What To Look For
Batteries for a solar system must be of a deep-cycle design, to withstand discharges to 50 percent of capacity on the occasional string of cloudy days. Among deep-cycle batteries, three designs stand out, in order of price from lowest to highest: flooded lead-acid, sealed AGM, and gelled electrolyte. Lead-acid batteries, while requiring occasional maintenance in the form of topping them off with distilled water, are both forgiving for the first-time owner and a good value in terms of capacity per dollar.
Common Pitfalls
The neophyte alternative power designer is often tempted to simply buy whatever deep cycle, 12-volt batteries are available locally. There are a couple of problems with this approach. First, consumer grade "deep cycle" batteries are more similar to automotive starting batteries than to real deep cycle batteries. Second, although their voltage matches the most common inverters, their storage capacity --expressed in amp-hours--is insufficient. This creates a need to connect multiple batteries in parallel to maintain 12 volts output while adding capacity.
It is far better to buy two L16 batteries at 6 volts and 350 amp-hours and connect them in series for 12 volts and 350 amp-hours, for example, than four 12-volt batteries at 90 amp-hours, connected in parallel for 12 volts and 360 amp-hours.
Where To Buy
Batteriesinaflash.com has the lowest price on the preferred Trojan brand L16 batteries. Batteries In a Flash offers truck freight shipping within 24 hours, and they have a toll-free telephone number for a freight quote. It is worthwhile to search for a battery outlet in your local area before buying, however; as such an outlet may be able to beat the online price plus shipping. Don't forget to account for sales taxes that will have to be paid.
Cost
Prices for a single Trojan L16 flooded lead-acid battery range from about $300 to $400. Two batteries needed for a 12-volt system make the cost $600 to $800. Better yet would be a 24-volt system, requiring four batteries at $1200 to $1600. Up to four batteries can be shipped within the continental U.S. for approximately $100 to $150.
Comparison Shopping
Prices can vary greatly based upon battery chemistry and manufacturer. For example, The Alternative Energy Store has MK brand L16s, which are similar in design and chemistry to the Trojan product, for about $260 to $270. Batteries in a Flash has the Fullriver 903, an absorbed glass mat maintenance free L16, for $450. That price is 50 percent higher than that for a flooded cell L16, but to some folks it may be worth it to never have to add water.
Accessories
One thing you will absolutely have to have is interconnection cables. Because these batteries are designed to be used as part of a battery bank rather than stand alone, each battery will need one cable to connect it to the next battery in the bank. Ask for these when you buy the batteries; they should cost no more than $15 each, and some suppliers may throw them in free.
Insider Tips
Although the siren song of the maintenance-free AGM and gelled electrolyte batteries tempt many buyers, they are much more expensive in the long run. Not only is the initial price higher, they also do not last as long as flooded-cell batteries, for one reason: all batteries degrade over time, and this degradation gradually reduces the storage capacity of the battery. But a flooded battery can be rejuvenated periodically by a controlled overcharge. This overcharge boils off some of the water, which must then be replaced. But sealed, maintenance-free batteries have no provision for replacing lost electrolyte, rendering the rejuvenating process impossible.
References
- "Power From the Sun: A Practical Guide to Solar Electricity"; Dan Chiras, Kurt Nelson; 2009
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