For off-grid systems, you have three options: (1) deep-cycle flooded lead-acid battery like those made by Trojan, Rolls and Deka (Figure 7.2), (2) forklift batteries, and (3) golf cart batteries. Car batteries won’t work. Their thin lead plates are not designed for the deep discharges that commonly occur in renewable energy systems. Although the lead sulfate crystals that form on the plates of a battery during deep discharge are removed when batteries are recharged, some crystals fall off before recharge occurs. The thin plates of a car battery are whittled away to nothing very quickly.
After twenty or so deep discharges, the batteries would be ruined — no longer able to accept a charge.
Battery-based wind and solar systems require deep-discharge lead-acid batteries with thick lead plates. It’s the thickness of the plates that allows them to withstand multiple deep discharges. Even though the plates lose a little lead over time, they are so thick that the small losses are insignificant. Consequently, deep-cycle batteries can be deeply discharged hundreds, sometimes a few thousand times, over their lifetime.
Fig. 7.2: Deep-Cycle Lead-Acid Batteries. These batteries contain thick lead plates and are used in many batterybased renewable energy systems. The thick plates permit deep cycling so long as the batteries are recharged soon after each deep discharge.
For optimum long-term performance, deep-cycle batteries still need to be recharged promptly after deep discharging. Don’t forget this! With proper care, these batteries could last for seven to ten years, perhaps longer.
Forklift batteries are high-capacity, deep-discharge batteries designed for a fairly long life and operate under fairly demanding conditions. They can withstand 1,000 to 2,000 deep discharges — more than many other deep-cycle batteries used in renewable energy systems — and thus work well. They are, however, rather heavy, bulky and expensive. If you can acquire them new at a decent price, you may want to use them.
Golf cart batteries may also work. Like forklift batteries, golf cart batteries are designed for deep discharge. However, they typically cost a lot less than other heavier duty deep-cycle batteries. While the lower cost may be appealing, golf cart batteries don’t store as much electricity and don’t last as long as the alternatives. They may last only five to seven years, if well cared for. Shorter lifespan means more frequent replacement. More frequent replacement means higher long-term costs and more hassle.
What about Used Batteries?
Another option is used batteries. Although they can often be purchased inexpensively, they’re rarely worth it. Used batteries are often being sold because they’ve failed or have experienced a serious decline in function. As a buyer, you also have no idea how well — or how poorly — they’ve been treated. Have they been deeply discharged many times? Have they been left in a state of deep discharge for long periods? Have they been filled with tap water rather than distilled water? Although there are exceptions, most people we know who’ve purchased used flooded lead-acid batteries have been disappointed.
When shopping for batteries for a renewable energy system, look for high-quality deep-cycle batteries. Although you might be able to save some money by purchasing cheaper alternatives, including used batteries, frequent replacement is time consuming.
Batteries are heavy and it takes quite a lot of time and effort to disconnect old batteries and rewire new ones. Bottom line: the longer a battery will last — because it’s the right battery for the job and it’s well made and well cared for — the better!
Sealed Batteries
Grid-connected systems with battery backup often incorporate another type of lead-acid battery, known as sealed lead-acid batteries or captive-electrolyte batteries. Sealed batteries are filled with electrolyte at the factory, charged, and then permanently sealed. This makes them easy to handle. They can be shipped without fear of leaking. They won’t leak even if the battery casing is cracked, and they can be installed in any orientation — even on their sides. But
most important, they never need to be watered.
Two types of sealed batteries are available: absorbed glass mat (AGM) batteries and gel cell batteries. In absorbed glass mat batteries, thin absorbent fiberglass mats are placed between the lead plates. The mat consists of a network of tiny pores that immobilize the battery acid. These tiny pockets also capture hydrogen and oxygen gases given off by the battery when it is charging. Unlike a flooded lead-acid battery, the gases can’t escape. Instead, they recombine in the pockets, reforming water. That’s why sealed AGM batteries never need watering.
In gel batteries, the sulfuric acid electrolyte is converted to a substance much like hardened Jell-O by the addition of a small amount of silica gel. The gel-like substance fills the spaces between the lead plates.
Sealed batteries are also known as “maintenance-free” batteries because fluid levels never need to be checked and because the batteries never need to be filled with water. They also never need to be (and should not be!) equalized, a process discussed shortly. Eliminating routine maintenance saves a lot of time and energy. It makes sealed batteries a good choice for grid-connected systems with battery backup. In these systems, batteries are rarely used and maintained like those in a grid-connected system. Sealed batteries are also ideal off-grid systems in remote locations where routine maintenance is problematic — for example, rarely occupied backwoods cabins.
Sealed batteries offer several additional advantages over flooded lead-acid batteries. They charge faster and do not release explosive gases, so there’s no need to vent battery rooms or battery boxes where they’re stored. In addition, sealed batteries are much more tolerant of low temperatures. They can even handle occasional freezing, although this is never recommended. Sealed batteries selfdischarge more slowly than flooded lead-acid batteries when not in use. (All batteries self-discharge when not in use.)
Unfortunately, sealed batteries are much more expensive, store less electricity, and have a shorter lifespan than flooded lead-acid batteries. They also can’t be rejuvenated (equalized) if left in a state of deep discharge for an extended period. During such times, lead sulfate crystals on the plates begin to grow. Large crystals reduce a
battery’s ability to store electricity. Batteries then take progressively less charge and have less to give back. Over time, entire cells may die, substantially reducing a battery’s storage capacity.
Large crystals on the plates of flooded lead-acid batteries can be removed by a controlled overcharge, a procedure known as equalization. Although equalization is safe in unsealed flooded lead-acid batteries, it results in pressure buildup inside a sealed battery. Pressure is vented through the pressure release valve on the sealed battery, which releases electrolyte and could destroy or seriously decrease the storage capacity of the sealed battery. So, while maintenance- free batteries may seem like a good idea, they are not suitable for many applications.
