Help! Choosing Batteries for a Solar System

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Our homestead comes with 3 15 watt solar panels, and no other equipment (I will test the panels tomorrow). I estimate that we will spend no more than 4 days at the property, in a 21 day period, for at least a year. I guess what I should ask is how many and what type of battery I would need for 17 days of sunlight, in winter in West Virginia, to charge them to last 4 days. I would like to have enough power to burn a 100 watt bulb (12 volt system if I don't have an inverter) for 4 hours hours, and a PC and 19 OR 15 inch monitorfor an hour. Of course we will add things beyond these if the power is there. I know we need a charge controller to protect the batteries, and it sounds like we will need an invereter unless someone knows a way to power a desk top pc from 12 volts. Any ideas out there? Would it be necessary to do battery maintenance (batteries will be protected and vented) more frequently than every 3 weeks? We leave tomorrow, so I'll be offline till Tuesday or Wednesday. Thanks in Advance.

-- rick K (rick_122@hotmail.com), November 08, 2001

Answers

Rick: Do you know what the electrical demand is for the puter and monitor? It should be fairly easy to figure out if the demand is known. BTW, consider compact florescent lites. Almost as much lite with alot less electrical demand.

-- john (natlivent@pcpros.net), November 08, 2001.

Well....... The tower was home made, so there is no "plate" on the back of it. The 19 inch monitor uses 2.5 amps (I just looked) to my amazement. The latest compact fluorescent bulbs have incredible output!!

-- rick K (rick_122@hotmail.com), November 08, 2001.

Rick: Here's a little input. 45watts x 6hrs sunlight x 21 days= 5670 watts at 12 vdc or 473 amp hrs.

Daily use = (I'm gonna arbitrarily cut your lighting in half and assume 50watts for compact flourescents) 200watts daily x 4days=800 watts.

That leaves you with 4870 watts left for the puter.

For the sake of illustration I'll assume the tower uses as much as the monitor so the amp draw will be about 5amps. A rule of thumb I use is 100watts = 1 amp at 110vac so that'll give you approx 9 hrs of puter operation.

As to batteries I'm guessing a pair of L16 would be adaquate. I think they're rated for 220 amp hrs.

Since you're not gonna be there for extended periods of time I'd suggest you get the hydro caps for the batteries. They'll prevent the batteries from drying out in your abscence.

Take all this with a grain of salt because I'm not an engineer but I suspect I'm in the ballpark with my estimates.

Used 12v inverters should be fairly inexpensive. Check out the numerous RE suppliers on line.

-- john (natlivent@pcpros.net), November 09, 2001.


John I have saved the formulas and thoughts. Thank you!!! I checked the output and specs on the panels. They are arco mod ASI 16-2000 (4 panels - about 12 inches wide each, joined together at 2 common leads). They displayed a warning "rated for system operation at 50 volts maximum. Operation at higher voltages may be hazardous. The joining harness showed no voltage. The individual panel "nuts" showed about 18 volts each with minimal tree branch obstruction (it was still bright and sunny there. Does this give you a better idea what I have. Do you have any idea if solar panels have a shelf life, or if their life expectancy is determined by use (these have been largely unused). Thanks again.

-- rick K (rick_122@hotmail.com), November 12, 2001.

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