Alternators use rotational energy from your boat's engine or
genset to provide electrical power that can be used to charge
batteries and run DC loads on board. They are generally the most
potent source of charging current on board, producing up to 200
amperes of current.
Alternators generate electrical current by rotating an
electromagnetic rotor inside a cocoon of wire (which is stationary,
and therefore called the stator). Since the rotor requires
electricity to make it magnetic, alternator outputs can be regulated
by varying the voltage passing through the rotor. Thus, an
alternator can be made to put out the right amount of electrical
energy, depending on the batteries' requirements.
Alternators are standard equipment on all inboard engines, and
most outboards over 20HP. But most engines come from the factory
with alternators in the 35- to 55-amp range, inadequate for the
loads most boats require. Furthermore, stock alternators are
equipped with simple, automotive-type regulators that are not
optimized for marine service.
Alternators in automotive applications are seldom called upon to
replace large amounts of energy in a short period, this is exactly
what marine alternators need to do. The goal, therefore is to return
the greatest number of amp-hours as quickly as possible, without
damaging the batteries. This usually requires high-output
alternators combined with modern multi-step regulators.
Ignition-Proof Design
For use on boats, especially those with gasoline engines, marine
alternators must be ignition-proof. Those from BALMAR are, but some
non-marine models are not.
Heavy-Duty Construction
While this is always desirable, marine alternators operate at full
output for much of their lives (or should). This requires
high-temperature grease, large cooling passages, large fans,
heavy-duty bearings, conservatively-rated diodes, etc. Since
high-performance regulators will demand the maximum output that the
alternator can deliver, less robust alternators will commonly fail
using these regulators.
High Output
Your alternator should be roughly 25-40% as large as your battery
capacity for effective charging. If you regularly operate large DC
loads underway, it could be even larger. Small-frame alternators
range from 75 to 150 amperes, while large-frame alternators produce
130 to 275 amps.
It's a waste of money to buy a large alternator to charge a small
battery bank, unless you have large DC loads operating. Batteries
need to have sufficient acceptance to utilize the alternator's
capabilities.
It Has to Fit!
This is the tough one. There are dozens of alternator/engine
variations, and it is hard to guarantee that a new alternator will
replace the original without a custom bracket. Here's where a marine
electrician can be a big help.
Most
High-output alternators use a lot of engine horsepower, which is
transferred via belts. Small alternators (<100A) can use one belt,
but larger alternators require two. This may necessitate a second
pulley on your engine, available from the engine distributor in most
cases.
Regulation
None of the alternators we sell are internally regulated. As such,
they require external regulators. Both Heart and BALMAR make
excellent multi-step requlators, and we encourage you to compare
them.
Number of Outputs
BALMAR makes patented dual-output alternators, which allow two
batteries to be charged without mutual discharge. One output can be
connected to the starting bank, while the other can charge the house
bank. This is a simple, relatively robust system that costs about
$100 more than a single-output model.
Temperature Ratings vary depending on the application. We
measure our alternators at 122°F, (50°C). This is the temperature
used by the military to rate alternators, and it strikes a balance
between the too-cool 80°F and too-hot 200°F ratings. The temperature
refers to the ambient air temperature in the vicinity of the
alternator. If your engine room is hotter than 122°F, our
alternators might put out slightly less.
Whether you have an N-type or P-type alternator
depends on where the regulator is placed in the circuit. Most
aftermarket regulators (BALMAR, Heart) only work with P-type
alternators, although N-type alternators can be converted by
experienced electricians. All BALMAR alternators are P-type.
Rotor RPM is the speed at which the alternator turns, not
the engine. Since most engines have a larger pulley than the
alternator, the alternator will turn faster than the engine.
Alternators are not damaged by speeds up to 10,000 rpm, and must
turn at least 5,000 rotor rpm to produce their rated amperage.
Large-frame and small-frame refer to the diameter of the
alternator housing. Small-frame alternators measure about 7" from
their mounting bolts to the adjustment bolt on the opposite side.
They are roughly 7" from front to back, and weigh about 13 lb.
Large-frame models measure about 8" from mounting bolt to
adjustment bolt, and are about 7" deep. Large-frame models are
generally more efficient and run cooler than small-frame
alternators.