There’s nothing sexy about off-grid storage batteries; they’re heavy, clunky and sometimes smelly. We off-gridders hide them away in dark places and don’t look at them any more than we have to. We think about them only when they’re getting low on charge or need a little maintenance, and we ignore them the rest of the time. But for all that neglect, those of us who rely on them to store solar and wind energy to power our homes need them every minute of every day.
That’s because there is a great disparity between the way we use solar energy and the way the sun doles it out. In a well-designed solar-electric system, the sun will provide, on average, appreciably more power during the daylight hours than the home requires. But the sun does its work slowly and steadily, whereas we often use solar energy in fits and starts to run microwaves, well pumps, hair dryers, table saws and coffee makers. And every time the combined house loads exceed the solar input, the batteries are called on to quickly make up the difference.
The batteries are happy to do this, of course; it’s what they are designed for. But a battery’s lifespan is not measured by ticks of the clock so much as by how many cycles of charging and discharging it must endure. Whenever power is drawn from a battery, the battery ages just a little more, even if the charge is quickly replenished. It’s unavoidable and nothing to become self-conscious about, but it is a useful fact to consider when choosing the right batteries to store your system’s solar and wind energy.
Batteries designed for use in residential solar-electric systems are heavy-duty behemoths designed to be discharged slowly and deeply. Typically they will range from two to twelve volts each, and can weigh up to several hundred pounds apiece. Which batteries are right for you? That depends on the type of system you have, your power requirements and, of course, your pocketbook.
Batteries for off-grid systems
As tempting as sealed batteries may be, in off-grid situations where the batteries are constantly cycled, liquid lead-acid batteriesthe kind you have to add water toare a must. This is because of the chemical transformations continually taking place within them. Whenever a battery discharges, the lead on the negative plate and the lead dioxide on the positive plate both interact with sulfuric acid to make lead sulfate. Once the battery begins charging again, the lead sulfate dissolves back into solution. The problem is, not all of it does. There is always a little lead sulfate remaining, and continuous charging and discharging will eventually leave substantial lead sulfate residues on the plates, residues that will begin to impair a battery’s performance by reducing the surface area available for the battery’s power-producing chemical reactions.
The remedy for this is fairly straightforward: overcharge the batteries until the sulfates are “cooked” off the plates. This is called equalization, and it’s a process controlled either by the charge controller (when equalizing by solar and/or wind sources) or the inverter (when using a generator).
An equalizing charge is generally several volts higher than the nominal voltage of the battery system (we equalize our 24-volt battery banks at 31.1 volts, for instance). It is a high-powered process that boils off some of the water in solution, and this water cannot be added back if the battery is sealed. Hence the reason sealed batteries will not perform satisfactorily in an off-grid environment where the batteries are always cycling.
Types of batteries
So, now that we’ve cleared the water, what battery choices are there for off-gridders?
Not too many years ago, there were basically two choices: T-105s or L16s. The T-105, made by the Trojan Battery Company (www.trojan-battery.com), is the basic golf-cart-style battery. These batteries are light (62 pounds), cheap (around $90 to $100), and will last anywhere from three to six years or longer, depending on how they’re maintained and how extensively they are used. We’ve had a bank of these in our guest cabin for eight years and, as a result of good maintenance and very little use, they’re still going strong.
The L16-style battery is the next step up the line. This battery is considerably more rugged than the T-105. Popularized by Trojan but manufactured by several different companies, these batteries generally weigh in at around 120 pounds and cost in the neighborhood of $220 each. If properly maintained, a hardworking bank of L16s can last eight years or longer. (We have two banks of 12 Trojan L16 batteries in our house, for a total of 24. Switching from bank to bank ensures that neither battery bank ever becomes more than 35 percent discharged, a system designed to greatly extend battery life.)
Surrette batteries, manufactured in Canada (www.surrette.com), are finding their way into more and more off-grid homes, particularly as off-grid systems become larger and more demanding. Big, heavy and rugged, these batteries are built to last 12 to 15 years or longer, as evidenced by the 10-year warranty they carry. Surrette offers a number of battery sizes and types from two to twelve volts over a wide range of capacities, so it’s possible to size a bank of Surrettes for almost any system.
If you’re really serious about going off grid with a large solar array and you insist on top-of-the-line batteries, then HUP Solar One batteries (www.hupsolarone.com), manufactured by General Battery, may be just the workhorses you’ve been looking for. General Battery has developed a patented process that uses Teflon to stabilize the lead paste that covers the positive plates (hence HUP for High Utilization Positive) of these huge batteries. The result of this innovation is a battery that can take intense use (which is not to say neglect or abuse) over many years. In fact, Solar One batteries are warranted to endure 2,100 cycles of 80-percent discharge, or 4,000 cycles of 50-percent discharge. This is equivalent to using half the energy in your batteries every day for nearly eleven years, something that would never ever happen with a properly sized solar/wind charging system.
As you might imagine, these are not small batteries. Solar One’s 12-volt batteriesthe basic building blocks of their 24- and 48-volt batteriesall stand 25 inches high and weigh from 742 pounds for the 845 amp-hour size, to 1,336 pounds for the 1,690 amp-hour giants. So in addition to a suitably large solar array, you’ll need either a forklift or a football team to set them in place.
Choosing the right battery
Selecting the brand, size and number of batteries you need can be a little tricky. It depends on the average daily output of your solar array (see Countryside May/June 2007, “Calculating Your Daily Solar Energy Harvest”); your home’s daily energy demands; the amount of stored energy you’d like to keep in reserve; and, of course, what you can comfortably afford. Generally you will want a battery bank sufficiently small that you can return it to a full charge at least every few days, but large enough that it will be able to hold the extra power reserves you’ll need to tide you over for two or three consecutive cloudy days.
In sunny Colorado, LaVonne and I can easily bring a 28,000 watt-hour (1,200 amp hours @ 24 volts) bank of 12 Trojan L16 batteries from a 70 percent charge to a full charge in less than two sunny, breezy days, without working particularly hard at saving energy. We do it with a 1,640-watt solar system capable of producing over nine kilowatt hours on a good day, and a 1,000 watt Bergey XL.1 wind turbine that’s usually good for a couple of kilowatt hours.
It’s a system we’re comfortable with. Some people we know get by fine with less storage in relation to their solar and wind output, others with more. In the end, you’ll find that you either adapt to your system, or you change it. And therein lies the key to successful off-grid living: be flexiblein the way you design your system, and the way you live within its limits.
Then be happy…you’re living off the grid. And living well.
Rex Ewing is the author of several renewable energy books, including Power With Nature, Got Sun? Go Solar, (available from the Countryside Bookstore) and the newly released 2nd edition of Hydrogen: Hot StuffCool Science (available from www.pixyjackpres.com). He lives with his wife, LaVonne, in a handcrafted log home powered solely by the sun and wind in the foothills of Colorado.