Solar(s) Panel(s), wich choose? - Page 4 - Fiberglass RV


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Old 03-21-2016, 04:50 PM   #43
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For a single 100 watt panel I would use 14 gauge wire. I would put a 10 amp fuse between the controller and the battery, close to the battery. This protects from the high battery current in the event of a short. For a single panel, I won't bother with a fuse between the panel and the controller but if it makes you fell better, a 10 amp fuse is fine.

Folks use large wire between the controller (source 1) and the battery (source 2) to reduce the voltage drop. But because this is a two source circuit, the voltage drop is always the controller voltage minus the battery voltage (Kirchhoff's voltage law) and is independent of the wire size. The wire size determines the charging current within the capabilities of the panel.

I charge = (V controller - Vbattery) / Rwire ohm's law

Example1 Controller voltage = 14 v. Battery = 12 v. Between them is 100 feet of 14 ga wire (50 ft. Two conductor). R = 0.26 ohms. The charging current, I = (14-12)/0.26 = 7.7 amps. Most likely this will exceed the capability of the panel. The controller output will go down slightly giving a charging current of 5 to 6 amp.

Example2 Same voltages but I'll use 100 feet of 6 gauge wire. Now R = 0.04 ohms. The charging current, I = (14-12)/0.04 = 50 amps. This will certainly exceed the capabilities of the panel and the controller voltage will drop (loading is what its called). Just like the 14 gauge wire, the charging current will be about 5 to 6 amps because that's the limit of the panel.

While it certainly doesn't hurt to use larger wire, there's not much advantage unless you wish to allowing for future expansion. Raz
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Old 03-21-2016, 06:01 PM   #44
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Quote:
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For a single 100 watt panel I would use 14 gauge wire. I would put a 10 amp fuse between the controller and the battery, close to the battery. This protects from the high battery current in the event of a short. For a single panel, I won't bother with a fuse between the panel and the controller but if it makes you fell better, a 10 amp fuse is fine.


While it certainly doesn't hurt to use larger wire, there's not much advantage unless you wish to allowing for future expansion. Raz
Raz thank you, this is the alternative that I see now.

For our purposes it should be enough, we just want to be able to do a few days in the same place even if there are no services.
Time to visit there regularly travels to the 3 or 4 days.

I should get the solar panel kit, March 28, I study the situation pending the hardware.
Thank you for the info,
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Old 03-23-2016, 06:12 AM   #45
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Additional thoughts

Looking at the specifications for this panel, I see an optimum current of 5.29 amps and a short circuit current of 5.75 amps. With these so close I don't think you can choose a fuse that will protect much.

Also, voltage drop is an issue between the panel and the controller, especially if more panels are added. Putting the controller close to the panel all but eliminates the problem. Raz
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Old 03-23-2016, 06:57 AM   #46
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...
Also, voltage drop is an issue between the panel and the controller, especially if more panels are added. Putting the controller close to the panel all but eliminates the problem. Raz
Renogy's online calculator addresses this and is a good resource for planning your solar install.
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Old 03-23-2016, 07:15 AM   #47
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Renogy's online calculator addresses this and is a good resource for planning your solar install.
I think folks would be better served with a good book on DC circuits. Raz
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Old 03-23-2016, 07:53 AM   #48
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I think folks would be better served with a good book on DC circuits. Raz
Or even a Masters Degree in Electrical Engendering.. but keeping this more simplistic serves the masses better, assuming of course it is done safely. Making it more complicated than required is only going to turn away many people who otherwise could benefit from solar energy.

Of course if one has the time and ability to undertake a serious study of electrical engineering (or any other topic), it can only benefit him or her. But even if you have that Master's Degree.. its good to see if the guidelines from Renogy match your assessment (which by the way, did agree with your suggestion fro wire gauge).
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Old 03-23-2016, 08:37 AM   #49
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its good to see if the guidelines from Renogy match your assessment (which by the way, did agree with your suggestion fro wire gauge).
And if they didn't agree, how would you know who was right? nuff said, Raz
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Old 03-23-2016, 10:06 AM   #50
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Ahhh Raz brings up the old "man with two watches" problem. If the watches have different times then which one is the correct time. Comes up fairly often when using the internet as a resource to learn about something that is unfamiliar or new. Like repairing fiberglass or adding solar.

Not always a bad thing to see different ways of looking at the problem, it does often force one to dig deeper. Or in the case of this forum ask questions. More work but broadening of ones knowledge too. Sometimes one just wants a simple answer to "which do I choose", other times one asks a broader but ultimately more informative question of how do I choose, or why do I choose?

