The open-circuit voltage from a solar panel can be quite high compared to the nominal 12V rating, but that doesn't mean a battery connected to it will ever see that high voltage. I've seen at least 19V, but as soon as the panel is connected to the battery, current flows, and the load pulls the panel voltage down to the battery voltage.

My "30 watt" panel will not produce more than a couple of amps under any condition under the sun (literally...), so there is no risk of excessive charge rate. There is a risk of over-charging eventually, so I use a simple on-off solar panel charge controller. With a small enough panel and large enough battery, indefinite charging would still be safe, which is why the panel suppliers make recommendations such as using a controller with panels of combined capacity of 15 watts or higher (they assume a typical RV/marine battery size).

The simple controllers like my under-$30 unit are the same circuit used in basic (not "smart") automatic battery chargers: it is a switch which turns on at some low battery voltage (eleven point something) and off at some higher voltage typical of a fully charged battery (maybe 14.2V). This type of controller does nothing to adjust the charging voltage or current flow, so it cannot optimize panel performance, limit charging current with large panels, or get useful charge voltage in low-light conditions.

Sorry Brian. I mis-read Joy's message. I missed the part where a controller was in the circuit...

My excuse is that it was a pre-coffee posting.

I like it!

I have actually used it twice. At Applewhite, in the San Bernadino Forest, It maintained the battery where it did not fall below 12.4 volts at any time. Definitely kept up with my 12 volt usage, even the furnace blower motor. I did not skimp on light or pump usage. The battery left the campground with as full a charge as it had when I arrived.

At Lake San Antonio, while we were on plug-in power, I separated the panels and their charge controller from the trailer converter's wiring. I charged a group 27 battery (my separate auxillary battery) that had been run down below 11 volts. It was useable that afternoon.

Herb,

I would suggest staying away from a fusible link and install an inline fuse instead. I have had the misfortune of trying to replace a fusible link on the road, could not find one and had to splice a wire in its place. A fuse will be much easier to find.

Liz,

A couple weeks ago when I camped on the two day weekend at Heartbar, then left the trailer for the 3 day..

I left the panels out all week while I was gone. I arrived back to a fully charged battery, and despite my stewpidity of leaving the heater on all nite Friday and running the battery down, I was back up with full power by mid day saturday.

Wanted for nothing power wise for 3 days, as usual.

Carrying an extra $4.00 fusible link is fairly cheap insurance. I've had fuse holders that got corroded and had diminished current carrying capacity.. Hopefully it will stay 'dry' inside my trailer but I'm dubious. Anyway, it's an experiment and we'll see how it goes.

My 45 watt solar array is up and running baby!

Complete with hinges; you're an inspiration Frederick!

The instructions mention something about not having the controller wired too far away from the battery...is there a max. length of wire one should stay under from the controller to the panels, or is it simply a matter of upping the gauge?

Bought the Harbor freight panels 3 weeks ago and went on a trip to fishing rivers of Idaho (will post later) They seem to work well for my needs. I did put a one way plug on each solar panel and have found out that on a very sunny day I run just one panel to keep our battery charged. I did this so I could test each panel and found out it was fun to see the charge rate of 1 2 or 3 panels.
I would recommend these panels and regulator for people who love to boondock like we do to find off beat fishing and camping.
We are back for a few days and will be taking off again.
Dave and Sharon Tharp

Greg, All you have to do is keep in mind that the 'smaller' the wire the more resistance there is and thus the more voltage you will loose while passing current through it. SO, use the largest diameter and the shortes length you can live with betweem the controller and the battery.
I use the same solar panels for my Ham Radio Station here in Florida and it works wonderfully. (Let's hope i don't have to use it for real this hurricane season".

