Battery type. Flooded lead-acid (FLA) batteries are the most common type used in RE systems, particularly off grid. They are the least expensive per capacity and, if well maintained, can have a relatively long life span. However, they require the most maintenance. Distilled water needs to be added to the cells on a regular basis, depending upon how often and how deeply the bank is cycled, and upon battery charging regimens.
Valve-regulated lead-acid batteries (VLRA, a.k.a. sealed batteries). Two general types of VRLA batteries are available for RE systems—absorbed glass mat and gel cells. Absorbed glass mat (AGM) lead-acid batteries are similar in chemistry to FLA cells. In their construction, glass mats, placed between the lead plates (anodes and cathodes), allow the electrolyte to be suspended close to the plates’ active material. These sealed batteries offer the advantage of not needing to be watered and greatly reduced gassing during charge cycles. This type of construction—adding glass mats, sealing the cells, and constructing the plates to operate with less electrolyte—increases cost while potentially shortening life span.
Gel cells use a “gel”-type electrolyte—with a silica additive that causes the liquid to stiffen. Gel-cell batteries are also sealed, which means no water to add—less maintenance and less gassing. However, because lost electrolyte cannot be replaced, they also have a shorter life. They are typically more expensive than FLA or AGM batteries.
Because AGMs can’t be watered, they have to be charged more lightly to avoid using up the finite amount of electrolyte they contain. Gel cells also aren’t watered but need to be charged even more lightly to avoid drying out the cell, which will kill it.
So why would you ever choose shorter-lived, more expensive batteries like AGM or gel cells? The reasons vary, but often portability, poor battery area ventilation, and maintenance are factors. AGM and gel cell batteries have no liquid electrolyte to spill, so they can be a good choice for mobile systems. And because they hardly gas, they can work well in places where adequate ventilation for FLA batteries isn’t possible. Because they are freeze-resistant, they may be a good choice in applications where extreme cold is a factor.
AGM batteries are often the best choice for grid-tied applications with battery backup, since they are designed for float or standby applications. Because low-capacity battery banks are typical in backup applications, both decreased cycle life and increased cost can be offset by the fact that these batteries are rarely cycled. Plus, users with grid-tied systems are usually less inclined to pay attention to the battery maintenance, since they are also unaccustomed to “maintaining” their grid power. Finally, VRLA batteries will outlast FLA batteries that are not maintained properly (i.e. not watered regularly). If batteries are to be deeply cycled (50% to 80% DOD), gel-cell batteries may offer a longer life (more overall cycles) than AGMs.
Nominal Battery Voltage. Lead-acid batteries are built from individual cells with a “nominal” voltage of 2 V. Battery packs for RE systems are made up of combinations of cells to achieve nominal battery bank voltages of 12, 24, or 48. When designing small systems (loads less than 1,000 WH per day), 12 VDC is often selected as a nominal battery bank voltage if that system is not projected to grow. So a system for a hunting cabin that isn’t going to become a vacation home will keep battery costs down by having this low-voltage design.
For systems with heavier load profiles, larger (and more electrically efficient) battery voltages of 24 and 48 are commonly used. With commercial deep-cycle batteries (like golf cart and L16), the basic unit is often a 6 V battery made up of three, 2 V cells. In the medium-to-large systems, these 6 V units are typically combined in series (four for a 24 V string; eight for a 48 V string). To get greater AH capacity at that voltage, additional strings are then paralleled or higher-capacity batteries are selected.
Amp-Hour Capacity. The sizing of the battery bank depends on the storage capacity required, the maximum discharge rate at any time, the maximum charge rate, and the temperatures at which the batteries will operate.