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Old 11-04-2014, 10:36 PM   #1
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Experiments with solar panels

I ran a little solar experiment this morning to answer a couple of questions that have been on my mind. The motivation behind this is to determine how much electricity I can realistically expect to produce under less than ideal circumstances. Those include having a non-movable horizontal panel, low winter sun and the possibility of shade on the fixed panel.

What I wanted to learn was:
1. What are the measurable benefits of tilting a panel.
2. What are the drawbacks of using different size (voltage) panels at the same time.

A quick search of the internet shows: #1 - good, and #2 - bad.

I didn't agree with #2. To elaborate, a couple of solar panel dealer web sites stated explicitly that a lower voltage panel in parallel with a higher voltage panel would "pull down the voltage" of the high voltage panel. My own thought process concluded that if the panels in question had the normal blocking diode then it was impossible for any panel to "pull down" another.

So, out comes the soldering iron, a handful of eBay solar cells, a two ohm power resistor as a load, and a couple of multi-meters to measure voltage and amperage. I assembled two panels from 18 identical cells, one 6 cell (3v) "auxiliary" panel and one 12 cell (6v) "main" panel. The 2:1 ratio was intentional to match the real world where big panels are typically 18v or 36v. ( ETI is using a 18v panel.) The parts in the photograph, starting at the bottom going clockwise: power resistor, meters, 3v and 6v panel.

Note that the main panel is horizontal and the small auxiliary panel is tipped. This is to model a real world example where the ETI panel is permanent on the roof, and a smaller panel is propped up near the trailer.

One big assumption is necessary. My two ohm resistor was a substitute for a discharged trailer battery. Unfortunately not exactly true because a resistor behaves according to Ohm's Law - E=IR, and a mostly discharged battery has a starting voltage of at least 10v and the voltage changes according to the lead-acid chemical reaction. But, that said, I didn't have a discharged battery available today.

Observations #1: The two ohm resistor is enough to pull down the horizontal panel voltage from a nominal 6v to 1.66v here in Denver on a sunny day at noon while pulling .84amps. Adding in the output from the tipped 3v panel raised the system voltage to 2.54v and 1.27amps. So much for the small panel pulling down the big one! Pure nonsense. But only pure nonsense when the load is bigger than the output of the bigger panel.

Observation #2: Under no load conditions, the small panel pulled down the voltage of the big panel by 0.002v. Essentially nothing, and I attribute the microscopic drop to leakage of the blocking diode. (FYI, a 5amp 30v Schottky unit. Schottkys leak more than traditional diodes but have a smaller voltage drop. Drifting off topic now...)

Observation #3: Any tip up from horizontal is a big, big help in fall/winter/early spring up here in Colorado, and more so the further north you go.

Observation #4: Any shade is a no-no. Putting just one cell of my setup into the shade results in a loss of 95%. A shaded cell acts like a blocking diode - not good.

My conclusions (and it sure took me long enough to get here). Mixed size panels in parallel are not an issue. Smaller (or bigger) than the roof mounted ETI panel is not a factor - if the load is "big". A pair of 6v ETI batteries drawn down to the 50% level do in fact represent a huge load on a solar panel. They will pull down the panel voltage to a nominal 11-13 volts until the charge gets up somewhere above 80%.

Second conclusion: The extra panel will benefit greatly from being tipped towards the sun. But it does not have to be exact - anything in the general direction will be a great help. Set it up once, pointed more or less south at 45 degrees tip, and you are in business. If you have any shade on the horizontal ETI panel then the auxiliary panel is doing all the work.

I hope this answers more questions than it raises.

--
Alan
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Old 11-04-2014, 10:55 PM   #2
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This was really interesting. We hope to be camping on Big Pine Key some this winter, and our campsite this year will be more shaded than in the past. I'm thinking about taking along a portable to add to the roof-mounted ETI panel, and I think you just convinced me that it's worth the effort.
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Old 11-04-2014, 11:17 PM   #3
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Alan, great work! Thanks for sharing the details! I may redesign my permanent roof mountings based on your study.

Steve
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Old 11-04-2014, 11:51 PM   #4
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Good job on quantifying the differences. I've seen lot's of emperical evidence that supports your findings but it's nice to see your results support my observations.

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Old 11-04-2014, 11:55 PM   #5
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I'll try to quantify 'the benefit of white paper in front of a vertical panel' tomorrow. Maybe worth the effort to put a reflector (snow??) on the ground, vs. grass or dirt.

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Old 11-05-2014, 12:31 AM   #6
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Often I wonder whether or not ground-based solar panel owners have dogs or not. That alone led me to install roof mounted units. Two tethered dogs could easily disturb optimum settings, or just plain trash the whole affair.
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Old 11-05-2014, 12:52 AM   #7
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Quote:
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Often I wonder whether or not ground-based solar panel owners have dogs or not. That alone led me to install roof mounted units. Two tethered dogs could easily disturb optimum settings, or just plain trash the whole affair.
We have dogs and a portable panel, but we use an x-pen as a temporary holding cell for the dogs if they need to be contained, so we don't have an issue with tethers. They're off leash a lot when we're camped places that's possible, which is our ideal.
Our 80w briefcase solar panel has been amazing over the last year and a half and never left us without a full charge even on dreary, cloudy northwest days.
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Old 11-05-2014, 08:36 AM   #8
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So, Alan, since it appears that you're measuring the performance of the panels directly, without a charge controller in between the panel and the load, what does this say about connecting two dissimilar panels in parallel before a single controller? I think it says that it's no problem. I've been advised by the Go Power! people that I can go either way.... parallel two dissimilar panels ahead of a single controller, or run each panel through its own controller, then parallel the two outputs to the battery. I questioned the latter idea, and he said it's no problem, probably because of blocking diodes in each controller. Sound reasonable?
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Old 11-05-2014, 09:43 AM   #9
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I am not personally interested in dealing with a portable panel , but a friend of mine engineered a adjustable tilting bracket for his roof panel that works really well. I will
See how we do with the factory 160 w setup next year and go from there
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Old 11-05-2014, 10:43 AM   #10
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Quote:
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... what does this say about connecting two dissimilar panels in parallel before a single controller? I think it says that it's no problem. I've been advised by the Go Power! people that I can go either way.... parallel two dissimilar panels ahead of a single controller, or run each panel through its own controller, then parallel the two outputs to the battery. I questioned the latter idea, and he said it's no problem, probably because of blocking diodes in each controller. Sound reasonable?
Good question. To paraphrase - two panels, one controller or two?

Let me start by saying that my primary interest is to draw conclusions from actual measurements, and I don't have one controller, much less two, to play with. But I'm always willing to guess...

The "which-is-better" question depends on many variables. For example, are the controllers absolutely identical output - voltage, amperage, charging algorithms, etc? Plus, how do they respond to different input voltages from the panels? Just too many variables.

I'm not sure how blocking diodes come into the picture when talking about
controllers. To my way of thinking, a controller should not allow reverse current to flow - without depending on a diode. But I could be completely wrong... Maybe a diode is necessary to prevent one controller from fooling the other?

Personally, I would go with a single controller with the two (or more) panels combined in parallel before the controller. If nothing else, it would be cheaper.

I can say with some certainty that a controller in the bulk charge mode (maximum charge) can be modeled reasonably well by a low value power resistor. Meaning simply, a heavy load on the panels, pulling down one panel in favor of the other due to sun angle, panel size, shade, etc. If the controller is not in maximum charge mode, say float or equalization, then the load is light and it doesn't matter that one panel is not playing its part. Or to say it differently; adding more solar won't charge an almost full battery any faster.

Hope that helps.
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