Carbon: The Answer to Partial State of Charge & Maximizing Deep-cycle Batteries
Deep-cycle batteries are critical components of any renewable energy (RE) and inverter backup system that’s operating in off-grid or unstable grid locations. To meet tough environments and project conditions, such batteries are often heavily cycled at partial states of charge (PSOC)—meaning, they are usually never fully recharged on a regular basis. Unfortunately, operating at PSOC can quickly diminish the overall life of a battery, which results in frequent, costly battery replacements.
Addressing the issue of PSOC and its impact on battery performance and overall life has remained a challenge, at least until recently. Advancements in carbon technology and its addition to lead-acid batteries have been shown to extend the life of deep-cycle batteries, which is good news for off-grid renewables.
The problem with PSOC
A partial state of charge is a reality of most off-grid and unstable grid RE systems. Often times, solar panels used in these applications are undersized, preventing the batteries from achieving a full recharge. The same is true with intermittent weather conditions or placement of solar panels in shady areas, which also affects a solar project’s ability to fully recharge batteries.
PSOC is also common in inverter backup systems when batteries are not fully charged because, in this case, the grid tends to frequently go down. Since the grid is the main charging source for the batteries, the consistent unavailability of the grid prevents deep-cycle batteries from being fully recharged on a regular basis, resulting in a diminished lifespan.
Even telecom applications, which operate off-grid, rely on an unstable grid, or depend on a hybrid RE/battery system for power, face the same PSOC issues as do many solar installations.
Studies in carbon
Deep-cycle flooded batteries have become the most widely used battery technology in RE and backup inverter applications, primarily due to their widespread availability and economical price point. Seems only natural, then, that the addition of carbon formulas to these types of batteries as a means of prolonging lifespan is gaining significant interest.
Over the last several years, battery manufacturers have done extensive research on various types of technologies and methods to lessen the impact PSOC has on deep-cycle batteries. Much of the carbon additive research available, however, was conducted in mobile and stationary applications, such as VRLA batteries for start-stop automotive applications.
But, more recent studies have shifted in focus. With the increased use of deep-cycle, lead-acid batteries in RE applications, research has been concentrating on the inclusion of carbon. More specifically, researchers have looked at how the use of carbon additives can successfully address the negative impacts of PSOC applications.
The conclusion? According to the Advanced Lead Acid Battery Consortium (ALABC), an international research cooperative dedicated to the enhanced performance of lead-acid batteries: “Lead carbon batteries provide better performance in partial state-of-charge operations, making them optimal for applications requiring high-rate and recharge.” (Read more at: www.alabc.org/publications/vrlas-in-stationary-energy-storage)
The Battery Council International (BCI), a trade association representing leading battery manufacturing companies, has also released similar findings, stating that: “Newly developed carbon-based advanced lead-acid technology has the ability to provide high-energy efficiency and absorb charge rapidly, making it ideal for applications that operate at a partial state of charge” (http://batterycouncil.org). Moreover, BCI maintains that advanced lead acid batteries will support these applications at one-third of the cost of nickel cadmium and one-quarter of the cost of lithium ion batteries.
Carbon & PSOC
Engineering teams have been aware of the potential of carbon for some time now, and have been experimenting with a number of different types of carbon in various formulas for several years. The goal: to find the right carbon mix to properly address the effects of PSOC in deep-cycle batteries.
Developing a formula that provides improved performance when RE batteries are unable to fully recharge on a regular basis has been accomplished. In fact, market and technology research firms have conducted several independent studies on the use of carbon in lead-acid batteries. They have determined that the use of carbon on a battery’s negative plates dramatically reduces sulfation at PSOC, which is the leading cause of shortened cycle life of lead acid batteries.
However, though success with carbon and deep-cycle batteries has been achieved, it’s important to note that not all carbon additives are alike or provide the same level of performance improvement. The key, as with most any successful renewable energy project, is to research the available options. Carefully select batteries that not only offer enhanced performance but, more specifically, an improved charge acceptance and faster recharge in PSOC applications.
Deciding Factors: Purchasing deep-cycle batteries
There are several factors to consider when selecting a deep-cycle battery...
- Technology. It’s important to select the right battery technology for a project application, especially in terms of battery maintenance. If a site is easily accessible, then deep-cycle flooded batteries are a good choice due to their low cost and widespread availability. If the installation is in a remote area where regular maintenance cannot be guaranteed, then maintenance-free, deep-cycle AGM or gel batteries are the best options despite their higher cost.
- IEC Testing. Look for IEC and third-party test results. Select a manufacturer that employs outside testing companies to ensure the accuracy and validity of cycle life data. Testing to the IEC 61427 standard ensures batteries meet the rigors of the deep discharge and recharge cycles of renewable energy applications.
- Cycle Life. This is measured as the number of discharge/charge cycles the battery can provide at a specified percentage of its rated capacity. Batteries from different manufacturers may have the same capacity rating, but design, materials, process, experience, and quality control also heavily influence cycle life performance.
- Capacity. Review the capacity of a battery to ensure it’s properly rated for its intended application. A battery with insufficient capacity will over-discharge on a regular basis, resulting in a shorter life. An oversized battery is also a poor choice because it will usually cost more for no added value.
- Price. A battery with a low price is attractive, but if obtained at the expense of quality and cycle life performance, the cost over time will be significantly higher due to frequent battery replacements. It’s important to consider issues other than price when deciding on a battery for renewable energy applications. Look for a brand that offers a history of engineering and manufacturing quality, deep-cycle batteries that are specific to renewable applications.
The carbon performance
Although PSOC cycling will continue to be an issue in off-grid renewable energy and backup applications, there are now special carbon formulas available from battery manufacturers to address the challenge of PSOC. Studies show that certain carbon additives can provide up to 15% improved cycle life of batteries used in PSOC applications, compared to batteries without carbon. Just be sure to research the best carbon, deep-cycle battery for your project to ensure quality, reliability, and longevity.
Dean Middleton is the global director of sales for Renewable Energy at Trojan Battery Company.
Trojan Battery Company
www.trojanbattery.com
Author: Dean Middleton
Volume: May/June 2014