In the end you either are an expert, ask someone you know is an expert, hire an expert, or sift through the information and weigh the information, considering the source (be tough to decide between Raz and Renology IMO) makes your choice and takes your chances. Confident if it doesn't work you now know enough to figure out or ask what went wrong.
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Old 03-23-2016, 11:42 AM   #51
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With a short circuit current rating of 5.75 amps # 18 wire would be sufficient with an ampacity of 7 amps. The ohms per 1000 feet = 6.48.
if you had a current draw of that 5.29 amps then the drop in say 20' of wire (19 feet on the + and 10 feet on the -) would be E=IR or 5.29 X (.00648 X 20) = .6855 volt drop.
Since this wire would be able to draw enough short circuit amps to blow a fuse a 7.5 amp. fuse would do.
Or you could use #16 wire and have less drop and protect the wire with a 10 amp fuse.
The panel will just sit there and not do anything but put out current if it were shorted and the wire would be the load and the fuse would never blow and the wire would not get that hot.
#16 wire has a resistance of 4 ohms per 1000' so that same 20 feet would be .004 x 20 or .08 ohms.
I think power is I squared R or 5.75 X 5.75 X .08 = 2.644 watts.
I think that this would b e a safe power dissipation for the panel and the wire.
You might want to run larger wire for the heck of it, but it might be overkill.
By the way the voltage at the solar panel terminals would be about .46 volts with the wire shorted at the far end.
Keep in mind I am old and forgetful and can easily make mistahes
For more safety use the next size larger wire and have less drop as well.
I think that Renology suggests #14 in their calculator.
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Old 03-23-2016, 12:09 PM   #52
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Well said, RogerDat!

The sum of my knowledge gained from this thread is this: Wire BIG ...Fuse small.
I appreciate the input of the posters here. It has inspired me to learn more.

"Less is more" is something of a credo for me but, that doesn't apply to wire sizes or solar panels. Safety, and adequate energy, are important, too.

I have 3 panels on my Scamp (fixed at different angles, dictated by the curve of the top), because it seems to be the easiest and most affordable way to overcome the many variables encountered when using the sun for energy. At any given time of day, I'm sure one panel is doing more work than the other two. I'd like to know if anyone reading this has tried sun tracking (passive or otherwise) on an RV. That would be fun.

I don't want to discourage anyone from considering solar. The science is pretty complicated, but the reality of setting up a small system is pretty simple and very commonly done by amateurs like me. Keep in mind that my 12v fridge is always "on", and I haven't seen a 120v campground hookup in years. Others have different needs, I'm sure. I won't waste space here by listing all my electrical devices, but I think I use about 50 amp hours per day, and that's probably more than the average FGRVer. This season, I'll take notes.

Next topic: "Batteries". Lord help us!

Happy camping (however You define it)

Gordon
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Old 03-23-2016, 12:58 PM   #53
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The cool thing about solar, as with a lot of other stuff, is that if you're willing to spend a bit more money, you don't really need to learn much at all. There are kits that have come set up with wire gauge, fuses, controllers and everything, so all you have to do is connect it to your battery.

Now of course the more you know the better, but if you just want enough power for water pumps and lights and charging the batteries in your devices and running fans on those days you boondock, a 100W panel kit is a very safe bet.

From there, you can learn as much or as little as you want. If you "need" to run AC appliances and can't live without air conditioning, TV, hair dryers and the like, then you'll need to actually calculate your usage and see how much solar you need. But even then, usage isn't that hard to calculate, and there are kits to match it, for a price.

It only really gets complicated when you're trying to design your own kit.
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Old 03-23-2016, 07:50 PM   #54
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The 100 watt panel has become sort of the "default" size but in my 13 ft. Scamp there is really no place to put something that big and awkward. I have considered roof mounting but I will not generally camp in the sun, winter camping is an exception but I seldom find myself doing that as the years go by and never did it with anything but a tent anyway.

Kits are a great choice but only if you can find a setup that matches your requirements. They do make 40 and I think 80 or 100 watt suitcase kits but they are a little expensive. Nice but price per watt is steep. If on the other hand I can learn enough from all the good questions and discussions on the subject I can set up a 30 or 40 watt panel for now but design the wiring and such for a nice big 100 watt mounted on the car roof rack for later, once retired I might get enough camping time to make the extra cost of a 100 watt panel useful.

Wire is expensive, and especially when building a system on a budget a "weak link" can really degrade performance. So one does want plenty but not to waste money on overkill.

I'm wondering about the suggestion made in a post awhile back to buy a heavy duty extension cord and cut the plugs off to use the cord as a line between panel and controller.

I think the suggestion was to wire an outlet plug onto the panel and the opposite plug on the controller so one could connect the two with an extension cord of whatever length was appropriate to get panel in good sunny location.

One cord cut with the ends wired in, another cord to connect the two.
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Old 03-23-2016, 08:17 PM   #55
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Gilles,

I have a 100 watts taped to my roof and it keeps my battery fully charged. It's enough for most RV situations, requires no storage, is very light (about 4 lbs) and does not even appear to be there unless you're flying over.
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Old 03-24-2016, 06:05 AM   #56
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Gilles,

I have a 100 watts taped to my roof and it keeps my battery fully charged. It's enough for most RV situations, requires no storage, is very light (about 4 lbs) and does not even appear to be there unless you're flying over.
Thank you Norm and Ginny, so I ordered a 100 watt panel is to happen today.

I ordered it with the possibility of increasing the number of solar panels, installing a charge controller 30 amps, in case needs increase in the future, which appears unlikely us currently.

Your comment reinforces my choice.

Thank You,
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