series alternators are usually appropriate to be used in place of standard alternators on most marine gasoline and diesel engines. Some larger engines may support large or extra-large case 94-, 95-, 97- or 98-Series alternators in the factory position, or may be mounted on a custom bracket.

Keep in mind each alternator series is unique in its dimensions. It is always wise to compare the dimensions of your existing alternator to the replacement alternator to ensure a proper fit.

2 thoughts on “Alternator Selection

  1. Thanks for the info of Alternator sizing. How should alternator sizing be considered for a lifepo4 house bank and flooded starter battery set up?

    1. Hello Roger,

      Thank you for your question. LiFePO4 batteries love to be charged fast. Most manufacturers recommend charging rates of 1/2 capacity or more. For a large bank, it can be difficult to get this capacity with alternators alone.

      For systems with separate starting batteries, we recommend using the Balmar Digital Duo Charge between the house bank and the starting battery. This way, the starter battery is charged with the correct profile, rather than the LiFePO4 charging profile.

      Happy cruising!

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