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Old 09-01-2017, 08:42 AM   #1
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Startup Voltage for Solar Charger and 12v solar panels

Hi,
I am in the planning stages of a solar install on our upcoming airstream purchase where-as I planned on using 100w solar panels and a BlueSolar 100/30 charge controller. The startup voltage for the controller is vbat + 5v. I am concerned that the 12v solar panels will not even start the controller unless they are in direct sunlight. The 100w panels I have looked at have optimum voltage of 17.6v and 18v. Does anyone with a similar installation have problems with their solar being able to start charging under less than optimum conditions?
Thanks!
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Old 09-01-2017, 09:29 AM   #2
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Hi

Your solar panels will only charge the battery when the output of the panel is greater than the battery voltage. It's a buck converter not a buck / boost. For a normal lead acid setup, charge voltage is going to be in the 11 to 14V range. Your battery is toast at 10V. If you are down around 5V, it's not just dead, it's long dead.

You are correct that solar panels do require direct sunlight to work well. Running them in the shade will give you very little (if any) energy. In addition, flat mounting them is not ideal. A more exotic "tracking" setup will give you better performance. In about 99% of all RV installations, people just go flat and put up with it.

Best case, you will get about 80% of the "300W" out of your panels at high noon in terms of amp hours into your batteries. You do most of the charging at 13.x V and most discharging at 12.x V. It's just the way lead acid batteries work. (... this is only one of many limitations).

Bob
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Old 09-01-2017, 09:42 AM   #3
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I know I can put the panels in series to get the higher voltage, but then you have a possible problem with shading of one panel causing the "in series" panel to be adversely affected. I may have to buy a single 12v panel to see what the voltage characteristics under different conditions to see what I think would work best.
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Old 09-01-2017, 09:44 AM   #4
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I guess a person could use a portable panel to find the sun in the morning to get the charger started, at which time it then takes less voltage to keep it charging.
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Old 09-01-2017, 10:12 AM   #5
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Quote:
Originally Posted by uncle_bob View Post
Best case, you will get about 80% of the "300W" out of your panels at high noon in terms of amp hours into your batteries.
I recently installed 2x100 flat on my roof. My best case measured in the last month or so that I've had the panels is 175w as recorded by my Victron solar controller (buy the bluetooth dongle to get this data). Most days it's around 145w-165w if I'm in the sun. This drops quickly when the sun is shaded.

My best day thus far has been 580 Wh for the day with an average battery voltage of 13.23 v. By my calculations that's about 44 amps in that day. Many a day in partial shade I struggle to reach 150 Wh, or about 11 amps in.

Just to give the OP an idea of some real world numbers.
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Old 09-01-2017, 10:27 AM   #6
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Thanks for your replies. The solar manufacturers tout how well their panels work in low light conditions. To me, depending on the details of the conditions, your numbers are not very encouraging. It looks like real life testing will be my best bet before goiing whole hog on the project.
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Old 09-01-2017, 11:41 AM   #7
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Originally Posted by ggreen63304 View Post
Thanks for your replies. The solar manufacturers tout how well their panels work in low light conditions. To me, depending on the details of the conditions, your numbers are not very encouraging. It looks like real life testing will be my best bet before goiing whole hog on the project.
Shade kills. At least with my renogy eclipse panels in parallel.

But I'm talking about real in the woods type shade.
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Old 09-01-2017, 11:45 AM   #8
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If your selected charge controller supports it you could run the panels in series so that the voltage would be that of the panels combined...so if the minimum output of the panel is 12V, then you would get 24V and the charge controller would step that down to lower voltage.
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Old 09-01-2017, 12:24 PM   #9
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SCOTTinNJ

Thanks for the clarification on shade. There are definitely different levels of shade. In the woods shade is really a low light condition.
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Old 09-01-2017, 12:41 PM   #10
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Well, let me be clear. In the woods you would get nothing. Zip.

I'm talking about a relatively shady spot in a state park type location. So trees around but somewhat clear directly above.

Even out in the open you will get minimal solar when the sun dips behind a cloud or it's overcast.
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Old 09-01-2017, 06:01 PM   #11
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I have the same setup that you are talking about. 2-100W panels with the BlueSolar 100/30 charge controller. I was also concerned about the startup voltage but have never had a issue with it starting and charging the batteries. I also use the Victron's bluetooth device to monitor what's going when charging. Can't promise your's will work as well but for me it's not been a problem.
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Old 09-01-2017, 06:58 PM   #12
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Off topic about solar panels.

Quote:
Originally Posted by ggreen63304 View Post
Thanks for the clarification on shade. There are definitely different levels of shade. In the woods shade is really a low light condition.
I know first hand the low light of the woods.
We were laying out a motorcycle endurance run and part of it was to go diagonally through a section that was wooded. )Section is a square mile.) We started in in broad daylight. We weren't through the woods by about 5:30pm. We could look up and see the bright sky but we literally could not see our hand in front of our face. We had run across a rotting log earlier that was glowing. I had a piece of it. When it got very dark I held that piece up so the other two could follow me. I ran into a small branch across out path. I put the glowing wood down in front of me. The guy who was about three feet behind me exclaimed, "Lyle, where'd you go?"

There's just no solar deep in old growth woods.
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Old 09-01-2017, 06:58 PM   #13
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Hi

Like it or not, solar panels work based on the amount of light that hits them. A cloudy day can be a 50% drop in light and still look very bright. Your eyes do *not* respond to light the same way the panels do. Shade that you can notice is *way* less light than full sunlight. Sorry, but that's the way it works.

Series connections are fine, but you aren't getting any amps out of the panel when it's at 1/3 voltage. Load it a little and you go to nothing. The amount of power (which is what matters) available is not much at all....

Bob
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