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adhyartha
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Parallel of DC Supply

09/26/2010 7:07 PM

I'm building a Photovoltaic system coupled with battery charging system from AC supply. What would happen if I just couple the dc output from the PV system, and the dc output from battery charging?

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dvmdsc
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#1

Re: Parallel of DC Supply

09/26/2010 10:00 PM

Your battery will be charged to the output voltage of the PV system.

This value can be too high and overload the battery.

Analogue to a too low voltage of the PV system, your battery will be undercharged. Working below and above admissible voltages will damage the battery and/or shorten its life.

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peterg7lyq
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#2

Re: Parallel of DC Supply

09/26/2010 11:40 PM

you will damage the PV at night when there is no output from the PV

You would need some form of blocking to make sure power only goes into battery and not out of battery into charging items
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PWSlack
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#4
In reply to #2

Re: Parallel of DC Supply

09/27/2010 12:45 AM

Quite. A silicon rectifier, of appropriate rating, in series with each of the two sources of power would be sufficient.

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dvmdsc
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#8
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Re: Parallel of DC Supply

09/27/2010 8:36 PM

Hi PW and Peter,

As promised here I am back with the results:

My lowest current measurement I can make with my DVM is 2 micro-ampere and I couldn't get anything to read. My multimeter felt very comfortable reading on a resistance scale 20 Mohm and showed still out of range, something I do not recommend to do when light is on the panel.

Two panels parallel didn't read anything too.

This leaves the question open: when is a blocking diode necessary?

And if needed, for what purpose?

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dvmdsc
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#5
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Re: Parallel of DC Supply

09/27/2010 4:07 AM

I don't know the panel gets damaged at night. Mine aren't. And they have a steady connection to a feed back system to the grid, above 14.2 Volts. The batteries are for reason connected straight to to the panels, without a diode.

Please explain. Of course I will do a double check tomorrow. But I think at night the internal resistance of the cells is that high that they hardly drain the battery.

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peterg7lyq
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#6
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Re: Parallel of DC Supply

09/27/2010 4:40 AM

I stand corrected, I thought there might be a backflow, but as you say if the resistance is high then it would seem not well that saves on blocking diodes then
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dvmdsc
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#7
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Re: Parallel of DC Supply

09/27/2010 12:15 PM

Peter, You just made me curious about it now. Tonight I will check for any dark current and come back on this topic. Not aware of it this drew my attention completely. Forward, I have about 8 ampere per panel @ 18 Volts. I have the batteries and the inverter parallel per 12 volts battery. The threshold for charging the batteries, I have put at 14.2 volts, from that point, I send the energy back into the grid (inverter). Maybe I lose some power at night. Thanks, D.

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peterg7lyq
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#3

Re: Parallel of DC Supply

09/26/2010 11:43 PM

any use ?

http://www.discovercircuits.com/B/batt-chrg.htm
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dvmdsc
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#9
In reply to #3

Re: Parallel of DC Supply

09/27/2010 8:40 PM

I have seen diagrams with and without the diode. On the other hand some smaller panels are for sale with the purpose to maintain a boat and/or car battery. Only 10 watts, and they just have 2 wires with clamp?

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lighthasmass
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#10

Re: Parallel of DC Supply

09/27/2010 11:56 PM

Your PV's will output too high a voltage and you risk damaging your batteries. Use a MPPT charge controller.

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