Simple solar battery charger

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Told a guy I'd do some research for him into a small simple solar system for use in maintaining the charge on a 12 volt automotive battery. Found this at Harbor Freight:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber418
It's a 12 volt, 15 watt panel for cheap ($70).
I'm thinking that all he needs to do is wire it up to his battery to trickle-charge it. I think I'd add a diode (IN200x) in series for protection. So will this work?
Since they don't list the output voltage for the panel, except to say "12 volts" in the description, I guess one could opt for this larger one:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber@085 (20 watt panel, made for charging auto & RV batteries, $190)
which says it maintains a charging voltage of 17.5 volts, which seems a little high: perhaps a string of 3 or 4 diodes in series would drop the voltage just enough to bring it down to the recommended 13.6 volts or so.
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On Thu, 26 Feb 2009 11:26:00 -0800, David Nebenzahl

A little more information is needed. How many amphours per day will he need? What type of battery? Is it flooded, gel or AGM? What size is the battery?
Solar panels rarely put out more than half the "rated" wattage.However, most have output voltage around 18 volts, and a charge controller is used to prevent overcharging. AGM's and especially GEL batteries will have dramatically shortened lives if charged at too high a voltage. For an AGM it's 14.7 volts max.
Passive voltage control won't work for this application. If you limit the maximum voltage enough to prevent overcharging or overvoltage, you will have no charging at all under average daylight conditions.
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On 2/26/2009 11:41 AM snipped-for-privacy@dog.com spake thus:

Not sure of current draw. Standard-size automotive battery; I already stated that.

So how about an active regulator (say, a 78xx) that would limit the voltage to the rated max for a lead-acid battery?
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On Thu, 26 Feb 2009 11:57:09 -0800, David Nebenzahl

There is no such animal as a "standard sized auto battery. To calculate what you need for proper charging, you need to know the capacity of the battery, as well as the current you intend to draw from it. It may even turn out that you need more battery capacity for the intended purpose. If the battery will regularly be drawn down before recharging, a standard car battery won't last very long at all. Even a proper deep cycle battery, which can be safely drawn down to HALF charge if immediately recharged fully, will last longer of you don't draw it down even that far. The less you draw down the battery before immediately topping it back up, the longer it lasts.
A solar charger is limited as it is without crippling it with jury rigged regulation. Charging a battery is not a simple matter of stuffing volts into it. There are phases for the charge process that have different requirements. A proper "smart" charge controller will get more performance out of the panel, in addition to doing a better job of maintaining the battery.
I'm a long time sailor, so this is a subject near and dear to me. Sailboats are extrely limited in the electrical department, and I have to keep track of every milliamp consumed. Sailors keep a written electrical budget.
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Andy comments:
You'd be better off with the "battery maintainer" that sells for 6.95 and is left connected to the battery. On sale, it is a few dollars less...
I've been using solar panels for many years, and if you don't aim them, they don't deliver the "promised" output. Plus clouds, rain, bird doo....
I'm near Dallas and we average 5.5 hours of full sun per day..... And get full panel output ONLY if the panel is aimed at those particular times.....
If you're going to pay $6 per watt, which is the going price for panels, make sure that you are familiar with all the ins and outs....
Still , $75 isn't so much to toss away while playing with a new toy..... And people like me will buy your panel for pennies on the dollar in a few years after it decorates you garage wall until the wife makes you sell it at a garage sale.......
Sorry.... but that's the way the cycle goes...
Andy in Eureka, Texas
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On Thu, 26 Feb 2009 11:26:00 -0800, David Nebenzahl

Without a charge controller, it will probably boil the battery. There are items made to do what you want, a plain solar panel with no regulation isn't one.
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to -maintain- a charge,all you need is a few 100milliamps into the battery. You won't boil it with that low a current.HF sells a small solar panel,IIRC,1-1.5W. It plugs into a cig lighter socket.
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You must mean this one:     http://www.harborfreight.com/cpi/ctaf/displayitem.taf?ItemnumberD768
If solar is not a groundrule a couple more possibilities:
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?ItemnumberB292
This one looks interesting since it's mounted under the hood and you just plug it in to an extenion cord.
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber857
Of course, I have no idea if any are worth anything or if they will trash the battery.
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David Nebenzahl wrote:

