helenl avatar image
helenl asked

Solar set up for a boat

I'm looking for some advice on setting up solar on our new to us boat - there's no solar in place so we're starting from scratch and will be having an arch built to accommodate the panels.

The best fit is 3 x 175w Monocrystalline Blue Solar panels - I've been advised to wire each of these with their own MPPT controller so if one panel is shaded but another not, it won't affect the output of the other panels. This is important as we'll have radar mounted above and will cast a shadow at some point in addition to the mast and boom. Are the 75/15 Mppt controllers the best fit for this?

At the moment, we have 2 x 180ah lead acid batteries and a 95ah engine starter battery.

I'm looking to upgrade these to 4 x 100ah lithium life po4 batteries + the engine starter battery but this may be a while after the solar is fitted.

We are running on a 12v system and have a 24v system when on shore power. We'd like to be able to use the 24v system when away from land as we'll be mostly self sufficient and will need to charge laptops etc. - which inverter will we need? And how would all of this be wired for the best efficiency? I would like all the panels to charge all the batteries, rather than one panel designated to one battery.

Many Thanks

Solar Panel
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2 Answers
packrat1969 avatar image
packrat1969 answered ·

Ok, just a novice's thoughts here...

Interesting question. Are the cost trade-offs of having (3) charge controllers (one for each panel) overcome by the expected return (total power accumulation)? Or is this more a case of making sure you HAVE power if things go sideways? How does this 1-for-1 arrangement compare electrically with connecting the three panels in parallel and using a higher-rated charge controller? I would expect the 1-for-1 setup to achieve higher power generation during partial shading conditions, but it will probably lighten your wallet. Another positive you would get out of the more expensive arrangement is having some sort of built-in redundancy if one panel or controller craps out.

What are the specs of the 175w Monocrystalline Blue Solar panels? The closer to 12v they are the closer you will come to the maximum current rating of the MPPT 75/15 charge controller. The MPPT charge controller calculator on this site recommended the 75/15, but the closest 12v panel in the preconfigured list was only 160 watts. If you haven't done so already, run the calculator with the Custom Panel option chosen and enter the specs of your panel.

What voltage are the current batteries? Are these (2) 180Ah 6v true deep cycle batteries in series or (2) 12v "marine" dual-purpose batteries in parallel? If they are 6v units, then the maximum capacity is 180Ah. If they are 12v batteries, then the total Ah capacity will double. In either event, usable capacity will remain at 50% of maximum. I'll assume, for the sake of this reply, that the 180AH is the maximum 20-hour rating. If the charge current is roughly calculated at 10% of the 20-hour capacity, then that puts max charging current around 18A.

I am still learning about the Victron hardware, but from what I've seen in the forums, the 75/15 unit can be connected in parallel with others of it's kind. What I don't know is whether and how the parallel-connected units communicate with each other to limit final charging current. I would hate for you to cook your batteries.


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JohnC avatar image
JohnC answered ·

Hi Helen. With your selective-shading issue the multiple mppt's is the way to roll. They can all be wired to charge the same batt bank.

Which is presumably 24V, the 12V being the engine batt. And you want to charge, and run a 24V system for 'house' needs? All good..

Into 24V, 75/10 mppt's would be plenty for each panel, with a nominal capability of 240W. But you can't just grab any-ole-panel to do this, it should have a Voc rating of ~40Voc or more. If you've sized up '12V' panels, they won't do. Check the specs when selecting panels.

Sizing an inverter is a little trickier, as it depends on the loads you want to run, and for how long, how much capacity in your batts, and whether the sun is contributing at the time. Perhaps a Multiplus 24/800 might do:

Charging your 12V engine batt could be done in various ways. Leave that aside for now..

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