Solar for small applications

[superdave]

I was wondering if anyone had any ideas on the cheapest way to charge a battery using solar power. I will only have one battery; the only jobs it has are running lights and a car stereo (nothing like a starter, bilge, etc). I guess it should be able to recharge an empty battery in roughly three days of sun. (My boat will be on a mooring, and I will simply want the battery for the stereo and occasional night cruises, and can't think of any other way to recharge it).

I was wondering if a couple of these would work: http://cgi.ebay.com/12V-Solar-Panel-...QQcmdZViewItem

If anyone could give me advice on a place to get inexpensive solar reacharging at this small scale, or any other advice, that would be great!

Thanks,
Dave

[LWinters]

Good quesiton superdave. I've been looking at something similar to help keep the dinks batteries topped up while cruising. We've got a hard dingy with a dedicated deep cycle to make sure we have the legal lighting when running. Anyone else found a good solution for small boats like Dave's and dinks like mine?

I've found the products below that include a 5watt panel + a charge controller to help keep from frying a lead acid deep cycle.

http://www.siliconsolar.com/shop/cat...r-p-16156.html
http://www.siliconsolar.com/shop/cat...Watt-p-48.html

[sailingdog]

You probably want a larger panel than that, especially if you're going to be using the batteries at all.

Figure out what your electrical use will be...by multiplying the amps required * hours used, for the running lights and stereo and then size the panels accordingly.

Something like this will probably do the job for you, given the three-days of sun average...but you don't want to be running the battery down to empty, as this will drastically shorten its lifespan.

Batteries on boats should only be used to 30-50% of their capacity. Using them more drastically starts to shorten their usable life.

You really need to do an electrical budget, and scale your battery bank to be about 2-3x the expected draw of your use period between chargings.

[Idiens]

I have one just like you pictured Superdave. Forget it for your application, I use it on one of my cars, whose various standby electronics flatten 80Ah in three months of disuse - its about up to that. You need something like LWinters suggestion. I have one on the boat, but get the big one. It just keeps 550Ah up around 12.6V between weekend general usages (which don't drop the batteries below 12V). The flexible ones are usually fully dioded, so a minor shadow (like from a stay) does not halve their capacity. Plus they are pretty tough compared to the rigid glass protected ones.(But not so efficient).

[txmatt]

superdave,
The seller of the item you linked to also offers a 400 mA version. Just running the nav lights for a few hours could probably be made up for in three days with that, almost certainly if you got two of them. If you are running the nav lights all night, plus blaring the stereo, and expecting to have your batteries charged in three days you might need something like SD linked to.

Allowing a 2 amp draw for 10 hours (say, running an anchor light over night) would be 20 Ah, which would require 50 hours of charging at .4 Amps. With two of those panels, that would bring it down to the equivalent of 25 hours of peak charge output which you might be able to get in three days, but it would be close, I imagine. I don't know whether those actually put out anything near the advertised current. I'm thinking of trying something similar in my boat, but haven't yet.

[sailingdog]

You're not going to get 8+ hours of peak charging a day... a more realistic estimate is about 5 hours of "peak" charging a day... unless you're using a sun-tracking panel.

[heinzir]

I have a small (5W?) solar panel mounted to the foredeck hatch of my 23' Venture of Newport cutter Chiquita. I bought it at Northern Tool & Equipment for about $60. See the picture at http://sports.webshots.com/photo/204...49039029oYLmYT

The wood frame is homemade.
As my boat is on a mooring, this is the only source of battery charging I have. I have had it for many years and it has always kept my battery (a group 27 deep cycle from Walmart) topped up. My loads are minimal; I sail about twice a week with very little night sailing. The vhf, depth sounder, and GPS are always on when under way. I added a Davis anchor light 2 years ago (after Chiquita was rammed and dismasted by a drunk powerboater late at night! But that's another story.) The light comes on automatically every night and turns off when the sun comes up. To minimize current draw, I replaced the bulb with a homemade LED array that draws only .06 amps. The solar panel has been able to keep the battery charged with no problems. So far so good!

Henry
Chiquita

[camaraderie]

Superdave...typical car stereo will draw 2 amps in operation. Figure 1.5 amps for each nav light unless you convert to LED's. Then I'm sure you'll have cabin lights too...so you could be in the range of 8-10 amps draw assuming you don't have any 12V powered instruments.
So if you run the stereo 10 hours a day...20 amps and the nav lights and cabin lights 6 hours....say another 30 amps...you MIGHT get up to a 50 amp use which on a Group 27 battery will be a 50% discharge.

To replace 50 amphours over a 3 day period you need 18 amphours a day. Assuming SUNNY conditions and 4 hours of full output + 4 hours of 1/2 output each day) this will require a panel that puts out 3 amps @ 12V in direct sunlight or a 36 watt range panel.

NOTE that a 12V/36 watt panel does not need regulation but neither will it charge your batteries PROPERLY . They will quickly die from undercharging.
Instead you need a higher VOLTAGE panel that can deliver at least 15 volts and higher is better along with a regulator to step it down to to the right charging voltage...usually around 14.4 for bulk charging of wet cells.

I have been conservative with my recommendations meaning that you probably WON'T use 50 amps on most trips and you can probably use a smaller panel as SDawg suggests....but you are only gonna get about 1/2 of my projected needed amps out of it in 3 days of charging so if your reality is closer to my projected needs, you may find yourself wanting two those panels from Defender that he suggests.

[superdave]

Heinzer - how often do you take your boat out? Better question - how many days do you feel it takes to recharge the battery from a day of regular use?

Also, cam: it seems that most panels are 12v - is it difficult to find 14-15v panels?

Thanks again all!

-Dave

[sailingdog]

Most of the larger, framed rigid panels are 16+ VDC on output... The smaller flexible panels, designed for battery maintenance are usually 12V or so..