Friday, November 18, 2011

A Better Solar Charging Mouse Trap.PART 2

Typical Bracket Mounting
And...we're back! The last posting introduced my solar charging system and how false starts and misinformation can lead to inspiration. Now let's talk about the semi-flexible solar panels.

Solar panels are, most commonly, aluminum framed, glass encased assemblies weighing around 30 lbs. They are usually mounted with "L" or "S" shaped brackets using holes drilled through the RV roof and then sealed. Some manufacturers and installers use what is essentially double-sided tape. Mind you, it's VERY strong tape, but still....tape.

170 Watt Panel
After I found out I couldn't really use any of the commonly available panels on my tiny roof and still have enough power to run everything I wanted for as long as I wanted, began searching high and low. I found a manufacturer in China that could (and would) make me almost any size I wanted in any voltage I wanted. Now all I needed to do was decide on the specs. Sounds easy?? I thought so.

If you'll recall I needed at least 360 WATTS of panels to get the job done. I sent the measurements I had to the manufacturer and they sent me back some basic specs. Based upon the sizes I sent they configured 3 panels. A 170 Watt version shown here and 2 140 Watt versions. 1 of the 140 Watt panels is the same width as the 170 Watt, and the other is almost square. Together they added up to 450 WATTS(!) given my limited roof real estate. I was thrilled, more than I needed and they only weighed between 5 and 8 lbs each! Since they are SEMI-flexible they would conform to the contours of my roof. Mostly. More on that later on.

Originally my specs called for around 18 volts. I figured since I had a 12 Volt battery that 18 Volts would run a charger just fine. Being new at all this I hadn't looked any further into the system. yet...

WARNING: Science Content Ahead -

Let's look at maximizing efficiency for a moment. DC (Direct Current) systems can have lots of losses. The idea is to put as much back into your batteries as you can without losing it along the way. In order for a 12 Volt battery to charge you need to have MORE than 12 Volts.

Imagine VOLTAGE is like water pressure in a pipe and AMPERAGE is the amount of water flowing. The Battery is like a water balloon you are filling. You need to have more pressure going into the balloon than the it has pushing back to get water to go in. A large quantity of water will fill the balloon faster. The longer the distance the water has to go the lower the pressure you have, so you get less water at the other end. Think of a long garden hose. At a lower pressure you can get a bigger hose to get the same flow. Ideally, you have enough Voltage to charge your battery and enough Amperage to fill it quickly.

Now that we have that straight, back to Solar Panels. In order to make my system work efficiently I had to figure out how far my power had to travel and make sure my "pipes" (wires) were big enough to get the job done. If you've even done any work to the 12 Volt side of your RV you'll know that wires come in all sizes. From tiny wires going to gauge lighting, up to the heavy duty ones connected to your battery.

Table Shows Wire Size in AWG
The basic rule is, the higher the current (AMPS) the bigger the wire needs to be at 12 V. The longer the wire run, the thicker the wire. This will reduce losses. However, you can easily get to the point that the wires are WAY too big to be used easily. The fix is simple. Raise the Voltage! Higher voltages lose less with distance. Now my 18 Volt panels were beginning to look wimpy, especially since I had to travel 14 Feet from one of them to the charger. (We'll get into wiring and cable types in PART 3) It was looking like I would be better off with higher Voltage panels. I checked with the manufacturer and found I could spec almost any reasonable Voltage. After some research into the Charge controllers I found that I could comfortably use 36 Volt panels. In fact I would derive some side benefits from this....we'll talk about charge controllers and MPPT later on.

Science Content Concluded (Whew!) 

So now I was ready to order my custom panels. After trading some English language (mostly!) e-mails with the manufacturer I negotiated a sample price of around $3.10/Watt. You can get better deals now, but that included DHL shipping from China.

Tune in Next Week for PART 3 where we will talk about mounting the panels and dealing with wire runs. How do monsoon rains figure into a "simple" solar installation? 

Be Seeing You...Down the Road

Richard "The Wanderman"

Read Part 6 of this series.
Read Part 5 of this series.
Read Part 4 of this series.
Read Part 3 of this series.

Read Part 1 of this series.


  1. being of simple mind with the skills of a gardener i feel like i've had the good fortune to run into sage that has gone deep into the wilderness and returned with tales of treasure and treasure to boot. i will wait for your next post and hope one day to have enough of my own knowledge to begin the journey to solar.
    what is an amp?

  2. Jimmy,
    Myself, I have more of a black thumb than a green one, so gardening seems a lot like magic to me!

    Amps measure the quantity of power (current) going through (and sometimes into) another device.

