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Old 07-25-1999
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Alternative Battery-Charging Systems

Onboard electrical systems have become increasingly complex. Refrigeration, sophisticated navigation centers, worldwide communications, and so forth, they can devour enormous quantities of power. Keeping up with that power demand is the challenge.

Daily battery charging off the engine or the generator can have drawbacks. Cozy anchorages are polluted by exhaust fumes and noise. Charging schedules prevent prolonged absence from the boat. Worst of all is the price tag. Considering fuel, maintenance and depreciation, I calculate that the typical sailboat auxiliary costs an average of $2.50 per hour to operate. At 1-1/2 hours per day, the total bill comes to nearly $1,400 for 365 days. It's no wonder that cruisers seek alternative ways to charge batteries.

Solar power and wind or water-driven generators fill this need. But choosing the proper size and mix of alternative power can be difficult as the weather patterns of your sailing area have to be anticipated. Many liveaboard sailors get around this by setting up a mix of all three. Here are some general considerations for when planning to install an alternative-power source:

  • Using the wind to turn a propeller and drive a generator is attractive where steady winds blow. In windless areas, or where harbors have little air motion, solar panels are more attractive.

  • Weather patterns are such that sunny days are often accompanied by little wind while a stiff breeze usually accompanies cloudy days. This fact reinforces why many cruisers have both wind and solar chargers.

  • Alternative power sources require a voltage regulator. Combination regulator and monitoring panel with internal diodes for charging independent battery banks are common and definitely worthwhile.

  • Towing a water generator propeller on a long line behind the boat requires setup and takedown time. This is OK for long passages but a nuisance on short hops.

    Solar panels
    There are three types of solar panels:

  • Amorphous silicon, or thin-film, panels have no glass covers and are available framed or flexible for mounting on canvas tops. While longevity and efficiency have improved with "spectrum splitting technology," these panels require more space for an equal output to that of their crystalline cousins. Amorphous silicon panels are much more tolerant of shade and shadows than standard panels.

  • Poly-crystalline silicon panels are more efficient and have slower loss of efficiency with age. If properly made, they have life spans of 12 years or more.

  • Single crystalline cell (SCC) panels are the most efficient and long-lived available. They do well in low sun angles and cloudy light, but not direct or partial shadow. As production volume has grown, prices of the SCC panels have come down.

    Solar panel mounting is important. Glass tops on crystalline panels can be broken if struck with a winch handle or heavy footstep. They are also sensitive to shading. Output can be significantly reduced by even the shadow of a shroud across their surface.

    Solar panels are quoted in maximum watts and amps. Watts divided by amps will give you the rated voltage output, usually between 15 and 17 volts. These numbers are for perfect conditions, that is: no clouds or shadows, clean glass and panel aimed 90 degrees to the source. Multiplying the panel's rated amperage output by six hours per day seems to work in practice-less in the north and winter, more in the tropics in summer. Thus a 60-watt panel rated at 3.55 amps has a no-load voltage of 16.90 volts and would give a working average of 21.3 amps per day.

    Wind generators
    Wind generators come in two basic types or sizes. The fans with multiple blades of less that 48-inch diameter offer limited output, but they are a good choice when little charging is needed or where strong winds prevail. They are lightweight and generally have little vibration or noise. The other type, the large two- or three-bladed wind generators, have diameters of up 72 inches and can provide outputs of more than 60 amps. These powerful units can be noisy and produce vibration if the blades have not been properly balanced and pitched.

    Wind generators should be mounted as high as possible, and away from wind shadows, sailhandling gear and crewmembers. Small units may be mounted on mizzen masts or pilothouse decks-we have seen them at the masthead and even on a bow pulpit. Large units require good support and are usually pole-mounted at the stern. They can be dangerous in a high wind so they must be high enough to clear a crew working on the aft deck.

    But since the wind is rarely steady, most cruisers tell us they average about half the output advertised for their unit. If you need 96 amps per day and wish to be 100 percent charged by wind power, pick a unit that offers eight amps output at the expected wind speed and do this calculation: eight amps x 24 hours x 0.50 = 96 amps per day. Remember to expect days with no output and others with double the requirement.

    Water generators
    Many wind generator manufacturers also make water generators. Some make kits to change the wind model into a towed water generator. They are capable of up to eight amps, 24 hours a day.

    Some cruisers have experimented with low-speed, high-output alternators driven by the prop shaft. If the transmission manufacturer allows a freewheeling prop, this seems sensible.

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