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post #1 of 12 Old 08-14-2014 Thread Starter
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Setting up a 24v solar charging system

My main (often only) power draw is a 24v 74-pound thrust trolling motor. The boat came with two group 27 batteries in series to provide 24v to the motor, and separate leads off just one of the batteries to provide 12v for the house. It has a rather hodgepodge solar charging setup with no controller that isn't keeping up with my needs.

I want to set this up right, so I'm looking at the Genasun 24v MPPT controller.

Genasun GV-Boost 105-350W Solar Boost Charge Controller with MPPT

Their website says "These controllers will take a standard 12V panel and boost the voltage to charge a 24V, 36V or 48V battery pack."

Can that be right? Is that really as efficient as using a 24v panel?

I thought I'd have to buy two panels and run them in series, but a single 60 watt panel is a lot cheaper than two 30 watt panels (although two smaller panels would be easier to stash in a cockpit locker when sailing).



Also I'll need a 24v to 12v converter for power to the house, any suggestions on that?

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post #2 of 12 Old 08-14-2014
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Re: Setting up a 24v solar charging system

Unless something has changed since I installed my solar system, solar panels do not produce 12 volts. They produce something in the neighborhood of 18 volts, which is why a charge controller is necessary. 18 volts is too much for a 12 volt system and not enough for a 24 volt system.
You only need to find a charge controller that will handle the max amps produced by your panel at it's rated output voltage and connect it to your system. I did not investigate 24 volts set up because I wasn't doing one, but each charge controller will have a data sheet and connection diagram for the voltage you wish to use, I believe up to 48 volts, but you will need to check.

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post #3 of 12 Old 08-15-2014
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Re: Setting up a 24v solar charging system

The Genasun boost controllers will charge your 24 volt system.

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post #4 of 12 Old 08-15-2014
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Re: Setting up a 24v solar charging system

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.... Also I'll need a 24v to 12v converter for power to the house, any suggestions on that?
Why?

You now have two group 27 batteries in series right? What's powering your house requirements now? I would have tapped your "house" across one of the two batteries as requirements would be minimal on a 22' boat.

Back to your original concept, a 60 watt panel will provide less than 4 amps at 18v. The MPPT controller will shift the voltage to 24v but the current drops to 2.5 amps. A group 27 battery is roughly 90 amp hours. You have 2 of them. You're not supposed to draw them down more than 50% ... so you're looking to recharge 90 amps. 90 amps/ 2.5 amps per hour= a very long time. But wait, there's more. The output of the panel is rated at 60 watts set exactly 90 degrees to the sun at all times, and in strong sunlight... no clouds, no shadows, In reality the 2.5 amps I mentioned above will not be seen. You probably could count on just 2 amps per daylight hour of sunlight, meaning it would take almost 6 full days to charge the batteries back up.

How's that going to work?
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post #5 of 12 Old 08-15-2014
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Re: Setting up a 24v solar charging system

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You only need to find a charge controller that will handle the max amps produced by your panel at it's rated output voltage and connect it to your system.
Just be aware that contrary to popular opinion most MPPT controllers will not boost voltage, only reduce it.
There are a very small number that do (and one of those is listed) and they are reasonably efficient although a higher voltage running via a conventional MPPT controller is preferable.

Beware some solar panels listed as "24v" are not suitable for a 24v battery system. The Voc needs to be in the order of 38v for a 24v system. A good check is that there should be 72 cells, or more.
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post #6 of 12 Old 08-15-2014
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Re: Setting up a 24v solar charging system

Assuming you have a fridge and are a typical user you need at least 250 watts TILTABLE solar to be self sufficient.

Don't bolt them to the coach roof and have the boom cast a shadow over them. If you have a step up controller then you can either have two 12 volt panels wired either in series of parallel. Assuming you have some shadowing of one panel I am trying to get my head around which is better.
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post #7 of 12 Old 08-15-2014
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Re: Setting up a 24v solar charging system

60w of solar is barely going to make a dent in the power requirements of an electric outboard motor. Your outboard probably used 300w to 1000w when in operation. If it is 300w (under half a horsepower) then you would need 5 hours of full sun (that is effectively a full day when it comes to solar) to make up for one hour of outboard operation. If it is 900w (a little over 1hp) you would need 15 hours of sun to make up for 1 hour of outboard use.

Electric outboards are great for day sailors, but if you use the boat regularly a shore power charger is a must.

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post #8 of 12 Old 08-15-2014 Thread Starter
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Re: Setting up a 24v solar charging system

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Assuming you have a fridge and are a typical user you need at least 250 watts TILTABLE solar to be self sufficient.
22' boat, mostly daysailed. No fridge


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Electric outboards are great for day sailors, but if you use the boat regularly a shore power charger is a must.
I guess I'll string a couple extension cords out to the mooring!


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Back to your original concept, a 60 watt panel will provide less than 4 amps at 18v. The MPPT controller will shift the voltage to 24v but the current drops to 2.5 amps. A group 27 battery is roughly 90 amp hours. You have 2 of them. You're not supposed to draw them down more than 50% ... so you're looking to recharge 90 amps. 90 amps/ 2.5 amps per hour= a very long time. But wait, there's more. The output of the panel is rated at 60 watts set exactly 90 degrees to the sun at all times, and in strong sunlight... no clouds, no shadows, In reality the 2.5 amps I mentioned above will not be seen. You probably could count on just 2 amps per daylight hour of sunlight, meaning it would take almost 6 full days to charge the batteries back up.

How's that going to work?
I think capacity will be OK. It's not like I run the batteries down to half every time I use the boat. I'm out two or three times a week and I run the motor less than fifteen minutes each time, to tow the tender from the mooring to the dock and back. 1200 watt motor running for fifteen minutes = 60 watt panel at full charge for five hours. In reality probably a couple days. It took two months of sailing with a 5 watt panel before the batteries got low enough that I pulled them and brought them home to charge.

Don't get me wrong, I'd love to have a bigger panel, but it's a small boat and my plan is to stow the panel(s) in a cockpit locker while sailing, so I need to size the panel(s) to be small enough to easily put away.


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You now have two group 27 batteries in series right? What's powering your house requirements now? I would have tapped your "house" across one of the two batteries as requirements would be minimal on a 22' boat.
That's how it's setup now. Is that OK? It seems kind of half-assed to me. I would like to be able to run the stereo, charge an iPad, turn on the cabin lights, eventually run a chartplotter (not yet installed), et cetera. It seems like batteries in series should be kept in series, running one of them harder than the other seems like a bad idea. On the other hand, what do I know? That's why I ask questions here.

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Re: Setting up a 24v solar charging system

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If you have a step up controller then you can either have two 12 volt panels wired either in series of parallel. Assuming you have some shadowing of one panel I am trying to get my head around which is better.
Yeah, good question!

Since I'm on a mooring I don't know which direction the boat will be facing, so I plan on leaving the solar panels flat across the cockpit.

I wonder about that shading question.

Would there be an experimental way to find out? I suppose I could deliberately shade one panel and see how fast it charges.

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post #10 of 12 Old 08-16-2014
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Re: Setting up a 24v solar charging system

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That's how it's setup now. Is that OK? It seems kind of half-assed to me....
Sure it will work. In time you discover that one of the batteries will bite the dust before the other does. However, make sure the MPPT controller you buy has different charge cycles. The one I have equalizes the batteries I think every two weeks. It just happens, and I don't worry about it. This is important for extending the life of inexpensive batteries to 4-6 years or even longer at your latitude. Here in Florida batteries fail faster it seems than any place I've ever been!!

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