How do you know which battery type is right for your battery-backed solar installation? In Solar Power World’s recent Webinar “Battery Backup for Grid-Tied PV Systems,” SimpleRay Solar’s Doug Pratt, Senior Engineer, explains how to properly choose and maintain batteries in solar applications. Here is a quick summary.
Doug Pratt, Senior Engineer, SimpleRay Solar
Battery Types
Battery-backed solar installation systems don’t need a large battery. In fact, because most utility outages are less than four hours, one to two days of battery capacity is probably sufficient. Typical life expectancy of a battery is 7 to 10 years, with top-of-the-line batteries reaching 12 to 15 years, but even those will eventually wear out. The key to remember is that all batteries need regular replacement, regardless of type.
Pratt warns consumers to stay away from nickel-iron batteries due to their inefficiency, even though they may seem like a viable option because of their long-life expectancies (50 to 70 years).
Having been around for more than a century, lead acid is the standard in solar battery selection, but lithium ion is starting to make its way into the marketplace too. Pratt says lithium ion is a great option when you need something small and lightweight, but it’s about four to five times more expensive than lead acid. For that reason, lead acid batteries are going to be the best option for standard battery-backed solar installations.
Consumers have a choice between two types of lead acid batteries: wet-cell or sealed, and a choice within sealed batteries between absorb glass matt (AGM) and gel. Sealed batteries, particularly AGM types, are able to withstand long periods of float charging and inactivity, making them ideal for backup systems that could spend years only float charging.
While wet-cell batteries are less expensive, they need to be run more frequently than sealed batteries, and typically have short life-spans when put into backup storage.
Of the two options, Pratt recommends using AGM batteries for best results with standard backed-up systems. Most of those systems recommend using four D size AGM batteries, which will generate about 9.5 kW hours of usable storage space, enough to cover an average American home consuming 20 kW of energy per day.
Maintenance
Keeping air away from the battery terminals is the best way to prevent buildup from forming, ensuring a long-life for your battery. Swabbing the terminals with Vaseline or grease creates a barrier that blocks the emission of sulfur compounds from the battery, which in turn prevents corrosion on the battery terminals.
Pratt also suggests deliberately cycling batteries every three months for optimal performance. Turn off the utility power to the inverters, switch off the PV and have the system run solely on the batteries until the charge goes down to about 50% to 60%.
Another thing to keep in mind regarding maintenance is that the terminals are lead, and lead is a soft metal that gradually shifts over time. To prevent this, “snug up” the terminals and wipe down the tops of the batteries with a wet sponge once a year.
“Lead acid batteries like to be exercised,” Pratt says. “Think of them like the muscles of your body. We all need modest amounts of exercise in order to stay health, and it’s the same with batteries.”
Want more information on backup batteries? Try these:
Webinar: Battery Backup For Grid-Tied PV Systems
The Basics Of Battery Back-up In Solar