Well, yes and no. The voltage lost is related to both the number of amps you run through the wire as well as the wire size and length. At 45 watts under full sun your solar panels generating something like three amps (45 watts divided by 15 volts = 3 amps). If you connected your solar panel to your charge controller with a 50' cable, here's how much voltage you'd loose:

20 Gauge Wire 3.1 Volts (15v to 11.9v)
18 Gauge Wire 1.9 Volts (15v to 13.1v)
16 Gauge Wire 1.3 Volts (15v to 13.7v)
14 Gauge Wire 0.8 Volts (15v to 14.2v)
12 Gauge Wire 0.5 Volts (15v to 14.5v)

And here are the figures for a 25' leash:

20 Gauge Wire 1.6 Volts (15v to 13.4v)
18 Gauge Wire 1.0 Volts (15v to 14.0v)
16 Gauge Wire 0.6 Volts (15v to 14.4v)
14 Gauge Wire 0.4 Volts (15v to 14.6v)

And for 12.5 feet:

20 Gauge Wire 0.8 Volts (15v to 14.2v)
18 Gauge Wire 0.5 Volts (15v to 14.5v)
16 Gauge Wire 0.3 Volts (15v to 14.7v)

Why does this matter? Since most charge controllers limit the charging voltage sent to the battery to 14.5 volts or less, any volts over 14.5v are lost anyway, so buying heavier, more expensive wire won't improve your solar system performance.

--Peter

I use a 10g 20ft pigtail.

The solar panels should be spitting money out at me...

Is there any difference when one is not under full sun? Not being too knowlegeable in this area, I would think that in less than perfect conditions, one would want to minimize voltage drop to keep the voltage heading into the controller up to 14.5 to facilitate charging.

Roy

You're confusing the number of volts the panel puts out with the amount of power the panel produces. That's not surprising: the relationship between volts, amps, and watts are kind of complex and non-intuitive.

The voltage from a solar panel and the amount of power it puts out are two different things. Once a solar panel starts getting enough light to produce power it'll generally register close to or the same voltage whether it's in the shade or under full sun. What changes as the sun gets brighter is the number of amps the panel produces at that voltage; the actual power output of the panel, measured in watts, is equal to volts the panel produces times the number of amps produced. So Volts * Amps = Watts.

Using the panel on my trailer's roof as an example, my panel pumps out 17 volts whether it's in full sun or in the shade. Under full sun it pumps out 3 amps for a total of 51 watts of power (17 volts * 3 amps = 51 watts); when it's sitting under the shade tree in my front yard it'll still put out 17 volts, but might well only manage 1 amp for a total output of 17 watts of power.

The sad news is, because pumping 3 amps at 17 volts into my 12 volt battery isn't good for it, my charge controller "down regulates" the voltage it sends the battery to a maximum of 14.5 volts. Doing the math, that means my battery only sees 44.5 of the 51 watts of power my panel pumps out under full sun.

--Peter

Peter, you're assuming that the panel can sustain enough output voltage to get 14.5V to the controller. If not, then the loss in the cabling does matter; in 8 of the 12 example calculations, the loss would matter.


This sounds like a PWM (pulse-width modulated) controller. It is the most common type of advanced controller, but not the only one. The panel is not likely producing 51 W, since it is not putting out 3A at 17V... it's pulsing at that rate, with zero current in the off times between the pulses. It's only putting out 44.5 W, plus some lost in the controller. If the 17V and 3A are average readings, then it's really pulsing at more than 3A, and the difference between 44.5W and 51W is all controller loss.

My simple controller is just on-off: it's a switch which allows the panel output to connect directly to the battery, or just turns it off. The switching off occurs at something like the 14.5 V level, and it comes back on when the battery drops to some lower level. Again in this case, if the panel can't maintain more than the switch-off voltage, less cable resistance will mean more charging current. Since it doesn't frantically switch on and off, I know the 30W panel is not maintaining anything like 17V of output while connected to the battery... it's not even maintaining 14.5V after a couple feet of wire.

The best controllers are MPPT - Maximum Power Point Tracking. They work as voltage converters, shifting the voltage up or down (and thus the current down or up) to maximize the panel output. An MPPT controller in Peter's 17V/3A situation would convert the panel output voltage down to meet the battery needs, and thus pump more than 3A into the battery, getting the full 51W from the panel... if the panel can really produce that much. I've only read about these... it would cost more than my little 30W panel is worth.

Just got my Harbor Freight mailer today ("Postmaster: please deliver between 9-4-07 & 9-6-07")
Hurry! Sale ends September 24, 2007
Click to view attachment
45 watt Solar Panel Kit, LOT # 90599
$199.99 regular price: $249.99
plus, there is a coupon to take an additional 15% off on the front page

Thanks for the explanation Peter, I know have a better understanding how the solar panels work.
Although I consider Volts * Amps = Watts to be a simple mathmatical relationship.



