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gcrank1
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# Posted: 21 Dec 2021 10:14pm
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Paul, it sounds like the perfect experiment!
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paulz
Member
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# Posted: 22 Dec 2021 09:30am
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Watts are watts I suppose, whether high V x low A or vice versa. How the SCC handles the difference maybe..
On the bright side I just finished running 250' of 12g romex down conduit from cabin to shop. Plugged it into the cabin inverter and ran the LED shop lights just fine. If it will run a table saw or drill press I wouldn't even need the solar down there anymore. 10g wire would have been better but I had the 12g roll.
In summer sun I know this array does put out something, I'll at least hang onto it until then and retest. If nothing else it's the perfect size for a shed roof. 
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travellerw
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# Posted: 22 Dec 2021 11:22am
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Quoting: paulz Watts are watts I suppose, whether high V x low A or vice versa. How the SCC handles the difference maybe
Higher voltage is better for MPPT controllers (usually). It allows the controller to perform more efficiently... but in cloudy or shaded conditions series connected panels perform much worse. There are tons of youtube videos explaining why this is (they explain it better than I can). So you try to strike a balance depending on your conditions (and type of panels).
Personally I think that array would perform better in the long term if you switched it to a more parallel arrangement. I wouldn't go completely parallel if the panels are only 12V as you loose the benefits of MPPT. Maybe 2 panels in series, then all connected parallel at the controller. I think doing that would probably double you monthly harvested power number!
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paulz
Member
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# Posted: 22 Dec 2021 12:12pm
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OK I'll give it a try like that. I can just test with external wiring and not disturb the wiring in the conduit. Probably next week, got rain rest of this week.
Thanks Trav!
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gcrank1
Member
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# Posted: 22 Dec 2021 01:14pm - Edited by: gcrank1
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Watch your scc input max!
Fwiw, the shading effect can be mitigated by having the panels first affected (mine are 2 in series together), then paralleled up with like strings in the array.
As the first shaded string drops 'out' (in producing) it doesnt affect the output of the next or remaining string(s).
This is why I have my six panels in series pairs, each pair at a slightly different orientation and all 3 pairs wired in parallel to the mppt scc.
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travellerw
Member
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# Posted: 22 Dec 2021 01:35pm
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Quoting: gcrank1 Watch your scc input max
Ahh yes.. Good Catch..
Wiring in parallel means the AMPs go up.. Make sure your controller can handle the new value.
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paulz
Member
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# Posted: 22 Dec 2021 04:19pm - Edited by: paulz
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Quoting: travellerw he fact that those panels have a 12 and 6 volt config, seem to verify they are pretty old.
Does the 6v config effectively split the panel into two separate parts, with the ability to wire in parallel to lower V and raise A? Here's a photo where I highlighted the config, hard to see in the other pic.
Also, if they are in fact two separate circuits, how are they connected in 12v config? Mine are wired to only the two outer terminals, nothing connecting them to the inner two externally.
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gcrank1
Member
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# Posted: 22 Dec 2021 04:57pm
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Wonder if there is something there like with wiring two 6v batteries in series to make a 12?
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paulz
Member
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# Posted: 22 Dec 2021 05:33pm
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Found a Siemens guide. Fuzzy but readable, says they are pre-wired internally for the 12v, so it was wired correctly, I think.
https://demo.fdocuments.in/img/749x1009/reader019/reader/2020042819/5c75358309d3f2872 28c4fc3/r-3.jpg
Still like to know if they could be wired as 2 6v in parallel.
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travellerw
Member
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# Posted: 22 Dec 2021 05:51pm
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Quoting: paulz Also, if they are in fact two separate circuits, how are they connected in 12v config? Mine are wired to only the two outer terminals, nothing connecting them to the inner two externally.
Since you are getting voltage, there has to be a connection. The image shows diodes in the config. I'm guessing the diodes are on the other side of the box connecting the terminals in the way they show. Thats unfortunate as I'm guessing that box is glued to the panel. It would make changing a dead diode difficult.
But that brings up another point.. You really should check the diodes on each panel. They all seem to be passing current as if one was open it will take down a whole string when wired in series. However, they can fail dead short. They seem to be bypass diodes and if failed dead short would contribute to poor performance in shade/cloudy situations.
