Solar panel hookup questions

[peb]

Perhaps some of you have experience with solar panels on boats. My current build will have a 48V electric motor. I am currently planning on having an two battery banks. A 48V bank for the engine and a seperat1 12 volt house battery bank (none of this is set in stone). It is a 25 ft yawl rig, so I will be somewhat limited on how I will put solar panels on the boat. I was planning on not installing solar panels initially and just manage keeping batteries charged with shore power until I get some experience with working the boat and a better feel for where I could possibly install solar panels. That is likely still the case. However, the more I think about it, the more it appears that the only place to install panels will be on top of the cabin trunk. There is not much space there, but I could put a pair of panels on each side (the flexible walk-on-top-of type). But this seems to present other problems. I have read that one should only wire the panels together before the MPPT if they are going to have very similar shading characteristics. So that would require a MPPT for both sides of the cabin. I also want to select if the panels are charging the engine bank or the house bank. And of course I need a different MPPT for the 12 volt and the 48 volt. So now I need 4 MPPTs? Is this all correct? I suppose it works out where I could use one side to charge the engines and one side the house batteries all of the time.

Just wondering if anyone has experience with this and I am on the right track.


Thanks.

ETA: one more question, that I have not found an answer for. When the batteries become fully charged, what happens to the energy produced by the panels. Does the MPPT just dissipate that electrical input as heat?

[rgthom]

I’m not an expert Peb, but my smaller solar setup is similar to yours.

For the 12 V house power, do you need a separate battery or can you just use a 12 V buck converter from the 48 V bank? Mine is a 26 V bank to run the motor, I bought a (cheap) 12 V converter at 200 W for the house power.

As to where the power goes when the bank is charged, the MPPT shuts off the current so no electrical power is dissipated anywhere. The solar power absorbed by the panels now gets completely turned into heat, instead of 20% going into electricity. The panels will be a little hotter.

I do not know about the shading, mine is just a single panel.

- Rick

[peb]

I’m not an expert Peb, but my smaller solar setup is similar to yours.

For the 12 V house power, do you need a separate battery or can you just use a 12 V buck converter from the 48 V bank? Mine is a 26 V bank to run the motor, I bought a (cheap) 12 V converter at 200 W for the house power.

As to where the power goes when the bank is charged, the MPPT shuts off the current so no electrical power is dissipated anywhere. The solar power absorbed by the panels now gets completely turned into heat, instead of 20% going into electricity. The panels will be a little hotter.

I do not know about the shading, mine is just a single panel.

- Rick

It would simplify things if I only got rid of the second bank and installed a DC-DC converter for 12 volt power. It seemed wise to keep the engine fuel seperate, but I am starting to think that is a mistake. Not only will it save on two MPPTs, but it will save on a battery charger also. And perhaps for the same money I can up the amp-hours of my engine and have much more flexibility. Thanks for the response.

[J.Madison]

I would not have 2 banks on a boat that size. I would have a separate MPPT for each panel, they are relatively cheap and the extra performance will be welcome on an electric boat.

I believe that the MPPT opens the circuit when the batteries are full so no current flows. The panels reach their open-circuit voltage and stay there, the solar energy then behaves like it is hitting any other object. Some bounces off, some heats the panel.

[Breakaway]

It would simplify things if I only got rid of the second bank and installed a DC-DC converter for 12 volt power

You can simply take 12-volts off the 48-volt bank. For instance, run an additional positive and ground cable from one battery in the 48-volt bank to a small panel off of which you power your 12 volt devices. It is good to have a dedicated electronics circuit anyway, IMHO.

FWIW, many production powerboats are wired thus from the factory, usually having a 24-volt DC system to save amps and wire size (weight, cost) and 12v for the electronics.

I have no experience with MPPTs, or solar in general, but cannot see why switches, rather than separate MPPT's might not be used to direct current where needed.

I am interested to see this come together.

Kevin

[WX]

If you are using one panel to change the engine battery and one to charge the house battery then you only need one MPPT and one standard 12v controller...assuming one panel is 48v and one is 12v.
Ex grid panels are usually 48v and dirt cheap to buy 2nd hand but they are quite big.

[rgthom]

What batteries? Are they to be 12 V LA or higher voltage LiFePO?

