Simple solar battery charger

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On 2/27/2009 6:59 PM The Daring Dufas spake thus:

That should be "what *species* are you?". Like homo sapiens, a pretty specious species that sometimes wears specs.
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David Nebenzahl wrote:

What!? You never heard of Norm Crosby?
TDD
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On 2/26/2009 11:26 AM David Nebenzahl spake thus:

Update: I'm going to recommend this small charger from Harbor Freight to the guy:
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumberA144
It's a 5-watt[1] panel for $37. Should be good for starters to see how he likes the idea, and to figure out how to mount it, etc. (The vehicle is a truck with a camper shell.)
Thanks to all those who answered with helpful advice. To those who warned of dire consequences of overcharging, I don't think we have to worry about that with the relatively small currents here. Keep in mind I'm not designing a mil-spec system, but a solution for someone who currently has to drag his batteries (physically) to be charged regularly; this should make life better for him.
[1] Yes, I know, advertised power output: the actual output (under load) is probably close enough for our purposes here.
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David Nebenzahl wrote:

You should really get a meter on the battery in question and find out what the standby load on it is. Then you can get a better idea of what sized panel would be needed to keep up with it.
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On 2/27/2009 12:27 PM Pete C. spake thus:

I can tell you that he wants to be able to run a small portable TV, a radio (not at the same time) and maybe a small light or two (this is at night). I have no way of actually measuring the actual power draw, as he doesn't have this stuff at this time. As I said, close enough is close enough. Plus, $$$ is limited.
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On Fri, 27 Feb 2009 16:38:15 -0800, David Nebenzahl

I can tell you the outcome of your experiment, but I already know you only want someone to agree with your suppositions.
For openers, a 10 watt lamp draws about 1 amp at 12 volts. That's a very small lamp, like the dome light in your car. A small TV is going to draw 5+ amps at 12 volts. So with a small TV and two very dim lights, you will be USING about 7 amps an hour. The TV may draw twice my estimate, but lets call it 5 amps.
A DEEP CYCLE battery can be drawn down to 50% and then recharged many times. A Group 24 Deep Cycle battery is about the size of a large car battery and is rated at around 75-80 amp hours. You can only use half that capacity at MOST. So 40 amp hours. Divide 40 by 7 and you know how long you can run the tv and lights before you need long and substantial charging. Remember, this is a DEEP CYCLE battery designed to do this. A "car battery" will be ruined very quickly by the same type of usage.
Charging is not a simple matter of replacing exactly what you used. It will take more amp hours of charging than you used.
A "car battery" isn't going to cut it at all. Neither will a trickle charger. You need a minimum of a 100 watt solar panel with a good controller, and probably 2, group 27 (100 amp hour each) DEEP CYCLE batteries to come anywhere close to doing what you propose. The far more expensive AGM deep cycle batteries take a charge much easier and faster, and will last even longer than a deep cycle flooded battery, but the up front cost will probably stop you cold. A decent flooded Group 27 deep cycle battery is about $100. An AGM that size will be between $200 and $400. You need two of whatever you select.
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