Imagine a BESS container installed in an industrial area, next to a warehouse where dozens of people work every day. Inside, there are hundreds of cells storing energy. Under normal conditions, everything runs smoothly. But what happens if a cell starts to heat up more than it should? This is where the battery safety it ceases to be an abstract topic and becomes a concrete responsibility for those who design, install and manage the systems.

In this article we answer a simple yet crucial question: how safe is a battery and what makes it so? The answer, in short, is that safety comes first and foremost from chemistry, and only afterwards from electronics.

Thermal runaway explained simply

The thermal runaway — known in Italian as “fuga termica” — is the chain reaction that everyone wants to avoid in a battery. It works like this: a cell overheats, beyond a certain threshold the internal chemical reactions accelerate on their own, the heat increases further, and the process becomes self-sustaining without ever stopping.

Think of a pan on the stove which, once a certain point is passed, keeps getting hotter even if you remove the flame. In the worst case, the cell releases gas, catches fire and can take neighbouring cells with it. Typical causes are an internal defect, mechanical damage, incorrect charging or an ambient temperature that is too high.

The key point is this: not all chemistries react in the same way. Some start to “run away” at relatively low temperatures, while others remain stable for much longer.

Why some chemistries are more stable than others

The crux of the matter lies in the cathode, which is one of the cell’s active materials. During thermal runaway, some cathodes release oxygen: it is like pouring petrol on a fire, because the oxygen fuels the combustion from within. Other cathodes, with more stable chemical structures, tend to trap the oxygen and do not fuel the reaction.

The so-called chemical substances polyanionic — a family to which many sodium-ion cells also belong — have a crystal structure in which the oxygen atoms are tightly bonded. This generally translates into a lower propensity to oxygen release and a higher ignition temperature. It doesn’t mean “zero risk”, but a wider margin of safety.

The exact ignition temperatures and comparative data between chemical families vary considerably depending on the cell design and must always be checked in the manufacturer’s reports [TO BE CHECKED: ignition temperatures and thermal stability data for the cathode].

The three factors that determine battery safety

When assessing the safety of a storage system, three parameters matter more than the others:

• Ignition temperaturethe threshold beyond which thermal runaway is triggered. The higher it is, the greater the margin before a problem occurs.
• Cathode oxygen evolution: the extent to which the chemical sustains the combustion on its own. The less oxygen it releases, the less intense the reaction.
• Intercell propagationthe ability of a “runaway” cell to infect neighbouring ones. Good pack design isolates the problem instead of letting it spread.

Taken together, these three factors tell us far more than a single figure from a technical data sheet. A cell with a high ignition temperature and low oxygen release already has a structural advantage.

Why safety is so critical near people

La BESS safety It does not carry the same weight in every context. It is one thing to have an isolated system in an open field, and quite another to have a storage system installed where there are people, assets and activities.

Consider these scenarios:

• Facilities residential, often in garages, cellars or plant rooms just a few metres from living spaces.
• Accumulate in warehouses and productive activities, where the risk adds to the industrial one.
• Containerised BESS in industrial areas, where tens or hundreds of kWh are concentrated within a small space.

In all these cases, reducing risk at the source — by choosing a more stable chemistry — means protecting not only the investment, but also those who live and work near the plant. This is why, for a designer or safety manager, the choice of chemistry is a strategic decision, not a detail.

The role of the BMS, pack design and certifications

Chemistry is the first line of defence, but it does not work alone. Around the cell are other levels of protection.

The BMS (Battery Management System) is the electronic system that constantly monitors the voltage, current and temperature of each cell. It intervenes before critical thresholds are exceeded, balancing the charge and disconnecting the system in the event of an anomaly. It is the “nervous system” of the battery.

La package design matter how much the BMS, spacings, insulating materials, heat dissipation systems and barriers between the cells are specifically designed to contain a potential problem and prevent propagation.

Finally there are the storage certifications, which certify the passing of standardised tests. Among the most relevant for this sector are IEC 62619 and UL 9540A, the latter specifically for thermal runaway propagation tests [TO BE VERIFIED: applicability and updated versions of the standards]. Ask for the reports of the thermal abuse test and of propagation It is good practice before any purchase.

Why stable chemistries start with an advantage

Putting the pieces together, the message is clear: you can add all the electronics you want, but if the basic chemistry is more stable you start from a position of advantage. Polyanionic and sodium-ion cells tend to offer greater thermal stability precisely because their structure is less prone to releasing oxygen and triggering low-temperature reactions.

To delve deeper into the technical aspects of the thermal stability of cathode materials, you can consult the studies published on ScienceDirect: first study e second study.

Naturally, safety is only part of the picture. How do these same cells behave under power stress or in intense cold? We will see that in a forthcoming in-depth article dedicated to performance in extreme conditions.

In summary

A battery is never “zero risk”, but there are choices that significantly increase its safety margin. Evaluating the trigger temperature, cathode behaviour, resistance to propagation, along with a robust BMS and serious certifications, allows you to install reliable storage systems even close to people.

Do you want to design safer storage systems? Discover the safety criteria to evaluate before choosing a battery.