Thirty years ago, an off-grid PV system was likely to consist of a couple of solar-electric modules and a pair of automotive or truck starting batteries. The system had no inverter, as reliability was still nearly a decade away, and the 12-volt system was likely to power little more than a few DC car-type tailights, a car stereo, and maybe a few RV appliances.
In those days without adequate how-to information, off-gridders were likely to use car batteries. The PV modules charged the batteries, but not being the right “tool” for the job, they often failed to hold their charge after only a year or two.
Home Power published its first issue in 1987, primarily to spread how-to and homebrew information for early solar pioneers. In the premier issue, Home Power founder Richard Perez writes, “After many battery failures and much time in the dark, we finally tried a real deep-cycle battery. These batteries were hard to find; we had to have them shipped in, as they were not available locally. In fact, the local battery shops didn’t seem to know they existed.”
The first deep-cycle batteries were not developed for remote home systems, but were adapted from other uses—golf carts, supermarket floor scrubbers, and mining cars. A few pioneers could afford to order industrial-grade “traction batteries” directly from battery manufacturers. Some off-grid pioneers reused batteries that were routinely replaced by railroads and telephone companies long before their life was up; these were once commonly available for a fraction of a new battery’s cost.
Our understanding of what works and lasts has improved considerably. Some of this information remains unchanged over the decades and some is newly emerging. This article will focus on batteries in off-grid residential applications, and their selection and sizing.
For off-grid applications, flooded lead-acid batteries are the most common, although sealed batteries are sometimes used (see “Basic Battery Concepts” sidebar).
Golf-cart batteries are mass-produced by the millions. Even with price increases in recent years, they remain the best low-cost choice for small systems. Seldom does a set last more than seven years; typically they last 4 to 5 years. However, they will stand up to remarkable abuse, including chronic undercharging and lack of equalization, still providing adequate service. For some systems, frequent replacement of a set is a reasonable, low-cost approach, and their size and weight (62 lbs., about the same as a large car battery) make them easy for a homeowner to handle.
At 225 Ah at 6 V, a 24 V set of four provides about 4 kWh of usable energy, so these batteries are only for small systems. Given their short life expectancy, I have seen the three-string limit (see “string sizing section”) exceeded without the shorter-life penalties of more expensive batteries, but even four strings yields only about 16 kWh. At 48 V, two strings offer the same kWh capacity; designing a relatively small off-grid system at high nominal voltage could be a good approach.
L16 batteries and similar “commercial” batteries were originally designed for use in supermarket floor scrubbers, where they were run all night and charged from the grid the next day. Their combination of size, deep-cycle performance, and relatively low cost makes them an acceptable choice for some RE systems. With three cells (6 V) and 350 to 400 Ah, these batteries are well-suited for small-to-medium systems. However, they are more expensive per Ah than golf-cart batteries without supplying substantially more cycle life; 5 to 8 years is typical. At 120 pounds, they can be moved by two people fairly easily.
Several manufacturers are now offering 2 V batteries using the same overall case dimensions as the 6 V models. Using a lower-voltage, higher-capacity battery means fewer strings in parallel for the same kWh storage (see “Battery String Sizing” section). Some are simply three cells connected internally in parallel rather than series; others are true 2 V cells with thicker plates and a longer cycle life.