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How Do I Choose The Proper Alternator For My Boat?
Once battery capacity and battery type have been determined, the size of your high-output marine alternator can be calculated. If the batteries used are standard or deep cycle flooded, the marine alternator output should be sized at about 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 battery technology is used, the alternator output may be increased to 35% of the combined battery capacity, as those battery technologies will accept 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:

  • 1” Single Foot (Motorola style) - Mounting arm at engine pivot point measures 1” from front to back. (See Image 1 at left)
  • 2” Single Foot (Delco style) - Mounting arm at engine pivot point measures 2” from front to back. (See Image 2 at left)
  • 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 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 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.

What About a Voltage Regulator?
Most high-output marine alternators are designed to be used with an external voltage regulator. We strongly recommend that you use a multi-stage "smart" voltage regulator to get the most out of your boat's alternator.

Typically, internal voltage regulators operate in two stages (ON and OFF), as they are designed to recharge the engine's starting battery. The internal voltage regulator senses the voltage at the alternator's output terminal, which does not compensate for resistance in the battery wiring. Additionally, internal voltage regulators are not adjustable, and may not be appropriate for AGM or gel cell batteries.

Multi-stage "smart" voltage regulators, on the other hand, are designed to maximize your battery life, while getting the most out of your high-output alternator. Most smart regulators can compensate for changes in your battery's temperator, and some can even keep your boat's alternator from overheating. These voltage regulators measure the system voltage at the house battery terminal, which compensates for voltage losses within the wiring. A smart regulator's charging algorithm includes three stages:

  • Stage One - "Bulk": The alternator produces its maximum current output until the system voltage reaches 13.8 VDC (on a 12 volt system).
  • Stage Two - "Accept": Once the voltage reaches 13.8 VDC, the regulator switches over to "accept" stage, where the alternator produces a constant voltage of 14.1-14.6 VDC (which is adjustable to match your battery type). The accept stage runs for a fixed period of time (typically one hour), to prevent excessive off-gassing of your battery bank. After the accept stage your battery bank is fully charged.
  • Stage Three - "Float": Once the battery bank is fully charged the multi-stage voltage regulator switches over to "float" stage, during which the alternator's output is regulated to 13.3 VDC (again, adjustable depending on battery type). If your system experiences a heavy-enough load to draw the battery voltage below 13.3 volts, the regulator starts again with Stage One.

How About a My Alternator Wiring?
The original wiring that came with your engine's wiring harness is probably not of sufficient wire gauge to handle the higher alternator output. The wire gauge needed depends on the length of cable between the alternator and the battery, as well as the maximum charging current available from the alternator. Be sure to use marine grade wire and cable, with a temperature rating of at least 105°C.

Happy cruising!

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