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post #21 of 45 Old 03-14-2011
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Cranking amps are for starting batteries. What you need to know is the "Group Size" of the batteries, or the Amp Hour capacity of them, nominally at the 20-hour discharge rate.

I'm guessing they are group 24 or group 27 with a nominal capcaity around 75AH each when new. If you only cycle them 50%, that means you are working around a 75AH useable capacity for the "bank".

How often you will draw them down, how fast you want to charge them...you have to run the numbers for an energy budget. Odds are that one 50A or 55A solar panel with a simple charge controller would work out pretty well for the minimal power needs you have. FWIW, if you invest in an MPPT controller, which may be overkill for you, they actually squeeze about 10-15% more effective power out of a solar panel, so they're actually useful.

Anything smaller, and a couple of gray days after a deeper than usual discharge is going to leave you looking for extension cords. Typically, in a full 12-hour day of bright sun, you'll get about four hours worth of "rated" full output fro a solar panel. (i.e. pretend the day is only 4 hours long but the panel is working at full output for those four hours.)
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post #22 of 45 Old 03-14-2011
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Originally Posted by MarkofSeaLife View Post
Also remember, when you are using power on your boat during the day you are still charging with your solar panel, so what you are using is 'free'. Your draw is only at night
That's a bit misleading. If you run a tiller pilot, start your motor, or whatever, during the day, those amp-hours have to be replaced, which delays the replacement of last night's draw.

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post #23 of 45 Old 03-14-2011
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Also remember that if you use your engine a couple of times each day you go sailing (in and out of the marina, and/or to anchor) you should probably run it at least 20 or 30 minutes or so each time you start it to get it up to full operating temp. In that case, once you've replaced the juice you used starting the engine, you'll be charging your batteries at a fairly high rate (probably something like 20-25 amps, at least. In other words, much of your electrical requirements will probably be met by the normal use of your engine.

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post #24 of 45 Old 03-14-2011
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I think it is being assumed here that you have these batteries set up, or switched, to run in parallel, meaning you have the amp hours of both available. Is that true? That will be important when figuring if you are going to be drawing them down by 50% etc.

There are a lot of threads here that cover this topic, if you can find them. Some good reading in there.

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post #25 of 45 Old 03-14-2011
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Just got a flier in the mail from West Marine for their Sunforce Solar Chargers:

125-mA - 1.5-watts - $17.49
350-mA - 5-watts - $49.99
1.0-A - 15-watts - $76.99

That translates to 30-percent off their regular price. That's a pretty good deal.

Gary
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post #26 of 45 Old 03-15-2011
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Quote:
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125-mA - 1.5-watts - $17.49

That translates to 30-percent off their regular price. That's a pretty good deal.
True but those panels are a bit small The cheapest one would take about six hours of full direct daylight to recharge a single AAA battery

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post #27 of 45 Old 03-15-2011 Thread Starter
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I know the 15 watt version is about 2 feet long!
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post #28 of 45 Old 03-15-2011
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Keep in mind that the solar panels are not for charging your batteries from dead flat to fully charged. These panels do a great job of keeping your battery topped off and ready to go when you are. Most folks on this, and other sailing forms, are weekend warriors, sailing one or two days a week at most. The remainder of the time their boats are tied to the docks. I had the 350-mA charger on my Catalina 27 and it kept too dual purpose batteries fully charged all the time. In fact, this past winter I did not remove the batteries from the boat because the panel kept them fully charged. I checked them weekly with a hydrometer and the SG always read 1.275, which is right where it should be. And, the panel worked just fine on overcast days, times when there was no direct sunlight whatsoever. Granted, it didn't pump out the maximum wattage, but it still put something into the battery, which is better than nothing at all.

My panel was (is) mounted on the back rail on an adjustable frame that allows you to change the position so you get the best benefit from the sun at any time of year. It has a hermetically sealed connector where the wire plugs into the panel, and a pair of battery clips at the other end. About once a month I clean the batter clips with a bit of WD-40 and a piece of cloth. I also wipe down the battery terminals as well with WD 40 to insure good continuity.

Yes, the 15-watt panel is about 2-feet long, which is relatively small when you consider what it does. I suggest mounting the panel somewhere other than on a deck surface. Think about it. If you mount that panel where someone would normally walk, you know someone will likely step on it. If they do, that's the end of the panel. The adjustable, rail mount is the most likely candidate, particularly if it's mounted on a stern quarter.

Having sold my Catalina 27 just a few weeks ago, and the solar panel with it, I just ordered the 15-watt panel for my Morgan 33 O.I.. The Morgan has a pair of starting batteries, plus three golf-cart batteries connected in a series/parallel rig. I'm fairly confident the panel will keep them fully charged.

The boat also has a large, wind generator, but that will only be used while the boat is attended or underway.

Good Luck,

Gary
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post #29 of 45 Old 03-15-2011
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Those little panels, 15W and under, probably would be better marketed as "battery maintainers" rather than chargers. They're best suited for maintaining the charge on a battery, preventing the normal self-discharge that ruins batteries. And eventually, maybe, if the sun shines and battery is small enough, charging them too. :-)
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post #30 of 45 Old 03-15-2011
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Hm... perhaps 15 watts could be enough. My first panel was a 7.5 watt panel that I kept on the deck and, as Gary predicted, sat on (not stepped on, sat on). So that's god advice there, as is the advice about regularly cleaning the connectors. Hadn't thought to use WD-40, though.

Still, last year I did a few 12+ hour singlehanded trips, so the tiller pilot tended to see some heavy use. I was happy to have the 30 W panel basically supplying its power for a good portion of the day and not have to worry about starting the motor (my batteries are also dual-use).

s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27
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