The 45W three panel package that Harbor Freight sells for ~$200 is the best value $/W and the panels are decent. You have to replace the junk charge controller they provide with a decent unit though.
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On 2/26/2009 1:13 PM Pete C. spake thus:

>

Any suggestions? I'm not familiar with these units (charge controllers), could use a little guidance here.
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On Thu, 26 Feb 2009 15:43:15 -0800, David Nebenzahl

Sunsei makes some relatively inexpensive ones that are pretty good. Under $100.
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David Nebenzahl wrote:

45W = 3.75A or so so you don't need a big one.
http://shop4.frys.com/product/4980091 ;jsessionid=w2TBIp2ERhLIbxMLNMXW3A**.node3?site=sr:SEARCH:MAIN_RSLT_PG
Sunforce 60012 7 amp Charge Controller Solar: FRYS.com #: 4980091 or
http://shop4.frys.com/product/5633001 ;jsessionid=w2TBIp2ERhLIbxMLNMXW3A**.node3?site=sr:SEARCH:MAIN_RSLT_PG
ICP CC-10000 10Amp Battery Charge Controller ICP: FRYS.com #: 5633001 Would do the job. I got the later one, I think it's a bit nicer. Either one gives some headroom to add a few more panels.
Fancier MPPT controllers will give the best efficiency, but the extra cost probably isn't worthwhile for a tiny array.
The HF kit also comes with a couple decent little 12V CFL lights which can be handy and a basic rack for the panels.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber599
The HF stores usually stock the panel kit, and if you sign up for the flyers and emails you can often get a 20% off coupon for the stores.
Good resources here as well: http://www.homepower.com/sample /
Look through the sample issue where you will find plenty of vendors. The electronic edition subscription (download pdf) is a decent value.
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snipped-for-privacy@invalid.com wrote:

P/V=I
45W/12V=3.75A
It won't be exactly that since the voltage won't be exactly 12V, it could be more like 3.21A @ 14V. It's also unlikely that it will get quite up to the 45W rating unless you have a MPPT charge controller on it.

~0.4A or so.

Entirely variable. You usually don't find a proper Ah rating on a car battery either, just a "group" and a "CCA" rating.

Not over rated, it is probably "CCA" or Cold Cranking Amps, a rating of the amperage the battery can supply for a short time in cold conditions. It doesn't really translate into an total capacity figure like Ampere Hours.

Roughly. Battery charging is a complex thing and as you may have noticed with a battery charger that has an amp meter on it, charging isn't constant current and the battery will only draw 10A at the start and then slowly drop off over time as the battery charges.

Exactly. They are intended mainly to compensate for the battery's self discharge and small loads like engine computers and radio memory.

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wrote:

Just for openers, the output voltage for the panel is more like 18 volts. And it only puts out 45 watts under perfect and optimal conditions in a Laboratory. In real conditions you may get 30 watts output from it for a period of a few hours on a sunny day.
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snipped-for-privacy@dog.com wrote:

The output voltage of the panel varies with the load on it. This is where the MPPT tracking controllers come in, finding and tracking the sweet spot where the panel can produce the highest power (wattage). At any rate, the straight P/V=I gets you into the correct charge controller current rating with some conservatism.
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wrote:

The rating used to sell it is the output without any load applied.

Yeah, if by "conservatism" you mean, "divide the rated output by about 2"! LOL
Most Solar panels sold by bargain outlets have specious specs.
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snipped-for-privacy@dog.com wrote:

No, it isn't as there is no power (power as in Watts) output with no load applied, only open circuit voltage (closer to 24V).

Have you actually used the panels in question? If not hold your babble until you have.
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wrote:

The advertised ratings are puffery, and are calculated based on unloaded voltage and theoretical output derived by calculation.

This is a subject I am VERY familiar with. The only babble I've heard so far is from someone who believes advertising copy.
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snipped-for-privacy@dog.com wrote:

I'd do some measurements on the panels, however there is a severe lack of sun today. Perhaps on the weekend if I get bored.
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snipped-for-privacy@dog.com wrote:

What specious are you and what kind of specs do you wear?
TDD
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