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Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664

Posted: 12:14am 22 May 2018

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Hi All,

I'll demonstrate my ignorance, where I think I know the answer...

I paralleled up my 2nd & 3nd strings yesterday.

When it came to plugging in the blocking diodes, I'd presume they'd go in the +Ve leg, but then thought there's aways some weird chance...., so I put one on each side.

Gut knowledge tells me it shouldn't matter, but better to sound stupid that to damage stuff & confirm it.

Phil

yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166

Posted: 07:06am 22 May 2018

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I have fitted blocking diodes in the past, but in the end I have turfed them all in the bin. yes, they go on the positive leg traditionally.

I thought something weird was going on with one system, when I went back I couldn't prove there was much, if any, leakage through the shaded panels.

certainly not enough to justify the losses of a diode.

As long as the charge controller disconnects at night it will probably work better without the diodes. [opinion]Edited by yahoo2 2018-05-23

I'm confused, no wait... maybe I'm not...

Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664

Posted: 08:31am 22 May 2018

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Thanks,

The 2 Strings here are:-

6x250=195Vmp @ 7.67A
11x250=380Vmp @ 7.08A

So vastly different in MPPT requirement, bat also different orientation & shading issues.

I was switching them manually into PV2 for the last week or so, but put the diodes in yesterday.

It was interesting to see that I did get a better yield in the transition zone of 12:00 to 2:20pm.

Was getting 6A @ 195V from the 6 & 2A @ 190V from the 11.

The 195V string gets good sun from 9:30, but starts shading at 2:30.
The 380V string faces SW @ 34.5� & being 30� South they don't get much direct sun till well after mid-day at this time of year.

Can just feel some slight warmth in the diodes carrying the bigger load.
Their rating is 55A @ 2000V.

I know it's not an ideal solution but not sure of the other alternatives to get the 3 separate strings into service with only 2 PV inputs.

Phil.

George65
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Joined: 18/09/2017
Location: Australia
Posts: 308

Posted: 11:54pm 22 May 2018

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Get another inverter.
There are some new single string 3 Kw ones on Dumbtree atm for $125+ delivery.

Wouldn't the Diodes in the panels stop any backfeeding as it were or are you trying to take the load off them?

I wasn't even aware you could mismatch strings like that on a single inverter. I realise some have 2 MPPT controllers but the ones I have share the same neg inputs across different connectors so I was of the belief they all had to be the same or near to it.

Learn something every day! :0)

Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664

Posted: 08:47pm 23 May 2018

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George65 said
Wouldn't the Diodes in the panels stop any back feeding as it were or are you trying to take the load off them?[/quote]

No, they are across the strings in the panels,
if you put say 50V across this panel, it couldn't flow thru the diodes as they would be reverse biased, so it would flow back thru the cell & generate heat.

A single diode at the end of the string will block it though.
Just needs the appropriate voltage & current rating.

[Quote]I wasn't even aware you could mismatch strings like that on a single inverter.[/quote]

Mines got 2 MPPT controllers, but I've now got the 3 strings,
& I really didn't know what to expect,
I'd been manually switching strings for a week or so, the 6x string working from about 9:00 to 2:30, while the 11x string provided out from about 12:00 to 4:00.

When I first plugged them together on Monday the 6x was putting out 6A around 12:00, and the 11x was adding 2 amps.

Too early to really know how successful it will be, but daily production is a bit up on what I got with manual switching.

George65
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Joined: 18/09/2017
Location: Australia
Posts: 308

Posted: 12:19am 24 May 2018

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Phil23 said

George65 said
A single diode at the end of the string will block it though.

Thanks.
Something else well worth knowing I have learned.

I have a couple of arrays with mis matched panels I have amassed from various places when going cheap. I don't worry too much about the wattages but I do keep the voltages as close as I can. Running them in parallel seems to work fine.
I have one string with I think 8 panels and the other has 11 but the voltage are within 3V of each other for that string as rated which in my experience, is never the actual voltage you get anyway.

If I switch each string in separately I get an output on each but If I switch both in together, I get more than the sum of the individual strings. NFI how that works but I really don't care. Shows me that they are not only working together but complimenting each other and I'm gaining rather than losing.

I Bought a bunch of 600V/ 10A diodes a while back. Would be interesting to splice them in and see if and what difference they made.

I also have 2 Broken and very mismatched panels on the shed roof I use to power a couple of radiator fans for air circulation.
They are just wired straight together and work well too. Removing either one from the fans causes noticeable drop in the fan speed even though the outputs in voltage and amperage are very different. Would be interesting and easy to put diodes in here as well and see If I can get anymore power from them.

Seems to me the flexibility of panels is very wide even if the efficiency may fall off taking them out of their ideal range.

Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664

Posted: 01:28am 24 May 2018

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George65 said I Bought a bunch of 600V/ 10A diodes a while back. Would be interesting to splice them in and see if and what difference they made.

Which ones?

I've got these...

Might be a bit of an overkill, but better than under rated.

Phil.

George65
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Joined: 18/09/2017
Location: Australia
Posts: 308

Posted: 12:31pm 24 May 2018

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Mine are just the regular Kind although I was thinking pretty much doing that, Putting a connector on each end so they would be easy to put in to test with.
I was also going to pair them up so I should have at least double the amp capacity I need.

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