The parlor logic for the supply side of a sailboat’s 12-volt electrical system typically goes something like this: I need
X amp-hours of electrical power daily to operate all the systems on my boat, so my new Behemoth High-Capacity Alternator, capable of charging at a rate of
2X amps in an hour, coupled with my new Einstein Brand smart regulator, will keep my batteries fully charged with only 30 minutes of engine time per day. And, heck, when I’m cruising, I’ll surely be motoring in or out a channel or something every day or every other day, so the batteries should stay fully charged more or less automatically.
It is a nice fantasy.
The reality is that while your regulator may be smart, your batteries are dumb—and stubborn. They rebel at a charging level above around 25 percent so a 200-amp alternator is only appropriate for charging a battery bank of at least 800 amp hours. A more typical battery bank of around 400 amp hours can take full advantage of an alternator rated at not more than 100 amps. Worse yet for the aforementioned parlor logic, the batteries only accept a 25-percent charge rate when they are deeply discharged.
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"Many first-time cruisers discover the shortcomings of engine-driven charging only after well-stocked chandleries and suppliers of alternative charging equipment are out of reach." |
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As their level of charge rises, they accept additional charging at a decreasing rate. For example, starting with a half-discharged 400-amp-hour battery bank (discharging batteries beyond the 50-percent level severely shortens their lives), the 100 amp-hour alternator is going to charge at full capacity for perhaps 10 minutes, then the regulator will begin to cut the output back to a level that matches the acceptance rate of the battery. So running the engine for 30 minutes puts 40 Ah into the batteries, not 50. After an hour you have gained only 70Ah, not 100, and by now the alternator is putting out less than 50 amps. To take the 50-percent discharged bank back up to the 90-percent level requires at least three and a half engine hours. If you aspire to a 100-percent charge level, expect to run the engine another two hours or more.
Unfortunately, a lot of first-time cruisers discover the shortcomings of engine-driven charging only after well-stocked chandleries and suppliers of alternative charging equipment are out of convenient reach. This is because most cruises, especially those headed south, tend to involve a lot of engine hours for moving the boat, so the batteries stay fully charged. Not until the boat stops or at least pauses do the realities of engine-driven charging become painfully apparent. Not only is running the engine just to spin the alternator not particularly kind to your diesel, but you are in effect shackled to your boat by your charging regimen. Sightseeing trips, excursions on another boat, or trips home are problematic, requiring that you either burden another cruiser with running your engine or that you decommission the boat for the duration of your absence. It doesn’t have to be this way.
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If you plan on taking your sailboat cruising, listen up. True cruising is a lot about sitting in one place. When you find a harbor, and island, or a country to your liking, you will want to stay awhile to explore or just experience the place. You will find yourself waiting on weather. You will sit out the storm season someplace safe. Running your engine at
anchor an hour or more a day grows old quickly. To avoid this, you should outfit your boat with an alternative charging source, either solar panels or a wind generator, or both.
How much alternative charging capacity do you need? That depends. You can size your alternative charging to handle all of your electrical demands, or you can simply take aboard enough capacity to reduce engine running to a more palatable frequency, perhaps once a week.
In either case, the first step in selecting either solar panels or a wind generator is to list all of the electrical equipment aboard and to calculate how much power each item uses daily. For example, if you have fans aboard that draw 0.2 amps and you expect to run them all night (plan on it in the tropics) and a portion of the day—let’s call it 12 hours—then each fan will use 2.4 amp-hours daily (0.2 amps x 12 hours). Burning a one-amp light from 8 until 11 p.m. requires three amp-hours. A cycling six-amp refrigerator will run at least 12 hours out of 24, using 72 amp-hours daily. Watching a two-hour movie on a television/VCR combination drawing five amps requires 10 Ah daily, or however often movies run aboard your boat.
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If you don’t know the amp draw of an appliance and you have a digital meter installed (Link 10 or similar), the change in the amperage reading of the meter when you turn on the appliance will give it to you. Or if the label or manual for the appliance lists only watts, divide that number by 12 to arrive at amps. For example, a coffee grinder rated at 180 watts will draw 15 amps at 12 volts (180 ÷ 12).
Actually, appliances that run from an inverter require a bit more amperage due to the inefficiencies of the inverter, but for this purpose, dividing by 12 will be close enough whether the appliance is connected directly to the battery or through an inverter. Also you can safely ignore the load of some appliances, this coffee grinder being a case in point. Grinding coffee takes perhaps 15 seconds, so the daily Ah draw is 15 amps times 1/240 hours, or 0.06 Ah, not enough to effect your charging requirement.
Add together the daily draw of all of your appliances to arrive at your daily amp-hour requirements for the boat. This number may surprise you, but do not doubt it. If your daily power consumption is 150 Ah, that plus about 20 percent for battery inefficiencies—making a total of 180 Ah—is how much daily charging capacity you need to keep up with the load.
Solar panels are usually rated in watts, but their output is typically around 16 volts, not 12. So to arrive at the amperage rating, divide solar panel wattage by 16. That means a 50 watt solar panel in ideal conditions has an output of around 3.1 amps. Count on averaging this output for not more than five hours on sunny days.Doing the math, that means 50 watts of solar power will provide around 15.5 Ah of charging daily. To replenish 150 Ah of discharge daily (plus the 20 percent battery inefficiency) would require twelve 50-watt panels or six 100 watt panels.
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Wind generators vary somewhat in efficiency, but as a general rule those with five-foot blades put out around four amps in 10 knots of wind, about eight amps in 15 knots. Generators with three-foot blades have about 1/3 the output of those with five-foot blades, or 1.3 and 2.6 amps respectively. The good news is that you get this output for 24 hours if the wind is blowing. The bad news is that most favored anchorages are wind protected, so wind velocities are likely to be in the 10 or under range. In a constant 10 knots of wind, a large-diameter wind generator will have a daily output of around 100 amps. A small-diameter unit in the same anchorage will generate around 35 Ah.
With this information, you should be able to arrive at the best charging configuration for your needs. For example, a large-diameter wind generator and 200 watts of solar panels should come close to keeping up with a daily demand of 150 Ah. If you reduce your consumption to 100 Ah, a small wind generator combined with 100 watts of solar power leaves you about 50 Ah short daily. With a 400 Ah battery bank, you could allow this shortfall to accumulate for four days, then run your engine—either at anchor or as a consequence of traveling—to fully recharge. A point worth pondering is that if you are off your boat, sightseeing, for example, your onboard demand should be significantly reduced so that even a relatively small amount of passive charging capacity will allow the boat to remain unattended for several days. This is a major convenience.
So if you are planning to go cruising and your only charging capacity is your main engine, the time to remedy this shortcoming is before you leave, not when you are down island.
