Lithium-ion batteries are attracting a lot of interest as an alternative to lead-acid batteries for use with data center UPS systems. One of the most attractive aspects of the technology is the projected life expectancy. They don’t just offer a slight improvement over lead-acid batteries; their longer life can potentially eliminate at least two replacement cycles.
The problem, of course, is that there are currently no lithium-ion batteries that have been operating in data centers anywhere near that length of time. The technology in this application is just too new. Without real-world operating data to reference, how can data center operators have confidence that lithium-ion batteries will deliver on their projected life expectancy?
That’s one of the most common questions we get from customers considering a transition to lithium-ion batteries. To address the concern, it’s necessary to understand a little bit about how lithium-ion batteries degrade in data center applications.
Lithium-Ion Battery Degradation
Lithium-ion batteries have two largely independent modes of degradation: cycle life and calendar life. In data center applications, battery cycling is typically infrequent and calendar life is the primary driver of useful life.
Calendar life describes how the capacity of the battery declines over time. As with other battery technologies, the temperature is an essential factor in calendar life. A lithium-ion battery operating in higher temperatures will degrade faster than the same battery operating at lower temperatures.
Since lithium-ion batteries have been used in automotive applications for more than a decade, battery manufacturers today have a large volume of calendar life data, both under controlled lab and in real-world conditions. This data shows that lithium-ion batteries degrade predictably over time and are not subject to sudden drop-offs below a certain capacity level. The data is also more than sufficient to enable high-confidence projections about lithium-ion battery life, particularly in data centers where the ambient temperature is far more controlled than in an automotive context.
So, while we can’t point to data center applications where lithium-ion batteries have been in operation for twelve or thirteen years, we can apply other experiences to set realistic expectations for battery life based on the specific battery system and application.
The Relationship Between Degradation and Runtimes
Another common question we get relates to the impact of degradation on runtimes as the battery ages. In other words, Is a lithium-ion battery susceptible to surprise failures late in life similar to lead-acid batteries?
As I mentioned previously, lithium-ion batteries degrade predictably, and the rate of degradation actually decreases as the battery ages. As a result, runtimes are not likely to be impacted by a rapid decline in capacity. However, lithium-ion batteries also experience increases in their internal resistance as they age, just as lead-acid batteries do. If the heat generated by that resistance creates temperatures that exceed the threshold established in the battery management system (BMS), the BMS will limit runtimes to protect the battery from overheating. Nevertheless, the projected changes in runtime with lithium-ion batteries are more predictable and less dramatic as they age than is the case with lead-acid batteries.
Lithium-Ion Battery Safety
It seems that everyone has read or heard about lithium-ion batteries in laptop computers or other devices catching on fire, and we often get questions on lithium-ion battery safety.
Lithium-ion batteries do have a few known conditions that can lead them to ignite or release gases if the internal pressure gets too high. Knowing these risk conditions and controlling them is the primary purpose of the BMS that is integrated into the lithium-ion battery system. Continuous monitoring for reliability is an extremely valuable, but secondary function of the BMS.
The riskiest abuse conditions are generally overcharge, overheating and short-circuiting of the battery cells. The BMS implements controls that keep the battery within its safe operating range based on continuous measurement of cell voltages, system temperatures, battery current and other parameters. UL or CE certification of the battery system verifies the functionality and effectiveness of the BMS. As an example of how capable the BMS is at performing this function, manufacturers typically have to disconnect the BMS when performing fire testing because they are unable to start a fire within the battery when the BMS is operational.
We address each of these issues in more detail, as well as other common questions, in our new white paper, Considerations for Evaluating Lithium-Ion Batteries in Stationary Applications and the companion piece, Frequently Asked Questions About Using Lithium-Ion Batteries in UPS Applications.