When I began putting together my first solar electric system way back in the last century, I soon noticed there was an easy part and a hard part to the installation. The initial tasks-welding the heavy steel frames for the solar panels, digging and pouring the concrete foundation piers, mounting the frames and the panels, building a battery box from plywood and 2 x 4s and lifting in and arranging the octet of 80-pound batteries-all fell into the easy category.
The hard part came when I had to figure out how to wire eight 6-volt batteries and six 12-volt solar panels for 24-volt operation. Up to that time my most noteworthy accomplishment with DC electricity was rewiring the lights and brakes on an old horse trailer, after my new puppy had endeavored to sate her chewing urge on the original wiring. Frankly, I was beginning to think I was in over my head. But, armed with the knowledge that parallel wiring is like to like (+ to +, or - to -) while series means connecting dissimilar poles (+ to - , or - to +), I forged ahead, ever so cautiously.
In the end-when I got the system up and running without frying any solar panels or blowing up any batteries-it all made perfect sense. But I'll never forget staring intently at the crude diagrams I'd scrawled out on a notepad, wondering if they would lead to redemption or disaster.
Does any of this sound familiar? One of the most confounding aspects of installing a renewable energy system is wiring the solar array and the battery bank to achieve the proper system voltage. It helps to understand the logic of what you're doing. Why, for instance, does a series circuit increase the system voltage, while a parallel circuit compounds the system's amperage?
Getting a grip on amps and volts
Although amps and volts are more often than not uttered in the same breath, they are very different phenomena. So what is an amp (more properly known as an ampere)? It's simply a measure of the volume of electrical flow or, to put it another way, an accounting of the number of electrons to pass a given point in a given amount of time (6.25 quintillion electrons per second, for those who keep track of these things). The value of an amp is not affected in the least by the voltage; the electron count stays the same whether the amp is at 1 volt or 100 volts.
A volt, by contrast, has nothing to do with the number of electrons and everything to do with the force which moves them. That's why voltage is usually referred to as electrical pressure-it's a measure of the difference in electrical potential between where the electrons are, and where they want to be. For simplicity's sake, you can think of 1 volt as the force behind a herd of electrons sauntering lazily down a very gentle slope, while 100 volts is the force behind that same electron herd stampeding down the side of Mount McKinley.
Series and parallel wiring in a battery bank
With those concepts firmly in mind, let's look at what happens when we wire four 6-volt batteries together to work in a 24-volt system. It's really quite easy-we simply connect the positive (+) terminal on the first battery to the negative (-) terminal on the second battery, while connecting the second battery's positive terminal to the third battery's negative terminal, and so on. When we're done, all the terminals will be connected except for the negative terminal on the first battery and the positive terminal on the last battery. And if we then connect a volt meter to these bare terminals we will see our batteries do indeed comprise a 24-volt system.
Why? Because each step along the way we added electrical potential. This is analogous to making the electron slope steeper with each additional battery. But by wiring positive to negative we created only a single path for the electrons to follow, so the amperage of the first battery could not be added to the amperage of the second. We were only adding electrical pressure, not actual electron volume.
To increase the amperage of the system without lowering the voltage, we have to add more batteries. So let's put two more rows of four batteries beside the single row of four we've just wired, giving us a total of 12 batteries. Once we wire the two additional rows like the first one, we'll have three unconnected 24-volt series strings, and all that remains is to connect together the three positive terminals on one end, and the three negative terminals on the other. With multiple paths for the electrons to follow (any of three identically wired rows of batteries), we have increased the battery bank's amperage three-fold, while keeping the voltage at 24 volts.
See how simple this is? Any number of batteries can be connected in the same way, so long as they are in multiples of the number of batteries in a series string. To avoid confusion, you should always make your series connections first. Once that's done, the parallel side of things is a straightforward operation, since all you have left are positive terminals on one end and negative ones on the other.
All that remains at this point is to attach the positive and negative leads that come in from the solar array and go out to the inverter. Pick two opposite corners of the battery bank that give you the shortest path to the inverter, and attach both your input and output cables there. Why? Two reasons. First, it creates the shortest possible path from the array to the inverter and, second, it's the only configuration that works all of the batteries equally, since current drawn from any of the batteries must be drawn from all of them.
The solar array
Series and parallel wiring of a solar array is identical in principle to wiring a battery bank, but there are a couple of additional tricks to keep in mind. For starters, you'll want to be able to isolate your series strings from one another to protect the array from a backward jolt of amperage from the battery bank (that dark, brooding place where all the power for your system is stored). Usually this is done with a combiner box, which is an electrical component that joins several series strings of panels together while providing a separate fuse or breaker for each one. For smaller arrays with only a couple of series strings, inexpensive, off-the-shelf breaker boxes will provide 2 or more circuits each and will probably save you some money. Breakers manufactured by Square-D are DC-rated and should pass muster with the electrical inspector.
You will also want to cover your solar array with a tarp while you're making the connections. This is not so much to protect you (unless your array is over 48 volts) but to protect the delicate wiring of your expensive array from any lapses of dexterity-or logic-on your part. You can also wire the array at night with a flashlight clenched between your teeth, but I personally never thought it was much fun.
And one last word of advice: many solar modules these days come with simple plug-n-play connections, but most still have junction boxes with multiple terminals inside. Study the literature from the manufacturer to see which terminal does what before you proceed. It'll save you a lot of anguish in the long run.
Long ago, when I was trying to make sense of the differences between series and parallel wiring, I came up with a quick-and-easy little memory cue to remind me which was which. To wit: a series circuit provides serious voltage.
If it works for you, use it; if not forget it. But now that you've heard it you can't forget it. Not ever. Sorry.
Rex Ewing, author of two books on renewable energy (Logs, Wind and Sun, and Power With Nature) has just completed his newest book on grid-tied solar energy (Got Sun? Go Solar, PixyJack Press, 2005). He lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind in the foothills of Colorado. His books can be purchased at the Countryside Bookstore or at www.pixyjackpress.com.
