The traditional way of avoiding this unhappy circumstance is to run the engine every couple of weeks. In theory, at least, this keeps the battery (or batteries) fully charged. A more accurate description is that it keeps the battery from becoming fully discharged. This difference may sound like semantics to you, but for your battery it is a death sentence.
Wet-cell batteries self-discharge at about one percent per day—more when the weather is hot. Miss three weekends of sailing and your battery will have lost nearly 30 percent of its charge. Of course that still leaves about 20 percent of useable capacity. (As a general rule, "useable" capacity is half of a battery's rated capacity.) And since you are going to fire up the engine and recharge you battery bank, what's the big deal?
In a word, sulfation. As a storage battery discharges—whether that's because you are using the electricity or it is "leaking" away—the acid inside the battery separates into hydrogen and sulfate ions. The sulfate ions combine with the lead inside the battery to form lead sulfate on the battery's plates (electrodes). This sulfate is initially soft and readily separated from the lead with a charging current, which will also cause it to recombine with the hydrogen into sulfuric acid. However, in the absence of a charging current, sulfates begin to crystallize (harden) in a matter of hours. Sulfate crystals are not easily reconverted and the battery suffers a permanent loss of capacity. Sooner, rather than later, the remaining capacity declines below what is necessary to operate your boat and the battery fails completely.
The formation of sulfate crystals on the plates occurs when a battery is left in a discharged state, even partially discharged. It is analogous to rust forming on idle machinery. How can you avoid this? A sure way is to keep the battery fully charged. Note, however, that I said sulfates begin to crystallize in the absence of a charging current. A charging current, even a small one, also inhibits plate sulfation.
Running the engine every week or two doesn't satisfy either of these requirements. If your battery was fully charged on Sunday, by Wednesday sulfates are already forming and hardening. By the second Wednesday capacity is already being lost.
Where an electrical outlet is available, a charger can keep the batteries fully charged in your absence. However, leaving an unattended boat "plugged in" introduces the real risk of the boat sinking due to stray-current corrosion. For boats on trailers, the problem is that the usual charger for such boats—a portable, automotive type—doesn't shut off completely, so most of the time it is overcharging, doing the battery far more harm than good.
If your boat is on a mooring, a solar panel is the only practical choice for maintaining batteries at full charge. But a solar panel is not simply the default choice. Appropriately sized, a solar panel is the best way to prolong the life of your batteries, no matter where your boat spends its idle hours.
Solar panels give maximum output when they are perpendicular to the sun's rays, but since boat movement makes inclining the panel toward the sun less than a sure thing, it is almost always best to mount the panel horizontally. But solar panels should not be shaded by booms, masts, or rigging since even the thin shadow of a shroud can drastically cut the output of the panel. Heat also reduces output, so deck mounted panels need to be raised enough for air to circulate beneath them.
One of the advantages of solar panels is that they start generating as soon as they are exposed to sunlight, but that means that you'll have to cover the panel during installation so that it will turn itself off and you won't cause a short circuit. Fortunately, wiring the panels is essentially a matter of connecting the panel's positive terminal (or red lead) to the positive battery post, and the negative terminal to the negative battery post. I recommend that you use at least 16-gauge (AWG) wire. Even though the currents are small, given the cost of solar panels, you don't want to lose any output to undersize wiring.
Solar panels with a maximum output current of no more than about one percent of battery capacity don't require regulation, nor will you need a blocking diode to keep current from flowing back to the panel at night. What is essential is having a fuse located close to the battery. Without a fuse, a fault in the wiring may become a dead short across the battery, with fire a likely consequence.
If your boat has more than one battery (bank), you can maintain them with a dedicated solar panel wired to each bank, or you can use a single panel sized according to total battery capacity. If you are floating two battery banks with a single panel, insert diodes in both positive legs of the circuit to keep the batteries isolated. Low-loss Schottky diodes are best. The marked end of the diode (or the point of the arrow) goes on the battery side.
Remember, a small solar charger can, without exaggeration, quadruple the life of the batteries on your boat. You do the math and I think you'll find that it's a sound investment.
Charging With Solar Power by Kevin Jeffrey
Choosing and Installing Solar Panels by Sue & Larry
Solar Panels and Regulators
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