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Keeping batteries charged when the vessel is unattended

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What is the issue?
Unattended wet cell batteries naturally discharge by approximately 1% per day in warm climates and significantly more in cold climates. Maintaining a charger in a marina is a possible solution but it is not prudent to leave a vessel plugged in unattended and this is not available to moored vessels.

Why address this?
Apart from not having power when next required, allowing the batteries to discharge degrades the batteries reducing their service life.

How to address this?
Invest in a solar panel to maintain the batteries when you are away from the vessel.
A rule-of-thumb is 3.5 watts per 100-amp-hour (Ah) battery. However, allow for 40 -50% extra to offer a better charge, circa 5 watts, for days with poor sunlight, charging inefficiencies and the occasional power usage around the vessel. Aim for a panel that has at least 33 cells.
The best charge is available when the panel is perpendicular to the sun but as the vessel is unattended and often moving on a mooring the best option is to lay it flat. Place the panel in a location that avoids shade. If any of the cells are covered by a shadow the power output drops by that percentage. This is important as any shadow could easily take it below the requisite voltage required to achieve a battery charge so that the remaining sunlit portion’s output is of no use. Most people insert a diode to stop power floating back across the panel during the night. If you are wiring up more than one battery it is prudent to insert both diodes and fuses. Make certain there is plenty of water in the batteries before leaving the vessel to support the floating charge.
Utilising a solar panel is an excellent way to keep your batteries top shape and having your vessel ready to go at the turn of a key.

With thanks to:
Michael Harpur, Yacht Obsession.

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