Thanks Frederick!

How can we get a coupon if we are not on the mailing list?
Are copies available in store or on line?
Seems worth the trip to cross the border ... might even get to check out the Niagara meet at the same time!

Unless of course someone here is crossing the border into Canada ... (hint hint)

I have seen copies of the ad available in the store, and at one time there were coupons available online, but not at the Harbor Freight site.

Harbor Freight 45 Watt Solar Panel Kit
http://www.harborfreight.com/cpi/ctaf/disp...temnumber=90599

My solar panel may differ from other panels, but my understanding is that the amps any solar panel produces will drop off more rapidly than the voltage does when the sun goes down. Curious about that, I tested my panel when it arrived. (This was to both confirm my "new" used panel was in working condition and operating as advertised, as well as to satisfy my curiosity about the functioning of a solar panel.) I measure my panel's output by hooking both leads from the panel to the 10 Amp leads on my multitester and reading the value -- in other words, I shorted the panel out to measure the amps it produces. In full summer sun my panel pumps out 2.99 amps at 17.5 volts (so my panel's output is actually 52.3 watts, right in the middle of the 50-55 watts range Seimens advertises.); under my shade tree it pumps just under an amp, and still registers just over 17 volts. When I flip the panel over at a 30 degree angle over my cement driveway it registers just over 7 volts and virtually no amps at all. This performance mirrors what I have read about solar panel current production (amps) falling off much more rapidly than its voltage. (If you test your panel by this method make sure you connect your multimeter leads to the 10 amp plugs and set the multimeter to the 10 amp test setting BEFORE you touch the probes to the panel leads, otherwise you'll blow your multimeter's internal fuse and it won't measure amps anymore until you replace the fuse. Trust me on this.)



You've pegged it. Mine is a Sunsei 10 amp (PWM) controller that lets on-off-on-off pulses of electricity through so the voltage value on the wires going to the battery average out to a maximum of 14.5 volts when charging at its highest rate.



The simple on/off charge controllers I looked at when I was building my system turned on at 13.0 volts and off at 14.2 volts, but remember that that voltage level doesn't represent the voltage of the panel, it represents the voltage of the panel + battery system, which will be lower. As for voltage loss due to wiring, that's easy to calculate if you know the resistance of the wire using Ohms Law, volts = amps * resistance. I used the resistance values and calculator on this web page to come up with my figures.

--Peter

Peter, maybe I'm just misreading your description, but it sounds like you're combining the open-circuit voltage (nothing but the voltmeter connected to the panel output; no current flow) with the short-circuit current (ammeter across the panel output with no other resistance). If you hook up both ammeter and voltmeter at the same time (call your friends and get your spare meter back...), I think you'll see that while delivering 2.99 amps, it is delivering much less than 17.5 volts. The two separate readings can't be multiplied to get power, because they're not happening at the same time, or under the same conditions.

For the mechanically inclined, you can think of open-circuit voltage like peak engine torque, and short-circuit current like redline speed: you can't multiply the two together to get engine power available. The peak torque (voltage) is only available at a lower speed (current), and at the maximum speed (current) much less torque (voltage) is available.

I agree that open-circuit panel voltage does not vary strongly with sun intensity, but the ability to deliver current does.

Thanks for the reminder of these cutoff levels, Peter; I think those are roughly the same limits as my controller, and simple automatic battery chargers.


But it is the panel voltage under the operating condition of being attached to the battery. The panel's voltage depends on the current passing through the panel, due to internal resistance, just as a battery's output voltage drops while it is delivering high current, and comes up to a higher level instantly when the current stops. So my "30W" panel puts out close to 20V in full sun when open-circuited, but when I connect it to a battery to be charged it delivers a couple of amps at a lower voltage determined by the battery's state of charge and internal resistance; the intervening controller sees that voltage is less than 14.2V, and stays on. Eventually, the battery's internal voltage comes up with charging, the resulting voltage at the terminals rises as the panel puts out more voltage and less current, and the controller shuts off at 14.2V.


