EastGreenfields: Batteries for Renewable Energy System

Understanding Batteries for Your RE System

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Last Updated:

Aug 22, 2013

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.

Battery Types

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).

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Batteries to Suit Your System

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.

Details Make a Difference

Although all battery manufacturers use the same general processes, all batteries are not created equal. There are significant differences in the battery components used during the manufacturing process as well as the quality control processes each battery manufacturer implements within their facilities. Evaluating these differences is key in determining a battery’s longevity and performance, and this information can usually be found on a deep-cycle battery manufacturer’s website. If not, ask your sales rep or the battery manufacturer directly about specific manufacturing processes.

The working components of lead-acid batteries are their plates, which are made of lead. To harden the lead for use in a battery, it is combined with other metals (calcium, in VRLA batteries; antimony, in flooded batteries).

Lead oxide is used to produce the pastes, which include the positive active material (PAM) or negative active material (NAM) that is applied to a grid of lead that becomes the battery’s positive and negative plates. The chemical compositions of PAM and NAM are proprietary. Deep-cycle batteries have PAM and NAM recipes tailored for applications such as RE, propulsion (in EVs), or equipment like electric forklifts. Therefore, it’s important that a user understand what they want to achieve from their batteries—long float life or long cycle life. A battery’s design and paste formulations are customized for one or the other.

The battery’s lead grids are designed to hold the active materials (PAM and NAM). The grids also provide an electrically conductive path that enables the electrons to move in and out of the battery. Positive and negative grids are cast from molten lead alloys, and the patterns used in the design of grids vary from manufacturer to manufacturer. Grid designs and configurations help the current flow through the grid network, enhancing overall battery performance.

Once the positive and negative grids are pasted with PAM or NAM, they undergo curing, in which the paste is dried slowly. During this process, a bond is created between the paste and metallic grid, to develop an electrochemically active mass. Curing must be accomplished under precisely controlled temperature and humidity to produce a high-quality battery. The curing time can take as long as 72 hours to fortify the interlocking crystal structure of the PAM and NAM.

After curing, the plates are arranged in groups, alternating positive and negative. A separator is placed between each plate that prevents them from touching and shorting. The separator material’s quality and design are important factors in determining the battery’s service life. High-quality batteries use glass mat or rubber separators that reduce the chance of stratification, which is a typical mode of battery failure in renewable energy systems. Stratification is the process in which the acid concentration at the bottom of the battery becomes greater than the concentration at the top. If not addressed, stratification will destroy a battery.

After the appropriate number of positive and negative plates are stacked, they are placed into a plastic case, which contains one cavity for a 2 V cell, three for a 6 V battery, or six for a 12 V battery. The number of plates depends on the specific capacity of the battery being built, i.e., the greater the amp-hour capacity, the larger the number of plates. The positive plates are then welded together at the top, as are the negative plates, forming a single cell assembly. In multiple-cell batteries, each cell assembly is welded in series or in parallel to determine the battery’s voltage.

A cover is then heat-sealed to the case. The quality of this seal is important—if poorly done, the seal’s integrity can be compromised, leading to future problems such as breakage and/or separation of the lid from the battery case.

Once battery assembly is complete, the battery goes through “formation.” The formation process involves filling the battery with a solution of sulfuric acid and distilled water. The battery is then given an initial charge, which preps the battery plates for operation. Formation usually takes several days to complete; the greater the battery capacity, the longer the formation period.

When the formation process is complete, the battery is tested to confirm it meets the manufacturer’s specifications. This is called “end of line” testing, which ensures all electrical connections are working properly and there are no obvious defects. The battery is then cleaned of any residual acid on its external surfaces, labeled to be in compliance with all necessary regulations, and prepared for shipping.

Quality Control

The differences in the manufacturing processes for a deep-cycle battery can have a profound effect on its performance and life, as seen in the images that show high- and lower-quality batteries.

Both had the same rated capacity, were the same age, and experienced identical service under controlled conditions. However, the battery on the left has aged far more gracefully than the one on the right. Why? The battery cell on the left has a moss shield on the top of the plates, which prevents the plates from expanding and shorting out at the top of the cell. The absence of a top shield on the other battery eventually caused short-circuiting, leading to lower capacity and a shorter life span. The battery on the left also featured a paste formulation specifically designed to optimize use of the battery’s active material, resulting in sustained battery performance over a longer period of time.

Besides the presence of a moss plate, the design and use of a battery’s grid and separators are equally important. Corrosion can quickly kill a battery, and the thicker a battery’s grid structure, the greater its resistance to corrosion. It also is important to ensure that the grids are cast, rather than stamped. Cast grids have no hairline fractures, which can cause the battery to fail prematurely.

In addition, separators between the battery plates should feature wide channels to increase the flow of acid, which enables optimum battery performance. Batteries that feature separators with wide-channel designs will also offer greater resistance to stratification.

Outside Parameters

An essential component of determining the quality of a deep-cycle battery is the manufacturer’s investment in independent third-party testing. This data provides valuable information on product performance, and validates a manufacturer’s product claims. Independent, third-party data obtained from testing batteries per industry-recognized standards, such as by the International Electrotechnical Commission (IEC), allows consumers to compare products from different manufacturers, and can be found on some battery manufacturers’ websites.

For an RE application, the two parameters of critical importance are amp-hour capacity (commonly at the 20-hour rate) and the number of cycles the battery can deliver to a given depth of discharge (DOD)—for example, 3,000 cycles at 50% DOD.

Once the short list of possible battery options has been created, the next task should be to ensure the battery manufacturer has not only a proven track record, but also builds quality into each battery it ships.

The type of battery you choose and the vendor you purchase from can have a significant impact on the performance, durability, and total cost of a renewable energy system. The battery bank in an RE system typically represents the largest equipment cost over the life of the system, so proper understanding of deep-cycle batteries is important to maximize the return on your investment.

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