When shopping for batteries for a renewable energy (RE) system, it is important to choose a battery designed for deep-cycling, and one with a track record of long-term reliability and performance. Make sure to evaluate both the battery manufacturer’s reputation for using quality components, as well as the manufacturing process employed to assemble a deep-cycle battery.
While buying cheap batteries may seem a good idea at first, it can cost you much more down the road. Cheap batteries will not withstand the rigorous deep-cycling that is inherent in RE applications. Over time, these batteries will fail and need to be replaced more often than quality deep-cycle batteries, costing you more money in the long run. Lower overall cost of ownership and reducing the need to constantly replace batteries should be the determining factors, not just the initial purchase price.
A battery uses an electrochemical reaction to store energy. Unlike primary batteries that cannot be recharged, secondary batteries can be recharged many times before they reach the end of their life. Several types of secondary batteries are available in sizes appropriate for an RE system, including lead-acid, lithium ion, nickel-cadmium, and nickel-iron. However, lead-acid, deep-cycle batteries, specifically designed to be deeply discharged to 50% to 80% of capacity, are most often used due to their relative low cost and wide availability.
Flooded lead-acid (FLA) and valve-regulated lead-acid (VRLA, or “sealed”) are two types of lead-acid batteries. FLA batteries lose electrolyte as the electrolyte is converted from a liquid to a gas during charging, so the individual cells must be periodically topped off with distilled water to avoid permanent damage. VRLA batteries can be either absorbed glass mat (AGM) or gel and are maintenance-free—they cannot accept the addition of water. AGM batteries feature individual cells that contain positive and negative plates separated by a glass mat separator (see “When to Use VRLAs” sidebar).
Lead-acid batteries are made for specific discharge duty cycles. For example, a starting-lighting-ignition (SLI) battery (used to start vehicles) is designed to deliver high amperage for short durations, and then be recharged quickly. Similarly, a lead-acid battery that is part of an uninterruptible power system (UPS) may need to provide energy for just a few minutes when the utility experiences a grid anomaly or outage—usually only a few amp-hours are taken out of the battery. In contrast, a battery in an RE application must run various electrical loads for long durations, and possibly over several days. So for an RE application, choose a battery designed for a marathon, not a sprint.
Charging characteristics also influence battery choice. A SLI battery is designed to charge for a few hours at most (via the vehicle’s alternator) before the next engine-cranking event. Since a UPS battery has to be ready for discharge at any time, it is on a trickle (“float”) charge whenever the source is available. A deep-cycling RE battery will experience constant charging and discharging over several years. Because of this, the ideal battery for an RE application must be capable of delivering many cycles over a longer time than SLI, and can have some charging flexibility—either more slowly from an RE source or more quickly from a backup generator.