Anyway, in a real panel-and-load combination, I would expect the operating voltage to be easily less than the controller's cutoff or controlling level, and so it seems that wire resistance both limits charge rate and dissipates energy.

A meter on the high-current setting should have extremely low resistance. An ideal ammeter has no resistance at all, but realistically it needs enough to produce an accurately measurable voltage internally... I just checked two of my (cheap digital) meters against each other; I measured the resistance with one (set to various ohms ranges) across the terminals of the other (set to various amps ranges). Meter resistance (of the meter set to a current range) in each case varied from about 2 ohms (on the highest low-range current setting of 200mA) to about 1000 ohms (on the lowest current setting of 200 microamps), and was too low to read (less than or about equal to 0.1 ohms) on the 10A setting (and using the 10A terminals).

To sustain 17.5 V across the ammeter at a current of 3 A, the meter would need to have about 6 ohms of resistance in 10A mode - I certainly hope it doesn't have that much, which would cause major interference with the operation of the circuit being measured.

I'm not questioning Peter's observed current - that's interesting information. It's just that the characteristics of the meter ensure that this is nearly a short-circuit current, and does not occur in combination with the open-circuit voltage.

Ok, Brian - Peter, great info but you guys lost me about 4 threads up.

In laymans terms if I can find the 15% off coupon and go get the Harbor Freight Panels for $169(pretty screaming deal) will they work well with my Group 27 deep cycle wally special battery right out of the box or will I need something else in addition to what is included?

The Harbor Freight system will charge your batteries just fine, though you'll need to make some adjustments to what lights and appliances you run when you boondock. (Things like using LED lights instead of the 12v incandescent bulbs most RVer use.) Gina's article on going solar has lots of good advice & ideas. The only think I'm not sure about is the charge controller that comes with the panels; the reviews I've read suggest you should buy a different charge controller.

Brian and I are being nerds; he's giving me something to think about and some more experiments with electricity to perform. With a little luck my valued mentor will teach me something.

--P

Greg,

It should work OK right out of the box, start back at the beginning of this thread to see what others have done.

I don't know how your system is wired, but you might want to check the diagram in this thread and see where a fuse is placed on the +ve line between your battery and the rest of your wiring in the trailer.

Considering this thread started off discussing the Harbour Freight Panels, the manual is online which gives the specs below.

Click to view attachment

Excellent find, Roy!
Such a radical idea... reading the manual

I downloaded the whole manual (over 8 minutes of dial-up ). Although Roy has already extracted the only real technical content for the panels, this setup does have an interesting controller/meter/adapter box, and I think this is a good opportunity for anyone considering buying the setup to have a look to see if it's what they want.

The Harbor Freight specs include:

• Peak Voltage - 23.57 OCV (Open Circuit Volts)
• Panel - 15 watts per panel, 14.5 volts output (working)
• Nominal Voltage - 14.5 volts battery output

In other words, placed in full sun with no load (open circuit, zero current) you should measure 23.57 volts from the panel, but the 15 watts is produced at 14.5 volts, and just over one amp, which is the condition normally encountered near the end of charging a battery.

My guess is that Peter is seeing a lower open-circuit voltage on his panel because he's not sitting in an equatorial desert at high noon on a perfectly clear day...

Thanks Brian,

I got one better for you, and it is a real big download (1.2 MB). Charlie C posted yesterday about the Coleman solar panel deal at Costco. That includes the Phocos charge regulator (manual onlline here), they just announced a new remote LCD display that works with this controller which displays panel current, load current and battery voltage.

I've got some questions that I'll be asking you and Peter about this which really should be in topic on its own in another area since this discussion has strayed Frederick's original topic.

And from what I see on the bottom of page 4 of the HF maual, Peter's readings might also be affected by the fact that PV panels can experience a 20% degradation within the 1st six months.

A separate thread sounds like a good idea. The Moderators could even move many of the recent posts in this thread to a new one, for that reason... maybe called something like "Solar Panel Performance and Wiring".

That dislay looks like a nicely packaged unit. The termination block can mount on a DIN rail, which will be familiar to many industrial electronics users. The controller can be mounted to a DIN rail, as well.

Hey, Per, you've got enough stuff in your trailer (and change it enough) that serious equipment management options might be of interest. Ready for DIN rail mounting?

