[Minnesail]

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?

 

[Minnesail]

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

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!

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.

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.

 

[Minnesail]

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.