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Old 07-27-2017, 09:38 PM   #1
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Reference book on solar power?

Could someone recommend a good book on solar power systems? My solar panels don't act the way that I think they should, and I need to learn more so I can understand what's going on and diagnose problems.

Example: I have 355 watts of solar panels mounted flat on my trailer's roof. Yesterday at noon I was parked downtown in Port Angeles, Washington, which is at lattitude 48 degrees N. My battery monitor read 10 amps charging. Well, that beats nothing, but it's half of what I thought I'd get. I figured:

Amps = 355W / 12V * sin(48) = 21.46

It's not long after the summer solstice, so that should give me a reasonable estimate, but it doesn't reflect what I'm getting. Something's not right, either with what's going on or my understanding of it. Does some kind of reactance come into play when charging batteries? I need to learn this stuff better, and a book would help. Thanks.
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Old 07-27-2017, 10:22 PM   #2
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Quote:
Originally Posted by Mike Lewis View Post
I figured:

Amps = 355W / 12V * sin(48) = 21.46

It's not long after the summer solstice, so that should give me a reasonable estimate, but it doesn't reflect what I'm getting.
It is a reasonable estimate if your panels were hooked up to a resistor that was the correct value to have a 12v voltage drop (meaning you could read 12v at one terminal with respect to ground.) But in the real world your solar is hooked up to a controller which in turn is hooked up to batteries. Now there are lots more variables than just the potential output and sun angle. At this point the math becomes so complicated it hurts.

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Old 07-27-2017, 11:50 PM   #3
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It may not answer your question, but I highly Recommend "The Battery Charging Puzzle" a blog written by "Handymen Bob". Google either him or his blog and I guarantee you will learn something. This guy lives in Montana completely off the grid and has for years. It is really a good "solar" read.
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Old 07-28-2017, 12:19 AM   #4
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Mike, start with the manual for your solar controller to understand the different charging cycles that it uses each day. I have the 2104 version GoPower controller. I forget the exact terminology, but there is a daily 1 hour bulk charge (up to 14.4V), followed by a trickle charge at 13.6V. If the controller is charging your batteries at the proper voltage, you will not see the maximum amperage and wattage being applied as your batteries approach a full charge. The bulk charge will happen and likely finish earlier in the day while the sun is lower before maximum solar energy is available. By noon time, your batteries are probably at close to full charge with that much solar charging capacity. At that point, the controller is maintaining a float charge on your batteries.

If you have an inverter, fire up your microwave or other heavy AC load, to make the battery voltage drop below what the controller is trying to maintain, then your solar controller will put out a higher amperage, allowing you to check the maximum output of your system for the angle of the sun and time of day.

I have the Escape factory system (85 or 90??) watts. The most I have seen on my system is just over 5A (13V x 5A = 65W). If it is a sunny day - the only time to expect max output, usually I am drawing less than 1A at 13.6V by noon, unless I made solar drip coffee that morning.
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Old 07-28-2017, 01:18 AM   #5
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Mike, start with the manual for your solar controller to understand the different charging cycles that it uses each day. ... By noon time, your batteries are probably at close to full charge with that much solar charging capacity. At that point, the controller is maintaining a float charge on your batteries.
Yes-- I should always read the fine manual. Good suggestion.

Thanks to Olympic National Park's inpenetrable rainforest, my batteries were at 57% capacity when I measured the 10 amp charge at noon yesterday. They had been much lower, which is why I left the park to find a campground with hookups. Thirty-two hours after plugging in, the batteries are at 99%. So the crisis is averted for the time being, but I need to better understand what I can expect from my panels.
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Old 07-28-2017, 02:25 AM   #6
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Amps = 355W / 12V * sin(48) = 21.46
Even having accounted for covering less effective area than the panel's full area (your sin(48°) factor), the calculation is missing a factor for the less-than-ideal amount of sunlight - the sky is rarely perfectly clear, etc. Panels also deteriorate with time, so there would ideally be a minor factor for that.

We keep time in zones, which are an hour wide and often far off of the nominal longitude, and we arbitrarily change it by another hour for most of the year (if using "daylight saving time") so when the clock says noon, the sun can be way off of the highest point in the sky... more two hours in some places. Port Angeles is at 123°26′27″W longitude, so the solar time is 8 hours and 14 minutes behind GMT, but Pacific Daylight Time is only 7 hours behind GMT, so at solar noon the clock (set for PDT) says 1:14 pm. To confirm this, just find the midpoint between sunrise and sunset, which (according to a random time-and-date website) are 5:43 am and 8:55 pm today in Port Angeles.

The other tweak would be to divide by the actual voltage, as already mentioned. If that 10 amps is into the battery, then substitute battery voltage (which could be over 14V depending on where you are in the charge cycle) to get the what the current would be if the panel were providing the rated power.

Another problem is that even with perfect sunlight and a known voltage at the load, the panel is unlikely to be operating at its ideal point - the maximum power point. That 355 watts is at one specific combination of voltage and current; that's VMP and IMP in the panel specs. If forced (by battery voltage) to run at any other voltage, the power will be less and the so the current will be less than what is required to make 355 watts at the operating voltage.

All this assumes that the charger is in the mode in which it is not restricting the charging voltage or current. Assuming it is the stock PWM controller (and not an MPPT type), this is the early state in which it is basically just a switched turned on, not changing the voltage or current going through it.

If all of these points are clearly covered in any reference material, then it's useful... they're all quite fundamental to solar power.

Quote:
Originally Posted by Mike Lewis View Post
Does some kind of reactance come into play when charging batteries?
Batteries have internal resistance, which combined with the voltage due the state of charge, determines the voltage required to charge the battery. That's how the not-12-volt charging voltage, and the resulting panel operating voltage, are determined.

You don't quite have an impedance mismatch as in RF applications, but there still is an issue of matching the source to the load. Maximum Power Point Tracking (MPPT) controllers exist to address this... but they add the issue of power lost in the conversion that they do.
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Old 07-28-2017, 03:18 AM   #7
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I picked up The Solar Electricity Handbook | Solar Photovoltaic Book
It's pretty good. Best part is they have free online calculators. If you can figure them out, the one on Solar Irradiance Solar Irradiance - calculate the solar energy available on your site shows what you could expect for a given location.
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Old 08-05-2017, 04:36 PM   #8
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While we are on the topic of solar panels, we just had ETI retrofit one of the 160W panels to our 17B. In our build sheet, having talked with ETI, we decided to have it prewired for solar, so the retrofit was an easy option. However I digress slightly.

In looking through the manual for the Go Power charge controller, I see that it covers off two batteries and that by pressing the appropriate series of buttons (mostly button B) you can monitor the charge status (and other information) in each battery, either battery 1 or 2. However, when I scroll through the status indictor, I can only see battery 1, even though we have two 6V batteries. I know that because I can see the two batteries on the back of the trailer.

I am assuming that as the two batteries are probably connected in series to provide the 12Volts that the trailer runs on, even though there are two batteries, they are essentially operating as one battery. Therefore, in cycling through the various status indicators, I am only ever going to see one battery.

Can someone confirm my thinking on that, or did I miss something and this is a dumb question?

Thanks much, in hope - especially that I am not dumb.
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Old 08-05-2017, 05:11 PM   #9
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^ I believe they are referring to banks of batteries, like the house / starting batteries one will find in a motor home.
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Old 08-05-2017, 05:14 PM   #10
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Reference book on solar power?

Bruce beat me to it....

You are not dumb, the two 6V batteries wired in series act as a single battery. The second battery would be for a completely separate battery like you'd have in a motor home to start the engine.
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