The meter shows panel current and "load" current separately, because the associated controllers are intended to have the loads (your trailer's lights, fans, pumps, etc) connected through the controller, not directly to the battery.

It is not, however, a serious battery monitor: it doesn't keep track of charge into and out of the battery, so it can't act as a battery "fuel gauge".

I noticed that, too...
My panel is a single-crystal silicon type, not amporphous silicon, and I don't know if there is a corresponding effect - the manual didn't say anything about it. I don't know what design Peter's Siemens panel uses.

Mine is a poly-crystalline panel. The mono- and poly- crystalline panels are supposed to see a gradual, 5% drop in efficiency over a ten year span with full exposure to the sun.

It's gonna be a hot day today, record-breaking, even. I'm hoping I can pull my trailer out and measure some volts and amps under the mid-day sun, then I'll report back.

There's some dumb saying that Kipling recites: "Mad Dogs and Englishmen . . ." But I'm sure that's just superstition. Has nothing to do with the fact I was born in England . . .

--P

Harbor Freight has a bunch of these up on their Ebay store and no one is bidding on them. The starting bid is $189.00 so you would only save $10 on it.

Solar

Well I broke down & ordered the 3, 45 watt solar panels from Harbor Freight.
Hope I made the right decision. Still have to get the battery. Should that be a 12v deep cycle?? Thanks, Colleen

Check out this link: What is the difference between a normal lead-acid car battery and a deep cycle battery?

Must not have been in a very British mood that day. Didn't get the trailer back from the storage place and check the panel . . .
Still need to do that . . .

--P

Click to view attachment
Today's Harbor Freight circular is advertising a $179 in-store price!

The website is posting much higher... $224.99

http://www.harborfreight.com/cpi/ctaf/disp...temnumber=90599

I've got two batteries hooked in series in my '81 Trillium. I'm just now trying to figure out where to attach the controller + & - wires. From what I've read... positive/positive on one battery, negative/negative to the other. Am I ok? Helen

If you're talking about two 6 volt batteries in series, then yes you are doing it correctly.

Yes, two 6 volt...thank you!

Went to HF today. The store price is $179. They will honor internet prices that are lower than the in store price. They also told me their pricing is different from online pricing, because they are two completely different groups. I now own aa observation camera, Two inverted tomato hanging bags, the tow alignment kit and a raincheck for the solar panels.


I was truly amazed. I have never been in HF during one of their big sales. Never, ever, have I seen that many men using shopping lists and coupon fliers. They were even comparison shopping. Guess, HF is a grocery store for guys.

I have had one of the 3 panel Coleman systems (45w@ 3a) for quite some time. Before I got them, I had (and still have) a small folding briefcase 13 watt@ 1a panel.

Altho the Coleman set works great and I have been satisfied with their electrical performance, I have been long annoyed by their size and weight. I had difficulty in moving the array as a set because of the weight. I could wrestle them into place, but it was not fun. In my 13, I could sort of tilt them up and slide them into the trailer as the last thing I packed, but the 17 sits much higher and they had to be lifted in, and the array took up too much space. Not convenient for those quick rest stops.

Click to view attachment

The guitar case that Fred has actually came from me. I broke down the set and intended to carry the panels in the case, but the frames on the panels were about 1/8" too wide to fit into it. OK, back to the drawing board.

Carrying one at a time is far easier than hefting all three. I removed the frames and came up with a poorly executed PVC frame with alternating sockets and extended ends so I could fit the panels together in an array and lock them together. Since I often leave the trailer during the day, I am deeply concerned about easy theft, so I don't want to leave them sitting on a frame as individual bits that can easily walk away. 3 bolted together would take longer to swipe, and the perma frame I had them on could be locked to my trailer frame or a tree.

Well the PVC idea was a bomb, but I only got as far as doing 2 of them before I came to the brilliant conclusion that "This ain't gonna work". I also tripped on one of them at Quartzsite and cracked it, fortunately, on the back, which is exposed glass. It still works fine, it was the back, but it shows the perils of having them exposed and movable.

