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Sheldon Haynie

Solar "Float" sizing?

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What's a good rule of thumb for sizing a solar float array to keep 12V banks up to charge?

Use is for SFBay climate with flooded CAT 8D (210 AH at 20 hrs) in parallel with 2x TROJAN T015 (225 AH at 20 hrs) on a boat that lives in a slip on a 3 stage shore power connected charger and has a Balmar regulator off the alternator for underway. . Typically powering 30 minutes to weekend day sail/ bay race and weeknight 2X 10 min for estuary racing.
Semi-Annual extended cruising with hours of motoring interspersed with hours of refrigeration/lighting use.

West suggests up to 1.5% of the AH rating can be supplied unregulated, and given the available flat space aboard would be a practical limit for 50-60 Watts of panel that would be easy to honor.

Solar charge controllers are cheap to price of a battery, and potentially a saving grace and a source of failure both, so I am of two minds on their use.


5 Replies

  • Solar controllers also dramatically increase the power you get on a cloudy day, by boosting the output voltage.

  • "cloudy day" ? What is this that you speak of?

    MPPT would be a reasonable improvement and an additional reason to "do it right"...

  • The DC output voltage of solar panels vary based on the intensity of the sunlight. You have a choice, if you put together solar cells that peak at voltages great for charging 12v batteries, they don't generate enough power on a cloudy day. If you put together solar cells that have enough voltage on cloudy days, the voltage is to high on sunny days. Solar controllers do away with this design problem.

  • Sheldon,

    West Marine is simply wrong on that and as a result of their horrendous guidance I have seen numerous destroyed batteries.

    If the vessel is left unattended then the batteries will eventually get to the point where they are accepting less current than the panel can provide, at which point the voltage climbs over 15V+ on batteries that should be a float votlage.

    Even small panels need a controller if the vessel will sit idle and unattended for periods of time. The link below will explain this in much greater depth.

    Do I Need a Solar Controller?

  • Thanks for responses, I appreciate what a max PowerPoint controller does, glad to have a refutation on the trickle/float before I toasted 440 AHr of battery. Looks like I'll need to provision some small controllers. Any specific recommendations for battery maintenance vs main power provisioning?