solar powered fan

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Location: Colorado

solar powered fan

Post by myp71 » Sun Jul 13, 2003 7:33 pm

I have a 12v8Ah battery to power a 3.9watt fan how huch would it take to run this fan and charge this battery off of a solar panel I have a 12v 120ma solar panel would that be enough?Or would it need to be more MA and how huch?<p>How long would this battery 12v 8Ah power this 3.9watt 12v fan during the night when there is no sun to power the solar panel.

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Location: Illinois

Re: solar powered fan

Post by Mike » Mon Jul 14, 2003 4:35 am

Why don't you use the solar panel to power it during the day and also a nicad battery at 12V that charges from the panel. If the fan could run off of 9V, I would reccomend using a 9V nicad battery, since the panel will not always produce 12V. You would simply connect the battery through diodes (to provent the battery from discharging through the panel) across the terminals of the panel, then the fan would connect to the connections on the battery. The battery would take probably about 6 hours to charge, as your panel will probably produce 150 or so MA. The rest of the time, the solar panel will power the fan, with the battery taking over as needed, and the battery will power it throgh the night.

Chris Foley
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Re: solar powered fan

Post by Chris Foley » Mon Jul 14, 2003 10:16 am

P = V * I
3.9W = 12V * Fan Current
Fan Current = .325A
Your fan requires 325mA current.<p>8Ah = .325A * h hours of fan operation
h = 24.6, meaning a fully charged battery will power your fan for 24 hours.<p>8Ah = .12A * h charging hours
66.6 charging hours to acieve full charge.<p>There are a couple of relevant facts here. For a .325A load on this battery and a .12A charging current, even in theory you would have to charge it for .325/.120 = 2.7 times longer than you would be running the load. There Aint No Such Thing As A Free Lunch. This has no chance to work except possibly in Alaska in the summer (very short nights). Second, with a 24 hour cycle, there's no way you can completely recharge this battery with .12A. You might want to try multiple solar cells in parallel. Also, different batteries have different recommended charging and discharging cycles. Ignoring the manufacturer specs can lead to poor battery performance. If your power cycle doesn't deep-discharge the battery, for some types of batteries that may also lead to degradation.<p>You might want to find out what kind of battery you have, and from that you can determine the recommended load to reach the required discharge, and the recommended charging cycle and go from there. This project is very doable with a different choice of battery or fan, or with more solar cells. That's been said a few times before ;-). A designed project of this type will start with the fan you require, then the selection of battery, then the recharging method, then number of solar cells. Happy hunting.

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