I will be taking the one with the original frame and attaching it with hefty hinges so it can be folded up INTO my rock guard for travel, then flipped out and supported by hefty telescoping poles for use. The poles will also double as a support for it when it's stowed. It will be hard wired into my controller, so I will need to do nothing but deploy it for quick afternoon and overnite stops etc. (The other two will get roof mounted permanently.) I will provide pics as I go along.

Since I still want the ability to adjust panels to the direction of the sun, I have decided to go back to my original briefcase panels.

Click to view attachment

I found a source on eBay that I can get them from for around 50 a piece + shipping combined, so that puts them at just a over the HF system for price per watt for 3 of them. They do not come with the charge controller the HF one does, which should be tossed in the trash anyway :-P. I already have a nice controller.

The bonus for the slight upcharge for me is.. I already have one and can vouch for their quality and durability. They also COME with their own carrying and stand solution and are actually MUCH smaller when folded, yet only a hair deeper than a single coleman panel. They weigh less as well, and they have a handle. I currently stow them in one of my rear cubbies. The large panels have to be stowed on the floor.

I will fashion an aluminum sliding bar that spans through the handles and locks down so the panels will be latched together and I will have a place to run the Kryptonite cable lock through. Anyone with a hacksaw could still pinch them, but I had the same situation with the heavy coleman array. It will keep impulse snatchers at bay, however.

Sounds complicated, but I have already built a connection box with 1/4" phone jacks and will be fitting the panels with 1/4" phone plugs and then tethered to the controller. It will all snap together and be running faster than the frame for the HF one can be erected.

and I am forever in your debt, Gina.

send cash....

So, Gina,

What's the link to your preferred seller for those briefcase solars? I like that idea better and being a small person was concerned about the weight and bulkiness of the Harbor Freight ones. Can't ignore the price,tho.



I am grateful you are going to explain step by step how you are going to wire them, as I am still more that a bit of a solar dummy.

Also, which controller do you recommend for this setup?

Yeah, I've done my homework and the array of possibilities is mind boggling. I'm not going to be able to get to a big egg gathering this year. but I have it in the plans for next year.

FYI, there's a really good tutorial on the solar on the left nav bar written by Jim Dunmyer:
Solar for Travel Trailers

Because of that excellent tutorial,I am willing to try this. I have also read Gina's personal blog about the 13'. Plus what seems like a million words on the subject. I'm concerned in putting all that knowledge into practical use, not so much understanding the technology. It's the doing part of the whole thing where I'm inexperienced.

Guess I should have made that more clear. Apologies for any confusion.

Never mind the seller link. I just ordered the 30 Watt Briefcase and charge controller package on Amazon. Not a bad deal, and now I've made the decision and I can get back to putting guts into the trailer.

30 Watt Briefcase Solar Panel and Controller

Click to view attachment Click to view attachment

Both, $319 with standard shipping.

Bonnie, I purchased one of the panels from this guy and when I could see that he delivered etc, I inquired if he could get more. The deal I have with him is a second chance offer from when I lost a bid on eBay. He does not have a warehouse full of them, he is checking into ordering 2 more for me at that price. But he has already delivered one. I do not know, and won't ask him about his source.

He only has one up right now. And I am bidding on it :-P There are LOTS of these on eBay, and the price varies wildly.

The 30 watt one is intriguing, but you can see, the price goes up per watt the bigger the panel gets. That is one of the reason why I opted to have it broken down into smaller bits, and it also allows me a consistent option for adding panels as I can afford them. The 13 watt panel is $3.84 a watt, (+ shipping) where the 30 watt panel is $8.30 per watt. With the 30 watt panel, you reduce set up time and get a larger current capability tho. The 30 watt panel also has Mono crystalline cells, which produce better in full sun, but the amorphous cells work better in low light cloudy stuff. (6 of one, 1/2 dozen of the other) I camp in foul weather a lot, so I prefer the amorphous cells.

For connections, I am using premade housings from a product at work and fitting them with standard plugs and receptacles I can get for free. I am using 1/4 phone jacks/plugs like what is on the end of guitar cords, but you can also use a variety of other types of robust connectors. I was using speakons, but they are a bit of overkill and rather large.

For a charge controller, I am currently using an ASC 12/4 I will have to add a second one soon, as I believe this controller is designed to handle 100 watts max. I can't seem to find that spec anywhere, but I vaguely remember it from when I bought it.