[peb]

The batteries will be LifePo4. Now, I do realize my idea for separate house batteries is expensive both in cost and space. From a space standpoint, it is a big boat for 25 ft. But still, its a 25 foot boat. However, for long weekends on the boat, I just didn't wanted to be able to manage engine usage without worrying about all of the 12 vote stuff on the boat. I do not like the idea of using one of the 4 engine batteries as a 12 volt system. As that will make effective engine range dependent on that single battery which may have been drained by the the 12 volt stuff. So, if I go with one bank, I will use a DC-DC converter. I would rather have the 15% efficiency loss of the converter than one battery that will reduce engine range in and of itself, and limit me to just one battery for the house.

I cannot use just one panel and one MPPT to the 48 volt system. The space for the panels is rather limited, in particular it is narrow. I have found a good 37 watt panel that is 15 by 21 inches. And I can install two of these on each side (the space between them is really handy for the base of the handrail). Lets simplify it for a second. On each side, the angle is very close to the same, but due to the boom and mainsail, the shading is not identical. Will wiring the two on one side to a single MPPT be acceptable. How important is it for the shading to be the same before you wire two panels into one MPPT?

And then, assuming I will only have one bank, still have to take output from the MPPT from each side (as the two sides of the cabin surely cannot be considered to be equal shading when considering both angles being opposed and the mainsail/boom effect) and combine them into the batteries. Is this correct?

[Breakaway]

I do not like the idea of using one of the 4 engine batteries as a 12 volt system. As that will make effective engine range dependent on that single battery which may have been drained by the the 12 volt stuff.

The 12v dc drain from electronics will be relatively small, but will still drain the bank to some degree and yes affect range minimally. I do not think it will affect charging much, if at all. Again, busses, big boats, other vehicles have been doing this for some time.

Also--many marine elex will run on 24 volts. Here is a example suite of small boat electronics, noting required operating voltage and estimated power consumption. ( No affiliation...I use a different brand)

Sounder

1406489239newcvs126.2.300.jpg
Works on 12-36 volts dc
consumes 10 w max at 12 volts ( .83 amps)

GPS

1410377488svs460.360.jpg
Operates on 12-36volts dc
Consumes 7w at 12 v (.6 amps)

VHF

GX2400_thumb.jpg



Operates on: 12v dc

Consumes: .9amps receive' .55a on standby

As you can see, running all out, total consumption @12 volts would be about 2.5 amp per hour.

Kevin

[peb]

Oh, I do realize the power consumption of the DC stuff is quite small. Probably won't make much difference at all. Just seems better to keep the 4 batteries as balanced as possible. Is there a reason why those boats do that as oppose to DC-DC converter?

I do think that if I go ahead and do solar, it pretty much negates any argument for 2 battery banks. And I will probably rarely have to rely on shore power to charge. So leaning towards a single 48V bank, two MPPTs (one for each side of the cabin trunk) and a DC-DC converter to get to 12 volts. I suppose I could go to 24 volt system. IRRC some of the nav light options also are good for 24 volts. Might give that some thought.

[Breakaway]

Is there a reason why those boats do that as oppose to DC-DC converter?

The reason for that sort of thing is often some combination of: it's less expensive/ it's one less line item to stock and warranty/ one less item to install reduces the build time/ it works fine.

Kevin

[J.Madison]

Thinking about it more, your biggest problem is going to be trying to charge a 48v bank with 12v panels. I read a bit on the Victron MPPT and it looks like you need a higher PV voltage than your battery voltage to charge, even with the MPPT.

48v panels are big.

[rgthom]

Genasun has the only MPPT I am aware of that has voltage boost. You can charge a 48 V bank from 12 V (or 24 V, etc.) solar panels. I am using one of their GVB-8-WP units to charge my 26 V LiFePO4 battery. It does not have a bluetooth phone interface like the Victron, just a simple LED status indicator, but it allows much more flexibility in panel selection.

https://sunforgellc.com/gvb-8-wp/

[peb]

From what I have seen, Genasun 48v MPPT seems like the go-to choice for electric auxiliaries with solar. That unit was the current plan. I was going to do the usual due diligence of seeing alternatives, but had not done so yet.