Correct sizing of solar panel for float charging

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rob-dean asked May 17, '22

Correct sizing of solar panel for float charging

I am going to add a solar panel to my boat just for providing float charge to my starter battery (12v 240ah) and bow thruster battery (24v 240ah). I will use a Smartsolar 75/15 controller and an Orion 12/24 to step up to the 24v battery.

My question is this: what current do I need the panel to provide to enable float charging of these batteries? I'm think a 115w panel will provide typically 8 amps in the sun but rather less most of the day and, of course, nothing at night.

Not worried about bulk charging - the alternator will do that and these batteries will only discharge at start or when the alternator is running

thanks

Rob

solar sizing float

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Answer 1 · 2022-05-17T19:21:53Z

Alexandra answered · May 17, 2022 at 07:21 PM

@Rob Dean

If the rebulk offset is set with a greater gap, then the mppt will not bulk charge in theory. Or you can just set the bulk and float charge to the same voltage. Then the mppt wont raise the voltage at all. Disable absorption.

I have never tried the above idea, but generally in normal use the mppt wont charge unless the battery needs a charge. It does not matter how much solar you install in that case.

I am not sure why you would not just set the mppt correctly for the battery and then leave the mppt to sort itself out then. With a small panel on it there would be no issues. Especially if the bulk is being sorted out by another source.

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rob-dean commented · May 17, 2022 at 07:30 PM

Thanks Alexandra,

I'm not trying to stop the mppt from bulk charging. It just isn't needed in the sizing decision. I'm just trying to decide how big (or little) a panel I need to be sure to maintain a float charge

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Answer 2 · 2022-05-17T21:28:08Z

kevgermany answered · May 17, 2022 at 09:28 PM

Not sure if this helps or not.

My starter battery is about 100Ah and 12V. A continuous (no nightime shut off) trickle runs under 1A and keeps it at 12.6V without a problem, even without starting the engine for a couple of weeks.

Based on that you'd need about 5A at 12V. So your 115W panel would be good.

You need to consider how to control the start/stop of the Orion. You want it to kick in when solar starts charging and cut out once solar stops, but not when the leisure battery is still charging. You should be able to do this via user defined switches on the load output of the mppt, connecting load positive to h on the Orion and configuring that to work from h input.

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