Building Solar Panels

A few months ago I built a few solar panels to add to the roof of my van. I posted a few pictures and some basic emails to the yahoo group Vandwellers, and since then have been inundated with a ton of questions regarding how I built my own 200 watt system for around $500.

I had wanted to add solar power for a while, especially since several of my places to park and plug in had no longer become hospitable and the only other way to charge my batteries was to run the engine since I don't have a generator. When I got my tax return I decided to look into it!

Seeing the cost of solar panels at over 5 dollars a watt even on the cheap end, I decided to build my own for about half that not just for cost, but the experience in building them as well. They've worked very well so far.

I want to let you know I had no idea how solar power worked, and to be honest, I still don't. Sun hits panel, electricity comes out the wires. It's still incredibly amazing to me. I just did a lot of research online the same way I did with any other system when I built the van and I'll guide you through the way I did it, and what I've learned.

Wiring a house for solar power is a lot different than wiring a van or an RV, so keep this in mind. The good news is, wiring up a 12 volt (off grid) system is a lot easier than a house (grid tie-in) system.

A solar system contains three different components:

1. 
   
   The panel(s)
2. 
   
   A charge controller
3. 
   
   The battery bank

Solar Panels

Panels are what produce your power when in the sunlight. They should be sturdy, weather resistant, and made out of materials that will protect the fragile solar cells inside. On a house, this isn't difficult to do. On a van or moving vehicle where a panel will encounter vibration and possible road debris, this requires the panels to be fairly overbuilt (but then again, I overbuilt everything on my van)

A solar panel consists of multiple fragile solar cells. These are wired together in either series or parallel depending on how many amps and volts are needed. The cells are encased in what is essentially a backed picture frame for them made out of wood or metal with a glass or acrylic front plate. There should be two wires coming out of the panel, one positive and one negative, that provide power.

Most solar panels designed for RVs (and the ones I designed for my van) and their 12 volt systems are actually designed to put out about 50% more than that in direct sunlight, or ~18 volts. This makes the panels more efficient on overcast or short days, and since these panels aren't in a fixed position facing the sun like they would be on a house, they provide more charging with their added flexibility.

Solar panels are great, but they also have a few drawbacks if you don't know about them. Solar panels put out different voltages and amps depending on how much sunlight they get. You never want to hook one up directly to your battery bank. Running 18 volts constantly into your batteries will accomplish nothing but boiling away your electrolyte and ruining the bank. 

Solar panels also have the added negative effect of actually draining your battery at night as they will draw power, not produce it, in the dark.

To combat this, there are two options. The first option is to install a diode in between the battery and the solar panel. A diode is a one way pathway for electricity – it allows current flow in one direction but not the other. This will prevent your batteries being drained at night but unfortunately will not do anything about overcharging your battery and boiling electrolyte. This brings us to the next part of the system...

The Charge Controller

The second option to control the voltage swings in the panels, and the much better one, is called a “charge controller” or a “solar controller.” Like the voltage regulator in your car or van, the solar controller will regulate the voltage going into your batteries to a constant charging rate and prevent them from overcharging. A good quality solar charger will also provide three-stage charging to your battery, and prevent the panels from drawing power at night.

You can find controllers online from a variety of sources. The controller I purchased off of eBay for $50 does all of those things I mentioned – in addition, it has a built in battery charge indicator which is more reliable than the one on my inverter.

When looking around for a solar controller, keep in mind that solar chargers aren't usually rated in watts, but in amps. Watts = amps * volts, so make sure you know approximately how many amps your system will have. 

If the system that you want will ideally be 200 watts and puts out 20 volts in direct sunlight, then your system will be producing a maximum of 10 amps. You may also find your system temporarily puts out more power than you expect it to or the numbers will tell you – solar cells, when wired together, produce some strange field effects that cause the entire array to be more efficient than the sum of their parts. 

Make sure you purchase a controller rated for at least the amount you're planning, and then some – if this piece of equipment gets overloaded, you can not only short it out but damage your batteries and solar panels as well. My charger for $50 is rated at 20 amps. This means at 20 volts the charge controller should be able to handle up to 400 watts which is almost double what my panels put out.

The third piece of equipment you already have, in your battery bank. This is used to store the power that the solar panels put out.

Purchasing Solar Cells

Solar cells themselves are incredibly thin. They're made out of silica so you're essentially holding a playing card made out of glass. 

