Demystifying safety for UPS Li-ion battery systems

Demystifying safety for UPS Li-ion battery systems

Gareth Hackett


Li-ion batteries can provide high-performance backup in uninterruptible power supply (UPS) systems at data centres, as well as savings in cost and carbon dioxide (CO2) emissions. However, some operators are holding back due to concerns over safety. 

Gareth Hackett, Saft’s expert in large-format lithium-ion (Li-ion) batteries, explains how to specify Li-ion batteries that provide safety as well as performance. 

Li-ion battery technology has the ideal performance to deliver high-performance backup power from a compact and lightweight footprint. However, as the Li-ion battery industry has come to maturity, there have been some high-profile safety incidents. For example, a fire in an energy storage system (ESS) at Arizona Public Service facility in 2019 and Top Gear presenter Richard Hammond’s escape from a burning supercar. 

What these incidents have in common is thermal runaway, where a short circuit inside one cell creates high temperatures. This propagates into other cells and causes them to break down, releasing hot flammable gases. 

Safety relies on avoiding thermal runaway from ever initiating. Selecting an inherently safe Li-ion chemistry is a critical step. However, it is also vital not to overlook the importance that good mechanical, electrical and electronic design and construction can have on managing battery performance and containing heat. 

Li-ion family 

What makes Li-ion complex is that it is not a single type of battery. It is an umbrella term for an entire family of chemistries. These chemistries each have their own performance profiles, with relative advantages and disadvantages between the parameters of calendar life, power density, energy density, high and low temperature performance, and safety.  

These chemistries can be used individually or blended together to achieve a suitable balance of properties for a given application. 

The largest branch of the Li-ion family tree covers metal oxides, with the other two branches in use being iron phosphates and titanates. Titanates are relatively niche and are currently used for railway traction and powerful off-road vehicles. 

Metal oxides versus phosphates

Metal oxides such as lithium nickel cobalt aluminium oxide (NCA) and lithium nickel manganese cobalt oxide (NMC) have particularly high energy density. This makes them suitable for high-power compact batteries such as electric vehicles, where it’s possible for people to have the need to escape. At Saft, we use NCA for satellite and space exploration batteries as it has the highest density. 

The safety drawback with metal oxides is that they contain oxygen. It can be released in the event of an internal short circuit, creating a thermal runaway situation as temperatures can reach 800 to 1000°C. The release of oxygen from the metal oxide blend makes it virtually impossible for a nitrogen-based or a fluoroketone fire suppression system to work effectively. 

Even with good design, the potential for thermal runaway leading to propagation makes metal oxides unsuitable for safety-critical sites such as data centres. 

When safety is critical, it’s essential to cut the risk by specifying phosphate chemistry such as lithium iron phosphate (LFP) or a blend of super lithium iron phosphate (SLFP). These do not contain oxygen and therefore the temperature never exceeds 250˚C, preventing thermal breakdown in the cell and making propagation to other cells extremely unlikely.  

In a data centre environment, a system based on phosphate technology will provide a calendar life of around 20 years. Even in cases where the ambient temperature reaches 35°C, the battery can be expected to last for 14 years. The drawback is that their energy density is around 30% lower than metal oxides and cell voltage is smaller. 

Building safety 

The other aspect of battery selection is whether a technology can meet building standards. 

Standards such as IFC 2018 and NFPA 855 control the potential risk by demanding that the energy content of Li-ion batteries is limited to 20 kWh per system or 600 kWh per installation and that an air gap of 3 feet (around one metre) separates each system. 

However, operators may need larger and more powerful systems and this is where stringent testing can be carried out under the UL 9540A test method for evaluating thermal runaway. It covers a demanding set of tests to evaluate battery systems and their ability to withstand failure at the level of individual cells, modules and strings, as well as overall installations. 

Battery systems based on metal oxides have been found to pass UL 9540A testing in spite of propagation between cells and modules. There have even been cases during this testing where the battery module and some small internal cell components have been ejected during thermal runaway.

An additional point is that a recommendation to come out of the Arizona Public Service fire was that air extraction systems should be able to handle the volume and temperature of gas generated during an incident. Phosphate-based systems produce significantly less gas at lower temperatures than oxide systems, making them more practical when it comes to ancillary systems. 

Battery system design

Battery safety also relies on the mechanical design and manufacture of battery cells, as well as the protective casings and electrical connectors. Cell format and design can have a large impact on energy density, power density, longevity and safety. 

In a battery system, the cells are combined in modules and strings with an electronic battery management system (BMS). Its role is to provide protection and communication with a UPS controller and also inside the battery system. 

An important function of the BMS is to monitor the voltage and temperature profile across the battery at the level of individual cells, as well as modules and strings. The BMS will then manage the charge or discharge of the cells with the aim of maintaining an even temperature across the entire system. 

The BMS controls safety risk by avoiding excessive temperatures, but it has the additional benefit of extending the battery life. Temperature affects the life of cells, so ensuring an even temperature will ensure all the cells will age at the same rate and will provide predictable performance. 

For large-scale batteries with strings housed in multiple cabinets, a master battery management system (MBMM) will oversee performance across all strings. 

Prioritising servers

Another important point for battery management is that it is worth specifying a battery that is dedicated to data centre service, rather than for energy storage system (ESS).  

ESS batteries are designed as valuable assets for generating income – and as such, their BMM contains a fail-safe that prevents excessive discharge. 

In comparison, a UPS battery’s job is to prevent the cost of power loss. The Uptime Institute Global Survey of IT and Data Center Managers 2020 found that four in 10 outages cost between $100,000 and $1 million – and about one in six costs over $1 million. The impact can potentially be even greater when indirect costs and reputational damage are considered. 

For data centres that are mission-critical, operators need a battery system that can be discharged completely, supporting every last second of server time possible, even if that sacrifices the battery. 

Ultimately, a well-specified Li-ion battery system can ensure there is no compromise between safety and backup performance. 

Gareth Hackett has written an in-depth white paper about safety aspects of Li-ion batteries in mission-critical UPS systems for data centres that can be downloaded here

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