If you moor your boat on the hard, on a trailer, or on the water at a marina you may have been confronted with having a dead battery when you turned the key to start the engine. And it always seems to happen when you have perfect boating weather with calm seas. A battery charger comes in pretty handy for times like these as it can keep a floating charge to keep the batteries topped off. With marine solar panels you can avoid these situations as they are a environmentally practical choice to maintain a full charge on your batteries.
As a rule of thumb, marine batteries dissipate an average of 1% of their charge on a daily basis dependent upon the weather conditions. So in order to maintain a float charge on the battery, a 100-amp battery would require a solar energy system that would be capable of 1 amp/day output.
Since a solar power system is actually rated in watts, to figure output amps, simply divide the wattage rating of the panels by 15. So a typical 5-watt panel would give you a maximum output of .33 amps a day. Because of the fact that the highest rate output occurs when the sun is directly overhead, expect an average rated output of about 5 hours a day. Thus, the 5-watt panel would give approximately 1.65 amp hours (5 x 0.33).
When designing a system for your boat, use 3.5 watts per 100 amp hours of battery capacity taking into account cloudy days as well as running essential systems like bilge pumps if you are in the water. As with normal battery maintenance, check your water level regularly!
Regardless of the panel size you decide to go with, make absolutely certain that you have a minimum of 33 cells. Fewer than 33 cells will be insufficient to charge the batteries. If cruising in an area where cloudy days seem to outnumber the cloudless ones, you may want to research thinner filmed panels versus the normal crystalline ones as this type deal better with low light conditions.
Mount the panels in an area that will be allow them direct access to the suns rays keeping in mind that as the boat moves (especially on anchor) you want to be able to angle the panels accordingly to maintain contact with the sun. Usually this would be the highest point on the boat like radar arches or rigged to sail masts. I have however seen a number of deck mounted installations but keep in mind that the panels do produce heat so leave room enough under the panels so that the heat can be dissipated.
Wire it all together with marine-graded wire. The positive (+) terminal from the panel will connect to the positive battery post and consequently the negative (-) terminal from the panel to the negative post on the battery. Although panels with a max output of no more than 1% of the battery power do not require a regulator, I would certainly advise for a regulator as a precaution and give you peace of mind when the boat is unattended. Essential will be an in-line fuse that is close to the battery this would protect against a short and prevent potential fires. If your boat has both a starter and house bank, you can have a solar electrical system that is dedicated for each one wired separately.
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