The standard cell that you'll want to look for online on eBay or Amazon is 6 inches long and 3 inches wide, and sometimes go under the name Evergreen Cells. They put out .5 volts at 3.5 amps (1.75 watts). On one side, they will be blue with many very thin lines running lengthwise across the cell with two thicker lines running down it, and on the other they will be gray with six small white squares.

The cells themselves are not all you need to make a panel. You will also need the following:

Tabbing wire – incredibly thin, flat wire designed for connecting the cells into strings

Bus wire – Wider wire designed for connecting strings of cells to each other

Flux pen – a pen full of solder flux which you'll need to connect to use to solder the cells

Solder – used for making the connections

The good news is, most online retailers will sell kits that contain all of this material. A kit will have a certain number of cells and the supplemental bus and tabbing wire needed to connect it together. Here's a picture of a typical solar cell kit that you can buy online. From left to right you can see the thin tabbing wire, the wider bus wire, the cells, and finally the flux pen and solder at the bottom.

A solar cell will still put out power if it is cracked or chipped or if a small piece of it is missing – I've noticed no power loss in any of the panels I've put together and I've cracked and chipped a few cells while working on mine. You WILL end up breaking a few no matter how careful you are, so most kits will contain a few extras.

One last thing: Make sure the kit you purchase have the cells PRE TABBED. The cells will have two thin wires coming off of the blue side. This will save you a lot of hassle in the long run (more on this later). They cost a little more, but are well worth it. Here's a pic of "pre-tabbed" solar cells:

When you receive your solar cells (and break the first one you ever hold because yes, they are that fragile) you'll need to identify a few things on them. First, the blue side with the two lines running the width of the cell is the negative side and the side that faces the sun. The two (relatively) thick lines on this side are the negative contacts of the cell. On a pretabbed cell, you will notice that there are thin wires soldered down the length of these lines with about three inches of wire hanging free off the bottom of the cell.

These wires are called tabbing wires, and without purchasing a pretabbed kit, you'd end up soldering those wires to the front of the cell by hand, at about a foot of wire per cell – very tedious and time consuming, not to mention incredibly fragile, work.

The grey side of the cell is the back and the positive side. The six white dots are the contacts for the positive side of the cell. You'll be connecting the cells together by soldering the tabbed wires from the front side of one cell directly to these white dots on a second cell. Connecting cells like this creates a strand, which like a battery, will have one end that is positive, and one end that is negative. But before you start soldering these strands together, you need to lay out and construct the panels themselves.

Laying out the Solar Panel construction

You have the dimensions of each cell and know you need 36 of them to fit inside a box you'll create. There are several ways to do this – you can do six strands of six cells each, four strands of nine cells, nine strands of four cells, etc. For my purpose, I decided on six strands of six cells each.

Layout your design any way you want, just make sure you leave at least a quarter inch gap between the cells in your layout since with road vibrations and heat they tend to expand/shrink/wiggle the cells a bit.

Up until this point, the information here has been pretty generic, and now I'll show you exactly how I build my own panels. You don't have to build them identical to mine, but the concepts and construction techniques should help if you make your own.

I chose to make my panels 24" wide by 48" long. There were several reasons for this; quarter sheets of plywood are already sold precut at the hardware store, as are sheets of acrylic. I purchased three 3/8” pieces of 24” x 48” plywood and some 1x1 stock (3/4” square) to make the boxes. I predrilled the plywood and screwed the 1x1 stock to the outside edges with some inch long wood screws and wood glue. I basically ended up turning the quarter sheet of plywood into two 2'x2' boxes ¾” deep to house my solar cells, which is really the only depth you need. Unfortunately I didn't get any pictures of this process, but hopefully the ones I did get along with the diagrams will help with this step.

Next, I drilled the holes I needed. There's one hole drilled in the corner of each cell for the positive and negative wires to be fed out, and there are two holes drilled in the “dividing” wall between the two boxes. You also want to drill some small holes along the sides of the boxes which will allow moisture and air pressure to equalize between the inside of the panel and the outside. Temperature and moisture differential will cause condensation on the inside of your panels which could cause some of the soldered connections and wires to rust. (I learned this the hard way)

After you build your boxes, you'll want to paint them inside and out with a high quality exterior paint to seal the wood. I probably put half a dozen coats on each of the three boxes I made. In between waiting for coats to dry, I worked on soldering the strings together.

