A second consequence of temperature’s effect on battery performance is that a cold battery must be charged to a higher voltage than a warm battery to achieve the same SOC. All modern, quality charge controllers have temperature compensation to adjust the charge voltage according to the battery’s temperature. In most cases, the sensor is attached to the battery terminal or case. Unless a battery’s temperature remains constant, a temperature sensor is an important component of an RE system. Without temperature compensation, cells will not reach 100% SOC in winter. In summer, they can be overcharged, reducing battery life.
While there are tremendous advances taking place in battery development, most are based around increasing a battery’s performance and capacity per pound—that is, lightweight, high-capacity batteries for electric vehicles and portable applications of all kinds. In the RE industry, weight isn’t a key factor; a more relevant figure has been energy density per dollar, and conventional flooded lead-acid batteries have filled this bill the best.
Battery choices have been slow to evolve, due to a unique quandary: their longevity. Since deep-cycle batteries can last 15 to 20 years, learning by experience what works best can take decades. Plus, there’s only a handful of long-time off-grid installers who have been selecting, installing, and maintaining batteries for long enough to actually compare and learn from entire battery life cycles. In the absence of long-term data, we tend to use what has worked previously, rather than trying new and possibly expensive approaches.
A relatively new shift is the use of high-quality sealed batteries in some off-grid applications, but there isn’t yet a large body of experience from which to draw conclusions and predict performance. The expectations of some old-timers are that high-quality, maintenance-free AGM batteries may last about seven to 10 years in full-time off-grid use, with good care.
Allan Sindelar installed his first off-grid PV system in 1988. In 1997, he founded Positive Energy of Santa Fe, New Mexico. He has lived off-grid since 1999. Allan is a licensed commercial electrician and a NABCEP-certified PV installer.
“Top Ten Battery Blunders: And How To Avoid Them” by Windy Dankoff in HP114
“Flooded Lead-Acid Battery Maintenance” by Richard Perez in HP98
“Designing a Stand-Alone PV System” by Khanti Munro in HP136
“Toast, Pancakes, and Waffles: Planning Wisely for Off-Grid Living” by Allan Sindelar in HP133
For a listing of battery manufacturers and their products, see “Choosing the Best Batteries” in HP127