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Compass Marine How To | all galleries >> Welcome To >> Installing A Small Marine Solar System > A Small 30W Panel
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A Small 30W Panel

I work on lots of boats that are sailed off of moorings and as such they tend to have very short battery life no matter what brand or how expensive. Some types tolerate this abuse, the lack of ever getting to 100% state of charge, better than others, but none like it. Batteries like to be topped up to 100% as often as possible. When batteries are not topped up sulfation forms on the battery plates and they begin to suffer and die.

When you are off cruising, or sailing from a mooring, your battery bank will rarely if ever get back to 100% state of charge. Most cruisers cycle their banks between 50% SOC and 80-85% SOC. This is due to what is called "battery acceptance". The long and short is that in order to top your bank off to 100% state of charge it would take many, many hours of running the engine. For an average sized battery bank this time to 100% SOC is in the range of 10+ hours. Unless you're running the "ditch" ten+ hour motor runs are pretty rare in sailboats.

As lead acid batteries charge they accept less and less charging current. Rapidly declining acceptance begins around *80% state of charge (* depends upon size of current source). Putting the last 20% into the bank, due to changes in resistance as the batteries approach full, is like walking up hill on ice. It will take a long time to get to the top or full, even in the case of batteries charged via the engine alternator. With an alternator trying to charge much beyond 85% state of charge simply becomes a waste of fuel.

If trying to charge your banks through idling at anchor it can become an even more expensive proposition. Diesels do not like to be run lightly loaded and idling the engine for long periods, to charge batteries, can lead to cylinder "bore glazing". With diesel engines running well over 10k for replacement, and over 5K just for a re-build, supplementing with solar becomes even more attractive to the boater budget. No one likes to budget for a re-build.

For a few hundred dollars you can greatly extend the life of your bank and start each weekend adventure with a battery bank at, or darn near, full.

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