Soldering the Solar Cells into Strings

Each string of six panels has a positive and a negative end. To start a string, place one cell face down on a piece of cardboard or cotton towel(don't use synthetics as the heat of the soldering gun could melt the plastic). Push the tip of the flux pen into a piece of paper a few times until the flux starts to flow and lightly go over each of the six white contact points with the tip, coating them in flux. 

Cut two pieces of tabbing wire about six inches each and solder them to the six contact points, similar to the way the tabbing wires are soldered on the negative side of the cell. When you're done, you'll have that one cell with two wires attached to the positive side (coming out the “top” of the cell) and two wires attached to the negative side (coming out the “bottom” side)

To continue the string, slide a second cell facedown underneath the negative tabbing wires of the first cell. Coat the contact points of the second cell with the flux, and solder the tabbing wires from the first cell to the back of the second one. It sounds more complicated than it is, just take a look at the pictures to see me soldering a string together.

I continued this until I had six cells strung together. When you solder on the last cell in the string, you'll have a string of six cells with two tabbing wires on each end. The end with the tabbing wires connected to the negative (blue) side of the panel is the negative end of the string, and the end with the wires coming from the backside of the cell is the positive end of the string. I continued making strings until I had finished all of them (18 strings, 6 per panel)

Laying out the Strings

Since each string of cells has a positive and a negative end, they need to be laid out and connected to each other. This can be a bit difficult to visualize, so the easiest way to do so is to imagine the strings of cells as large batteries you're putting together in a power hungry appliance or toy. Here's a diagram of how the cell strings themselves are going to be laid out in the box:

 As you can see, the middle string in each row is “flipped” so that each string alternates between having positive and negative connections near the top or bottom of the box – just like installing batteries.

To secure a string inside, I laid down two beads of caulking in the box where the string would go, after I had finished painting the boxes. I very gently laid down the string on top of the beads and pressed down with my fingertips. The caulking provides a very secure attachment which is somewhat flexible, allowing the cells to vibrate slightly and expand/contract with heat. I continued this with all six strands of cells, three per each half of the box.

Connecting the Strings Together

This is where you use the bus wire. It's basically a wider version of the tabbing wire, and is used to connect the strings together to themselves and to the thicker wires which will carry the current from the panel. You need to secure pieces of this wire to the box – you can either use caulking, or do what I did and just use a staple gun since wood doesn't conduct electricity. Once you see the diagram with how to place them, you'll see how the strands are wired up to create more voltage.

The green lines in the following diagram are where you put the bus wire:

The next step, which I do have pictures of, involves soldering the tab wires to the bus wires where they overlap. This will provide solid electrical connections from one strand to the next. Here's me soldering the strands to the bus wires after they had been caulked down. (The pictures were taken as I was working on several panels, so you may see some things which are already completed)

The last two steps for wiring the panels involve connecting the strings in both boxes together and running the power wires out the back of the panel itself. The thickness of the bus wire helps this process, as you can see. I just stapled the wires where they were supposed to go inside the box, and then made sure all of the connections were soldered.

If you look to the right of the following diagram, you'll see that the two strands on the right side have ends near the center of the box that aren't connected to any other strands. Remember the two holes I mentioned drilled in the center wall earlier? You'll want to run a small length of wire through the hole between those two strands, soldering the ends to the bus wires at the ends of that string, as shown:

Finally, run, staple, and solder the last wires in the panel, as shown in the last diagram. (You can see in the previous pictures I did this out of order and connected these wires first)

 These will carry the power that the panel produces. Test these wires with a multimeter – you should be getting voltage. I moved this panel into direct sunlight and it was producing 20 volts at 3.6 amps.

Once you know it works, its time to seal it up. I sealed my panels by running a bead of caulk along the top faces of the 1x1 walls, and laid down 24” x 48” sheets of acrylic. To secure the acrylic to the panel, I used some type of deck screws I picked up at Home Depot. They are an inch long and they have a rubber washer already adhered to the back of the screw head – just remember to predrill and go slowly, or you may crack the acrylic. Tadaaaaa! Finished panel!

As an afterthought I ran waterproofing tape over the outside edges of the panels.

Tadaa! If you did it the way I did it, you now have a solar panel! I built two more in identical fashion.

