Safety in Li-ion batteries for UPS is a matter of choice

When a supply outage strikes, back-up batteries keep the data centre operating until the UPS can apply an alternative power source such as a diesel generator. If serious damage to the business and its reputation is to be avoided, confidence in battery reliability and performance is essential. Given their well-established superiority over the VRLA (valve-regulated lead-acid) alternative, Li-ion batteries make perfect sense in this crucial role.

An important point to recognise at the outset is that ‘Li-ion’ is an umbrella term for a whole family of batteries whose electrochemistry varies enormously. Specifying the most appropriate chemistry, design and set-up is the key to achieving each application’s optimum battery characteristics. For data centres, that includes maximising safety.

Why is Li-ion better?

Li-ion is more reliable and offers much higher performance. Its total cost of ownership (TCO) is lower, thanks to a combination of longer calendar life, minimal maintenance needs, high energy efficiency and tolerance of higher temperatures. In comparison, the reliability and lifetime of VRLA (valve-regulated lead-acid) batteries reduces drastically at elevated temperatures, which in turn means more cooling, with higher energy consumption and higher CO2 emissions.

High power and energy density also enable further savings on real estate and infrastructure. Being smaller in size, and six times lighter, they take up less space and require less structural support.

When called upon, they discharge power rapidly to meet the UPS system’s needs. Between outages, they charge quickly – ready for the next emergency. As their electronic monitoring and management systems integrate easily with those of the building, operators are kept fully aware of their condition and availability. At any time, you can be sure of their state of charge (SOC) and remaining calendar life, also known as state of health (SOH). Their electronics also enable easy scalability and optimisation for the application, in terms of voltage, power and energy.

What can go wrong?

Damage or misuse may lead to a short circuit in a Li-ion cell, leading to a chain reaction, known as thermal runaway. This produces a large discharge of heat which, if propagated into neighbouring cells, can initiate their breakdown and to release of hot flammable gases.

Widely reported thermal runaway incidents include an Arizona battery energy storage system fire in 2019 and TV presenter Richard Hammond’s escape from a burning supercar in 2017 during shooting for Amazon’s The Grand Tour. Importantly, the Li-ion battery specification in both cases was different to that which would be recommended for data centres.

Which chemistries are safest?

While we talk about Li-ion batteries, the term actually covers a whole family of chemistries with different characteristics, such as lifespan, power and energy density, and ability to operate across a wide temperature range. These chemistries can be selected and even blended to adjust the balance of these properties. A battery’s ability to manage performance and contain heat is also heavily influenced by its mechanical, electrical, and electronic design.

There are two main Li-ion battery types, named according to their cathode material: metal oxides and iron phosphates. Another group, with titanate anode material, is used in high-power applications with frequent charge and discharge cycles, like railway traction or underground mining vehicles.

Metal oxides, including lithium nickel cobalt aluminium oxide (NCA), lithium nickel manganese cobalt oxide (NMC) and lithium manganese oxide (LMO), offer the highest energy density. They are ideal in electric vehicles, for instance, as their chemistry is highly active – but the downside is that in a thermal runaway event, they release oxygen. This can feed the fire, potentially enabling temperatures to reach 800 or even 1,000⁰C. And because they release oxygen, oxygen-reducing fire suppression systems and advanced extinguishing agents like fluoroketones are not effective in this situation.

Iron phosphates, such as lithium iron phosphate (LFP) and Saft’s own proprietary Super Lithium Iron Phosphate (SLFP), are inherently much safer. The oxygen in their phosphate molecules is tightly bonded and is not released in combustion. This limits potential temperatures of thermal runaway events to around 200 to 250⁰C, making propagation between cells unlikely. The downside is a lower energy density (about 30% less) and lower cell voltage compared to metal oxides, but they are ideal when safety is critical. They also offer long service life, even at high temperatures, and good discharge/recharge cyclability.

What UL and IEC safety certification applies?

International standards IFC 2018 and NFPA 855 for fire safety in buildings seek to reduce risk by limiting Li-ion battery energy content to 20 kWh per system or 600 kWh per installation. They also demand an air gap of around a metre between cabinets.

However, many data centre operators need larger systems. Approval for unlimited energy content, without spaces, can be gained by passing rigorous UL 9540A tests on thermal runaway potential.

When tested to failure, some metal oxide systems have been reported in UL 9540A test records as experiencing cell-to-cell and module-to-module heat propagation, flaming, thermal runaway, and even ejection of small internal cell components.

By contrast, Saft’s Flex’ion phosphate-based system passed UL 9540A testing without cell-to-cell propagation or flaming under similar conditions.

During an incident, phosphate-based systems vent much lower amounts of combustible and other harmful gases, and these remain cooler, allowing easier and safer removal by gas extraction systems.

For further safety assurance, Flex’ion has achieved UL 1973 certification, passed IEC 62619 testing, and complies fully with IEC 62485-5.

What role does battery management play in safety?

Aside from their electrochemistry, there are physical differences between Li-ion battery systems which affect their properties. Each Li-ion battery consists of a collection of cells, together with battery management system (BMS) electrical circuitry, enclosed within a protective case.

The BMS is vitally important to optimising safety, reliability and TCO.

A good BMS will monitor both voltage in each individual cell as indicators of its charge status, health, and safe condition. Through control of charging and discharging at the cell level, it evens out temperatures across the battery to maximise longevity as temperature is closely related to battery ageing. It should also monitor and manage the temperature of the power connections, which can be higher than cell temperature during discharge.

Each single cabinet has one Battery Management Module (BMM) to oversee multiple battery modules. However, when scaling up, the BMS may combine multiple BMMs across multiple cabinets to establish a master battery management module (MBMM) governing the whole system..

How do UPS batteries differ from other applications?

A further point is that ideally, Li-ion battery systems used for data centre UPS applications should be specifically designed for that role. Their primary purpose is very different from that in a battery energy storage system (BESS), for example. BESS batteries are income-generating assets which must be protected, so their BMS will prevent them ever being discharged fully – a state that Li-ion batteries cannot recover from.

However, batteries in a data centre UPS are there to protect the business by ensuring continuity of power supply. If necessary, their BMS will permit complete discharge to gain vital extra seconds of power in an emergency, even if this means sacrificing the battery. In back-up provision as well as safety, there must be no compromise.

More information is available in Saft’s white paper on safety aspects of Li-ion batteries in mission-critical UPS systems for data centres.