    Rich "The Wanderman"

  3. Wow Richard you certaintly went deep! Good thing that you did because I have a 16' travel trailer and this is my first one. Since I want to do lots of boondocking I checked out two ads selling portable solar panels at 120 amps which you do not to install on the roof. That being said since I do not want to place them on my roof and the ability to move them where the sun hits them and then once done put them in their folding case, I felt it was and is still a good deal for my needs. However after reading you posts I wonder if they are sufficient for my needs. The trailer has a small microwave, small fridge and the usual "parasites" you mentioned. I have no tv, no radio/cd and the others pacman eaters found in larger RV's and trailers. So I wonder and post the other than my laptop and not being a mathematician or scientist, will the 120 hold water? Ray

  4. Ray,
    First off, thanks for stopping by!
    I'm happy to help, but need a bit more info. the "120 Amp" panels you found, I assume are actually "120 WATT" panels. They will likely generate around 77% of that in normal use. (92.4 WATTS) this gives you around 7.7 Amps per hour going back into your battery. Not a huge amount. How many batteries do you have in the trailer and what size are they in Ah? (amp/hours) If you look around online you can easily find the draw from all your devices and add them up. If you subtract what is being used constantly, that figure is that's left is what will be going back into your batteries for every hour in direct sun.

    Hope that helps!

    Rich "The Wanderman"

  5. I have four 125W panels. Two on roof mounted on carousel and can be raised up to 45 degrees to track the sun. They can be raised and rotated from inside with a shaft through the ceiling.

    The other two are placed on the ground and have 50' of 10 gage cable with push-on connector to side of camper. Those on ground are placed in sunny, open space if I park under a tree. So, that cuts my charging by half when camping in the shade.

    Charge controller is 40 amps. My monitor checks for battery voltage, amps, time-to-charge, A/H and some other stuff.

    Have four 75 A/H deep-cycle AGM batteries.

    I don't need A/C running up north or in mountains, but sometimes I use roof vent fans for a few hours in the afternoon. The water heater and refrigerator are put on propane and I conserve on electrical use.

    The inverter is sine-wave 2200 watts for use of satellite TV, laptop and intermittant microwave in addition to a few 12V lights.

  6. The rotating shaft idea sounds quite interesting. I don't like the idea of drilling more holes in the roof as that is a primary source of failure that leads to leaks. about a carousel style mount for ground deployment....

    Rich "The Wanderman"

  7. It is a good idea to over design. One issue I didn't see mentioned in detail is how shadows reduce current. If any part of a cell in a series string is shadowed, the output of the whole string is limited to the current of the shadowed cell. As you go higher in voltage, the series string get longer so your chances of a shadowed cell increase. Great article, though. I'm glad there are alternatives to the fixed dimension panels I've seen up to now. Are you going to publish your China sources? I'm looking forward to to seeing the complete design.

  8. Anonymous,
    Shadowing is an evil that flat mounted panels on an RV roof simply cannot avoid. The best you can do is up the panel voltage to somewhat mitigate the loss with a bit of earlier morning and late afternoon charging.

    Stay tuned to the article, I'll be getting into the who/what/where/when and how.


    Rich "The Wanderman"

  9. I wish thin-film solar cell modules were available back when I fabricated my system. I would have done it very differently. I'm very interested to see how the rest of your project goes.

    The raiseable, rotating shaft through the roof is a home fabricated device using, for example, a brass toilet flange as one piece off-the-hardware-store-shelf. The roof has not leaked in 6 years.

    The two panels on the ground are not bolted together and are just rotated by hand from east to south to west in three stages. The tilt up from the ground is maintained with a simple, hinged support shaft. I didn't do a carousel for them due to limited storage while driving. The two ground panels are stored face to face inside a rectangular bag made from a used awning, supported by a platform mounted on the rear bumper and velcro-strapped to the rear ladder. I reinforced the ladder with extra supports riveted to the outer wall.

    Since the thin-film modules are mega-lighter than the old glass and framed modules, I am working on a motorized auto-tracking mount for the next system.

  10. desertmanfred,
    I'd like to get a closer look at that setup!

    Just FYI, my panels are NOT thin-film solar. They are plain mono-crystalline style, just affixed to a semi-flexible substrate and have a polycarbonate top. In the sandwich is an EPDM membrane for shock absorbency and additions flexibility and torsional stability.


    Rich "The Wanderman"

  11. Hi Rich -

    I love that you managed to track down a place to get custom sized flexible panels - I'd love to get more details on your sources since we'll soon be working on adding solar to our bus to go along with our existing LiFePO4 battery bank project.

    I look forward to swapping notes with you.


  12. Chris,
    Oddest coincidence! One of my readers turned me on to your LFP project...just finished reading through it. Way cool, and some GREAT info. I've got a line on some US sourced Custom LFP packs...albeit a bit small than yours!

    I worked with a manufacturer in China directly to develop and assemble the panels themselves. It took quite awhile, but was worth it. One of the main issues was shipping....DHL is expensive, but for small quantities in a reasonable time it's the only way to go.

    I'll send you an email with my contact info and I'll help as much as I am able.

    Hope your Dad is better!

    Rich "The Wanderman"

  13. What brand of adhesive did you use to mount your solar panels down and what store did you get them from. You said they were 12.00 a tube. If good product, then well worth the price.

    I use THRU THE ROOF to seal my cracks and such, and it is also expensive, but worth it because it works!

    1. Anon,
      If you keep reading through the 6 part series you'll find out I used 3M 760 Paralastic adhesive. This is not really designed for crack sealing.

      The panels were custom made by a manufacturer in china, there was no "store" just many emails back and forth to solidify the specs and the deal.


      Rich "The Wanderman"


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