Wiring Multiple Panels Together

If you're only making one panel, you can skip this step.We wired the cells together in series to produce more volts, now we'll wire the panels in parallel to create more amps. To do this, you need to connect all the positive leads from all your panels together and all negative leads together in a similar fashion. While you could do this with some heavy duty wire nuts, the safest way is with a terminal block.All three of my panels are wired to a terminal block which then has thicker gauge wire running down into the van itself.

I purchased a terminal block from the electronic store along with a project enclosure for it, and ended up connecting all three panels together so only two wires carry all the current into the van. 

If you did it right, if you built the panels the way I did a multimeter reading will show 18~20 volts @ ~11 amps in direct sunlight. If you wired them in series by mistake you'll instead read 60 volts at 3.5 amps. Both of these systems by definition have the same wattage, but only the first of them is useful to the controller and batteries.

Connecting to the Charge Controller

By now your solar array is mounted to the roof in any way you like (I just used angle brackets and self tapping metal screws with a dollop of waterproof caulking to secure mine in place) and you have the two wires running down into your van ready to hook up to the solar controller.

The solar controller, if it's like mine and most others you'll find online for these relatively low watt applications, will have six screw posts. Two are for the positive and negative wires coming from the solar panel(s), two are for the positive and negative wires running to the battery, and the final two are for the positive and negative connections to the "load."

The "load" circuit is just a simple 12 volt outlet circuit that some people use for running lights or a small fan. After doing alot of research online and testing it, you don't need to have anything connected to these posts.  

NEVER HOOK AN INVERTER UP TO THE "LOAD" TERMINALS OF YOUR SOLAR CONTROLLER! 

It is not designed for something with a large power draw, and your batteries should already be hooked up to your inverter directly.

Run wires from your battery bank to your controller. If you have more than one battery in your battery bank, connect the positive wire to the positive post of your first battery, and connect the negative wire to the negative post of the last battery in your bank (like you did when wiring the inverter). This will help evenly distribute the charge from the solar cells across all of your batteries.

Read the instruction manual for your solar controller to know what order to hook up your wires. For mine, it involved connecting the battery first, then the solar panels. As mentioned, the “load” connections can remain untouched.

Getting Used to the New System

At first, I thought I would need to wire some complex system to switch between the now three charging methods in my van – shore power, alternator, and solar. My batteries charge while I drive the van, when I'm parked plugged in somewhere, and now whenever there's sunlight. It turns out I didn't need to do anything at all!

The controller I purchased monitors the state of the battery and how charged it is, and once it's fully charged, will automatically break the connection between the battery and the solar panels to prevent the battery from overcharging. When driving, the batteries are charging from both the solar panels and the vehicle's alternator until they reach the desired full charge voltage, and it's the same when I'm plugged in, having the batteries charge from both the solar panel and my inverter.

I get about ten hours a day of sunlight in southern California, and the panels provide me with all the power I need, enough to keep my roof vent fan going 24/7 which has been a lifesaver at night. In fact, the one or two times I've been stupid enough to run my batteries flat haven't been a problem – just wait two or three hours in the sun, and suddenly there's enough juice in the batteries to turn the engine over!

The total cost of the project, keep in mind this was for three panels:

Solar cell kit on ebay (108 cells + tabbing wire + bus wire + flux + shipping): $210

3 2'x4' Acrylic panels: $80

Solder, wire, and various odds and ends at the electronics store: $50ish

Lumber, hardware, paint, and other supplies to build the panels: $120ish

Solar controller: $50 shipped

Total cost: Around $500 for 200 watts ($2.50 per watt) or less than half the cost of commercial RV systems ($5-$6 a watt, and that doesn't even include the charge controller or wire or other components!)

I couldn't be happier with the results – and once I add two more panels, bringing my solar system up to 330 watts, I'll have enough juice to keep the fridge going nonstop too! I'll be able to do it for another $250, so my total system will cost me ~$750, with the cheapest comparable 300+ watt system on the market being in excess of $1700.

I hope this page goes out and inspires someone who reads it to go out and make their own panels, or prove to them it's really not as complicated as you might think. I knew nothing about solar power or how to wire panels or make my own before I jumped in and made these, and it seems everyone who claims to have instructions on how to build solar panels is selling the information online in the form of ebooks.

Steve