How Do I Choose The Proper Alternator For My Boat?
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?
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
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
- 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.