Deep Cycle Batteries

Check out the watering caps on this baby!

Flooded Lead-Acid (FLA) Batteries

Flooded lead-acid batteries, also called “wet cells”, are commonly used in solar, wind and hydroelectric RE systems. They are often the least expensive type of deep cycle battery and can last the longest. They also come in a wide range of sizes. In FLA batteries, a sulphuric acid solution is the electrolyte which reacts with the lead plates in the cells to produce electricity. When FLA batteries are recharged or equalized, the electrolyte fluid boils, producing hydrogen and oxygen gases in a normal process called “outgassing”. These gases may escape the cells through the filler/vent caps, creating two problems: the fluid loss means that the fluid level in the battery goes down, potentially exposing the normally “flooded” lead plates—not a good thing. So using FLA batteries in your system means that you are committed to regularly monitoring and maintaining them by adding distilled water to each cell as needed. Water Miser vent caps can help prevent some moisture escape, thus reducing watering frequency. The other problem is the presence and accumulation of explosive gases. Hydrogen and oxygen gases are extremely dangerous and must be properly vented—to the outside air. So it’s clear that with FLA batteries, battery maintenance is an essential part of system maintenance.

Sealed batteries

Don't try putting a FLA battery on its side like this!

One great advantage of sealed batteries is that they can be placed in any orientation: upright, on their sides, and, in some cases, even upside-down. In a sealed battery, the electrolyte is suspended in either an absorbed glass mat (AGM) or in a gel. Sealed batteries cost more than flooded lead-acid batteries and don’t last as many charging cycles; however, they are the preferred choice for applications requiring frequent battery handling, or where the system needs to be left unattended in a remote location. Like FLA batteries, sealed batteries have vents to allow hydrogen to escape when necessary, though this should not normally occur. The difference is that because the batteries are sealed, there is no way to replace the escaped moisture by adding water to the cells; that’s why a sealed battery won’t last as long as an FLA battery. Gel cell batteries have traditionally been a bit more expensive than AGM batteries; however, they have the unique advantage of performing better in very cold temperatures or in very deep-discharge applications.