# Solar question

Discussion in 'General Electronics Chat' started by iONic, Jul 15, 2011.

1. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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Given enough power from the solar array, could one attach a regulator to the Cells to run a 12V 400ma Water pump and also send to the PV wires to the solar charge controller to charge a 12V Lead Acid battery. Would there be any special hookup or additional circuitry to accomplish this?

Last edited: Jul 16, 2011
2. ### tom66 Senior Member

May 9, 2009
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Do the math.

Sunlight hitting the earth is about 1400W/m^2.

Best solar cells on the market today are 15% efficient - that will net you 210W/m^2.

You need 12V * 0.4A = 4.8W.

More than doable even in shade .

3. ### pistnbroke Member

May 9, 2011
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for such a low power system a diode in series with the output will be all you need to stop the battery discharging into the panels at night .

4. ### Bernard AAC Fanatic!

Aug 7, 2008
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I would use a buck converter to reduce panel V of 12 to 17V down to 12 for pump for best efficiency. A 45 W panel would be in right range to run pump and supply 2 A for charging.
?? battery Ahr rating??

5. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
1,422
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The question was essentially, not that I have enough power, but rather could I parallel the two systems, 1) a solar charge controller to charge a battery and convert the ~17V to 12V to operate a pump.

Bernard, I assume that the buck converter would be more efficient than a simple linear 12V Voltage regulator?!

6. ### Bernard AAC Fanatic!

Aug 7, 2008
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12V regulator power loss about 2W, buck converter about 300mW; but with no power limitation I would go for 12 reg & run battery charging & pump in parallel.

7. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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So I could save 1.7W of power. The buck regulator would be helpful. Any suggestion for an appropriate buck converter? What about the LM2595?

8. ### Bernard AAC Fanatic!

Aug 7, 2008
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I think we need SGT. Wookie on this as I have no experience with buck/boost converters- need to change that sometime.

9. ### SgtWookie Expert

Jul 17, 2007
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What's your goal on this? Run the pump 24 hours a day? Or just during daylight?

I tried using National Semiconductors' Webbench, 13v-18v in, 12v @ 0.4A out, and it couldn't produce a workable design with ANY of their products. I'da thunk they'd at least have one buck-boost design out of that whole thing, but nooOOooo....

The shutdown function was backwards from what you needed anyway. I'll suggest you'll need a regulator that'll shut off when Vin < 9v or thereabouts, otherwise it'll just be spinning its' wheels and maybe break something in the process. Boosting 9v to 12v @ 400mA with 80% efficiency means about 667mA current required from the input.

10. ### nickelflipper Active Member

Jun 2, 2010
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RV's or Caravans parallel their battery charging system (i.e. converter, solar, or both) with the load circuits, including the water pump. Running an RV Shurflo water pump at 14.4V, at least on an intermittent basis, poses no problems. The battery should never be run down more than 10.5V, which is basically empty and means reduced longevity of the battery.The water pump will still work, but be be kinda sluggish.

If you buy a solar charge controller, then their is no need to build a buck controller, unless it's for the fun of it of course.

11. ### iONic Thread Starter AAC Fanatic!

Nov 16, 2007
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Nice going "National!"
Sgt. I would plan to run the pump "While supplies last" or in the daylight only.
a cutoff at a predetermined voltage sounds like a good idea also, both for morning and night low voltages.

Perhaps a simple Linear LDO Vreg would be sufficient.

12. ### Bernard AAC Fanatic!

Aug 7, 2008
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A few last thoughts:
Measure lowest pump V that allows it operate after start. [ PVR]
Add a large cap to panel output, 10,000 μF to assist start.
Delay pump start untill panel output = PVR
Delay battery charging untill panel V = 12V, or greater than PVR.
A temporary load on panel [100 to 200 mA ] will be needed to get reliable V measurements, & start Vs re calibrated accordingly.
Does this make sense??

Nov 16, 2007
1,422
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Yes, Thanks!