Solar Gurus - A little help please

oh I see

I'll have to unbolt it from the roof and have a look

thanks
deano

Okay so I have pulled the panel off the roof.

no brand names as such on the back but the info label reads:

Model: PMW250PW
Watt: 250W
Voltage: (VMP36.0V) 24.0V
Current: 6.95A

I also pulled the cover off the back and took a photo.

It looks to me that the panel has 2 sections wired in series with what looks like diodes

can this be rewired to give me 12V?

sorry about the crappy photo. the other half has our nice dslr.a

Park it in the sun, measure the voltage across each diode. You may have a 3 section panel with a nominal 8 volts per section (around 11V open circuit). If you get 16V, 0V and 16V you have a 2 section panel. Remove the centre diode and connect the two sections in parallel, terminal 1 to 3 and terminal 2 to 4.

The diodes are blocking diodes for when the panel is part of a string of panels on the roof of a house, any section being shaded goes down to zero voltage and the diodes allow the current to flow through the rest of the panel string.

thanks bee utey.

just before reading your post I pulled the centre diode and wired terminal 1 to 3 and terminal 2 to 4.

tested the panel on the regulator and was getting around 13 to 13.5V in the late afternoon (5 pm) sun but only about 1 amp of output.

will try it again tomorrow when the sun is a bit higher.

fingers crossed I get some good results.

Quote:

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Originally Posted by bee utey

Park it in the sun, measure the voltage across each diode. You may have a 3 section panel with a nominal 8 volts per section (around 11V open circuit). If you get 16V, 0V and 16V you have a 2 section panel. Remove the centre diode and connect the two sections in parallel, terminal 1 to 3 and terminal 2 to 4.

The diodes are blocking diodes for when the panel is part of a string of panels on the roof of a house, any section being shaded goes down to zero voltage and the diodes allow the current to flow through the rest of the panel string.

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This (highlighted bit) is the important bit here. 0 volts across the centre diode. Though the open circuit voltage on a '12 volt' panel in good sun can be around the 20 volt DC mark.

If you read (in good sun) an open circuit voltage of around 10-14 volts across all three diodes then you have a 3 X '8 volt' panel as bee utey described.

Still not too bad, If this is the case (in theory) I'd 'drop off ' one '8 volt' panel and run the other two into a suitable reg and end up with a good 160+ watt supply. :)

Deano :)

Quote:

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Originally Posted by DeanoH

This (highlighted bit) is the important bit here. 0 volts across the centre diode. Though the open circuit voltage on a '12 volt' panel in good sun can be around the 20 volt DC mark.

If you read (in good sun) an open circuit voltage of around 10-14 volts across all three diodes then you have a 3 X '8 volt' panel as bee utey described.

Still not too bad, If this is the case (in theory) I'd 'drop off ' one '8 volt' panel and run the other two into a suitable reg and end up with a good 160+ watt supply. :)

Deano :)

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yeah just confirmed the panel is a 3 x 8v one.

this will mean I cant use it in conjunction with a separate folding panel on the same regulator right?

depends on the voltage of the other panel and the style of regulator, without seeing all of it Id advise not doing so.

if its reading 14v open circuit you might be able to just parallel the 3 internal panels on that one and use that to drive the regulator but it'll cost you effeciency.

I suggest you either sell the panel to a 24V vehicle owner or purchase a suitable MPPT regulator, such as this one:

Powertech MPPT Solar Regulator Charge Controller 30A 12/24V | eBay

Quote:

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NOTE: This unit WILL charge a 12V battery from 24V solar panels, but not vice versa.

Specifications
Max PV array : 360W(12V) or 720W(24V)
Battery system voltage : 12VDC or 24VDC
Max PV open circuit voltage : 65V
Max input current : 30A
Rated load current : 30A
Ambient temp range : -20ºC - 50ºC
Efficiency : 97%
Dimensions : 205(L) x 145(W) x 55(H)mm

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Run any other panels with their own regulator.

Quote:

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Originally Posted by bee utey

I suggest you either sell the panel to a 24V vehicle owner or purchase a suitable MPPT regulator, such as this one:

Powertech MPPT Solar Regulator Charge Controller 30A 12/24V | eBay

GSL MPPT12-1 12/24V 12A Solar Charge Controller Regulator. Aust made, 2yr wnty | eBay

Run any other panels with their own regulator.

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same tech as whats in the jaycar sold MPPT unit if your not willing to risk an ebay purchase

Quote:

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Originally Posted by Benz

...................................this will mean I cant use it in conjunction with a separate folding panel on the same regulator right?

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Correct.

bee uteys solution is the best technically, but at $188 for an MPPT reg that will do 3 panels of the size you've got I'd be looking for a smaller (hopefully cheaper) version if they exist.

Don't know if I'd parallel the three '8 volt' panels together. Too little room to move in less than full sunlight and correctly aligned.

As a cheap (not technically best) solution you could use 2 '8 volt' segments in series into a standard '12 volt' reg. If the voltage was too high for the regs input I would put a string of diodes in series with the output of the panels to reduce the panel voltage to get it in spec with the regs input. A silicon diode drops about 0.6 volt across itself REGARDLESS of the current flowing through it, so to lose 4 volts use 7 diodes. and 10 Amp power diodes are cheap.

10pcs 10A10 10 Amp 1000V 10A 1KV Axial Rectifier Diode | eBay

Sort of messy and not technically 'elegant' but doable to get a cheap result.

Deano :)