Solar Panels

[CampDavid]

Any thoughts on a good brand of solar panels for charging 4- 6 volt golf cart batterys (440 amp hrs). The boat is located on the Georgian Bay on the great lakes.

The main purpose of the panel or panels is to keep up with a fridge system that draws about 120 amps per day.

I am not very good at this stuff, I believe the panels should be int he area of 17.70 to 18.70 volts and I think I need about 12 0amp hours or 225 watts+.

Is the MPPT the way to go on the controller?

Any and all comments are appreciated.


Fletch

[onestepcsy37]

kyocera seems to be the brand most widely used on sailboats. mine is about three years old and has worked flawlessly since i installed it. i charge four golf cart batteries and a size 27 'starting' battery with it and the principle drain on the system is an adler barbour fridge freezer of about six cubic feet - uses about 60 amps a day in the florida heat.

since my panel is only 135 watts and i only turn it on when the fridge is in use i don't have any controller at all, so i can't comment on solar controllers.

if i had two of these panels i would need a controller since i would then get more power than i use and might overcharge the batteries.

[Jon Hacking]

Originally Posted by CampDavid

Any thoughts on a good brand of solar panels for charging 4- 6 volt golf cart batteries (440 amp hrs)... The main purpose of the panel or panels is to keep up with a fridge system that draws about 120 amps per day...Fletch

Hi Fletch, We've had excellent service from Kyocera - they replaced our panels (in Malaysia!) when they were 8 years old! All for free, gratis, & for no $ on our part

If you want 120 amp-hrs (Ah) into your batteries, you'll need at least 360W (nominal) of solar panels. But that's for the tropics, where the sun is bright & right overhead. At higher latitudes, you'll need more as you'll lose a lot to reflection & shorter days in the winter.

I've published a page on solar panels here - you might find it useful.

ALL good quality 12v (nominal) solar panels will have 36 cells, so their maximum power point will come in at ~17.7v. A MPPT controller will boost the panels output by 10-20% depending on who you talk to (real users or the company selling it - remember that these folks lie like dogs). We found a MPPT controller worthwhile, even though they're expensive.

But 120Ah for a fridge is a lot (ours uses half that, & it's a fridge/freezer). You might think about improving your insulation. Often cheaper than buying more solar panels.

[SailFastTri]

That's a huge draw. Is that number measured or calculated? If calculated, how did you determine it?

[CampDavid]

In reflecting on our trip last summer, our draw was closer to 70 amps per day. I was thinking 225 to 250 watts would do it.

Fletch

[henryv]

I have three 75 watt Siemens panels on my PDQ32 which has only outboards with limited charging ability. The panels were installed in 1999 so they are probably not to current standards in terms of output. The original charger was also old technology so the batteries would need a weekly plug in to bring them back up. This year I installed a Blue Sky MPPT charger and the difference was remarkable. The fridge ran all summer and I only plugged in twice.

[jdoe71]

We have a 220 watt Trina solar panel. Big panels like this put out well over 30 volts, I think ours is 37.5. You need an MPPT controller or you will lose half your power. We have a Morningstar Sunsaver MPPT controller. Best case scenario with a flat panel (not angled into sun) is about 13 amps and that is only going to be for about 6 hours a day in summer. You should have two of them and preferably some method of angling them into the sun at least somewhat. Also consider wind, it fills in nicely at night or cloudy days, though it is not lovely silent power like solar. We are running 4 X 6 volts 464 amps total on Lake Ontario and never are short of power, but our reefer only uses 5 amps and it runs only 20 minutes out of an hour.

[thinwater]

Our expereince:
Sail Delmarva: Solar Panels

I used a non-MPPT controller, but I think I was near the economic tipping, point (better controller vs more panels). You should go MPPT, since you have 50% more panels.

I would suggest getting help designing this, if you are not part engineer. I notice that forum postings have a lot of errors and misinformation. A good supplier should be able to spec out a sytem where the components match, even if you do install DIY.

Remember seasonal variation, cloudy days, and shading. Actual out-put is less. For example, at anchor you can generally get shade free, while with sails up, it depends on the course; head south and you may see little charging all day.

[speakeasy]

I picked up 2 - 210 watt evergreen panels from this guy in Indiana who seems to keep picking up factory seconds and ebaying 'em off. 4-270 watt solar panel B grade made with 72 solar cells | eBay

They were seconds w/full test rated output, and I added a sb-50 mppt controller. They are cranking out now and i'm very happy with them. The panels went for only 325 or 350 each so it was a big savings over retail.