I'm guessing that whomever you bought the panels from may have gotten them at a discount as old stock. Usually when I have seen panels with a 12V and 6V config they have been much older than 2008 (more late 90s). In the old days when electronics were more expensive, panels used to be connected directly to the batteries. It was very inefficient as the sun had to rise high enough to drive the panel voltage above the battery voltage before amps would flow. Panels were also super expensive back then and combined with the small solar window there was little risk to overcharge the battery. Once electronics and panels came down in price, that type of install was largely abandoned (however, I still know people that do it down in the Caribbean).
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travellerw
Member
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# Posted: 22 Dec 2021 05:54pm
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Quoting: paulz Still like to know if they could be wired as 2 6v in parallel.
I can't think of a scenario where you would want to do that today! There would be no benefit and the wire sizes would have to go up as you would be pushing more amps!
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paulz
Member
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# Posted: 22 Dec 2021 06:16pm
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Quoting: travellerw I can't think of a scenario where you would want to do that today! There would be no benefit and the wire sizes would have to go up as you would be pushing more amps!
Wouldn't you have in effect two panels in parallel instead of two in series?
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travellerw
Member
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# Posted: 22 Dec 2021 10:43pm
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Quoting: paulz Wouldn't you have in effect two panels in parallel instead of two in series?
You would.. but once you drop the voltage below about 18V, you lose many advantages. If you look at how panels actually function, they are essentially many little panels already in series. Manufacturers used to make them 12V (ish) so you could hook them directly to a battery, but once controllers got cheap almost all panels jumped to 18+V. With bypass diodes, that was a great voltage for PWM controllers. Once MPPT started getting popular, then panels jumped again as higher voltages make MPPT more efficient (48V panels are pretty common now).
Its about striking a balance with the factors of your system (panels, controller, wiring, ect). You don't want to go way to one side (all series, or all parallel). You want to find that sweet spot. In your current system.. My opinion is its too much in series for those old panels. A more parallel config will be more efficient and harvest more overall power. Often people go with series panels as the higher voltage means smaller wires. People are cheap and wire is expensive.. So they build a system that performs poorly in shade or clouds, but save money on wire! Its fools gold if you are trying to rely on the system for real power! BUT.. You need to keep the voltage at or above about 20V to make your MPPT controller work best!
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paulz
Member
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# Posted: 23 Dec 2021 08:24am - Edited by: paulz
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Quoting: travellerw You would.. but once you drop the voltage below about 18V, you lose many advantages
I was thinking of wiring several in series to raise the voltage, like this chicken scratch shows. Took me awhile to realize the amperage would be the same, in fact it's just doing what the panels do themselves internally. Oh well.
I'll test it 6x2 parallel/serial, opposite of what it is now, next week.
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paulz
Member
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# Posted: 1 Jan 2022 03:15pm
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Too wet and muddy to crawl under this array and rewire, but since you mentioned eliminating the SCC on older panels I got curious about these:
I have these two panels kicking around. I have tried them in series with a Renogy Rover MPPT SCC to charge car batteries but always seemed to do more harm that good. I'd come back a few days later and the SCC had a low voltage shutoff error and batteries were low. I should try them in parallel but..
I just hooked up one direct to a battery. It measures 19v open. The battery was at 12.6, on the panel it's gone up to 14.x, amps is .05. It's in the shade, as usual, even though in the photo I took there's a touch of momentary sunlight. If I hooked up the other panel in parallel I expect I'd get .1A. I know the voltage is too high, but what's going to happen with that little amperage? If the diodes are working, it should lose voltage overnight.
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paulz
Member
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# Posted: 7 Jan 2022 12:22pm
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Ha, so I was moving firewood this morning and happened by this plot, been gone from the cabin for several days and forgot all about it. Still hooked up direct to one panel just as I left it, no SCC. Been overcast and rainy, I doubt it's had 5 minutes of sun on it since Jan 1. Hmm, wonder what's happened? 12.73v, even after disconnecting! Next step is to try both panels in parallel on a weaker battery.
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