Sophistication versus Simplicity – System Design Considerations for Lithium-ion Batteries in Standby Power Applications

System architecture for traditional lead-acid and nickel-cadmium batteries has evolved in particular ways to meet specific application requirements. For example, valve-regulated lead-acid (VRLA) batteries for telecom outside plant (OSP) are deployed in multiple parallel strings, while vented batteries in utility substations are installed as single series strings. As users in these applications start to consider new technologies such as lithium-ion (Li-ion) there will be a natural tendency to maintain the same system architecture that has worked so well for them over the years. But is this the right decision? Li-ion battery systems include built-in monitoring and communication functions that are required to ensure operational safety. These functions provide a level of sophistication that is not seen in traditional batteries, and, combined with the increased energy density of Li-ion, can seem to make a compelling argument in favor of this new technology. However, with increased sophistication come new failure modes and new complications in battery application, and these issues can have a serious impact if the system architecture and battery integration are not modified accordingly. This paper addresses two applications – telecom OSP and utility substations – comparing existing battery solutions with Liion, and how the dc power system can be adapted and optimized to provide for successful application of Li-ion batteries. Recommendations are provided for the user to evaluate competing technologies, with particular reference to a soon-to-be published IEEE standards document. LITHIUM-ION BACKGROUND First shipped commercially around 1993, Li-ion technology has come to dominate successive applications for portable devices, first taking over the market for laptop computers, then mobile phones, then high-end digital cameras. Now this technology is penetrating the power tool market alongside the incumbent technologies. The earlier successes were driven by the superior energy density of Li-ion, and market gains in high-value applications such as laptops led to reduced costs, which in turn created new value propositions in the lower-end devices. More recently, the advent of high-power designs drove the adoption of Li-ion in the power-tool market. The electrochemistry of choice for most portable applications is based on lithium cobalt oxide in the positives and graphite in the negatives. However, Li-ion technology covers a broad range of electrochemical systems, as discussed in a recent Battcon paper. Longer lasting, more durable Li-ion technologies for electric vehicles have been under development for many years and are now starting to appear in production vehicles of various types, including hybrids, plug-in hybrids and full electric vehicles. At the same time the battery industry itself is being transformed. The US Department of Energy recently awarded grants totaling $1.5 billion under the American Recovery and Reinvestment Act of 2009 (ARRA) for manufacturers of battery packs and cells, and suppliers of battery components, raw materials and recycling services, mostly related to Li-ion technologies. So we have a situation in which consumers have become familiar with Li-ion technology, while the industry will soon experience a large increase in capacity in anticipation of the introduction of industrial type Li-ion batteries not only for automotive use but also for a broad range of stationary standby battery applications. The transition will be characterized by two major enhancements: • Battery size: shifting from batteries of less than 1 kWh to systems of tens or hundreds of kWh, with the first megawattscale systems already on test today.