[Jon Hacking]

Originally Posted by CampDavid

In reflecting on our trip last summer, our draw was closer to 70 amps per day. I was thinking 225 to 250 watts would do it. Fletch

Folks - Amps are an instantaneous measure of current. A typical 12v fridge draws ~5A when it's on. If that fridge is on for 1/3 of the time (20 minutes/hr) then it's on for 8 hours/day, so the current draw out of the battery is 5Ax8hrs = 40Amp-Hrs/day (usually abbreviated 40Ah/day).

Amp-Hrs is also how batteries are rated. So a fully charged 600Ah battery bank should put out 6A for 100Hrs, or 60A for 10Hrs, or 4A for 150Hrs, or any other combination that multiplies to 600 (this is a VERY simplified description for the sole purpose of helping folks get units straight, so battery geeks please don't get technical on me). The point is that the unit "amps/day" doesn't really mean anything (& probably upsets a bit of learning/understanding).

As part of our kids schooling, our son worked up an Energy Budget for Ocelot. One of the very interesting numbers that came out of that exercise is that a typical 120W horizontally mounted panel will deliver 40Ah of usable current over the course of a day in the tropics. That is, divide the nominal wattage of your panel by 3 to get an idea of how many Amp-hrs that panel will give you in a day. Lots of ifs, ands, & buts here, but it's empirical data so probably a very good first cut.

Fletch, you're right that 225-250W of solar panels should give you 70Ah/day (in the tropics) but it doesn't leave you with much excess to carry you through cloudy or short days.

One thing I found when researching solar panels is that the bigger panels generally give you the best bang for the buck (best W/$). But a 12v panel will have 36 cells. If you get a panel with 72 cells, that's really a 24v panel. That's OK to use with a 12v system IF you use an MPPT controller.

[s/vPainkiller]

Originally Posted by Jon Hacking

One thing I found when researching solar panels is that the bigger panels generally give you the best bang for the buck (best W/$). But a 12v panel will have 36 cells. If you get a panel with 72 cells, that's really a 24v panel. That's OK to use with a 12v system IF you use an MPPT controller.

Is there an advantage to using 24V panels in a 12V system? assuming that I'm going to use an MPPT controller than can handle this... is there a benefit to using 24V panels - either in efficiency, reliability or cost?

Thanks,
Scott

[noelex 77]

Originally Posted by s/vPainkiller

Is there an advantage to using 24V panels in a 12V system? assuming that I'm going to use an MPPT controller than can handle this... is there a benefit to using 24V panels - either in efficiency, reliability or cost?
t

No, go for the maximium watts,best quality, for the lowest cost that will fit in the available space. The voltage will not have mch impact.

[senormechanico]

Originally Posted by s/vPainkiller

Is there an advantage to using 24V panels in a 12V system? assuming that I'm going to use an MPPT controller than can handle this... is there a benefit to using 24V panels - either in efficiency, reliability or cost?

Thanks,
Scott

Actually, yes. You can use smaller gauge wiring from the panels to the MPPT controller and get the same energy transfer.
Power = volts x amps.
With the voltage twice as high, you need only to deliver half the amps.
Therefore, the wiring can be smaller, lighter and cheaper.
This is why the power grid's electrical distribution systems are very high voltage for the long distance delivery areas.

Whether or not it's worth it in your particular scenario will depend on how far away your MPPT is from the panels, and the relative costs of the panels.

[hellosailor]

Some of the MPPT controllers claim a slightly higher efficiency at higher voltages, i.e. 4% loss at 12v, 2% loss at 24v, so it might buy you something--IF all the panel inputs were at that voltage and IF the MPPT controller was rated for higher efficiency.

On the down side, if the controller fails you can use the 12v panels without it. You cna't do that with 24v panels.

[btrayfors]

Voltage makes a difference also on total AH provided, since with a good MPPT controller you can increase your low light output.

Here's a basic setup which might do for you:

1. two 135-watt Kyocera panels with 17.7vdc open circuit output, wired in series to give 35.4vdc open circuit voltage; and

2. a good MPPT controller.

With this setup, you'll see significant output even on very cloudy days (I measured 9-10amps output in the midst of a major thunderstorm).

Bill