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"
http://www.thewanderman.com

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.