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Old 28-07-2010, 13:49   #1
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Determining Size of Solar Panel/Battery

I'm trying to design the electrical system for my sailboat. The electrical requirements are very modest--approximately 8.56 daily amp-hours: LED running lights, two LED cabin lights, the compass light, cabin fan, and a VHF radio. According to a book I have, the house battery capacity should be about 2.5 times consumption, in amp-hours, between charges. That would be 21.4 amp-hours of battery capacity.

My plan is for a 12v deep cycle house battery and a solar panel to keep the battery charged. On another boat, I used two 6v Trojan T-105 golf cart batteries wired in series with good results. This has a capacity of 225 amp-hours (which seems like overkill for my new electrical requirements).

1. Is there any reason NOT to use a battery that has considerably more capacity than the 21.4 amp-hours required by my daily electrical needs?

2. To use a solar panel to keep a battery charged that is supplying 8.56 daily amp-hours of electricity, does the panel simply need to put out at least 8.56 amps in a day? If so, that doesn't mean the panel has to have an output of 8.56 amps, but could have a lower output? For example, if it outputs 2 amps, and you have 4 hours of direct sunlight, then it will pump 8 amps into the battery, replacing what I used during the day. Yes? No?

Thanks.
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Old 28-07-2010, 13:56   #2
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Quote:
Originally Posted by Triton318 View Post
For example, if it outputs 2 amps, and you have 4 hours of direct sunlight, then it will pump 8 amps into the battery, replacing what I used during the day. Yes? No?

Thanks.
yes. think of it in terms of amp hours. two amps for four hours equals 8 amp hours.
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Old 28-07-2010, 14:11   #3
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To answer your last question, yes.

8+ amphours each day is very, very small. But given that, figure that an X watt panel will put out about 1/3X amphours on a sunny day. So that would mean that you would need about 25 watts, but I would go for at least 50 for cloudy days depending of course on where you are cruising.

A group 27, 31 or two GC batteries in series would be plenty of battery capacity to see you through the night and cloudy days. You will need a minimal solar controller, like Morningstar's. Don't bother with a MPPT controller for your small system.

And put a fuse near the battery. 14 gauge wire will be enough for a 50 watt panel.

David
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