Lithium-ion battery storage (BESS) — thermal runaway, cell propagation, and why an ordinary hand extinguisher is not enough

The guide on home batteries & energy storage systems covers the installation and registration requirements of a battery energy storage system (BESS) in general terms, and mentions thermal runaway as the underlying safety risk addressed by IEC 62933-5-2. This article looks deeper at that specific risk: what actually happens during thermal runaway, and why the usual response to an electrical fire — a CO₂ or powder extinguisher — is not the appropriate first response for a lithium-ion BESS.

What thermal runaway is

Thermal runaway is a self-reinforcing, exothermic reaction inside a lithium-ion cell: once the internal temperature of a cell crosses a critical threshold due to a trigger (mechanical damage, overcharging, an internal short circuit, an external heat source, or a manufacturing defect), the cell's internal materials break down at an accelerating rate — a reaction that itself generates heat, which raises the temperature further, which accelerates the reaction further. Once begun in a single cell, this process typically can no longer be stopped by simply switching off the current.

Cell propagation: why one cell does not stay one cell

A lithium-ion battery pack consists of many cells packed closely together. The heat that one cell in thermal runaway produces can transfer by conduction to neighbouring cells and push those past their own critical temperature too — cell propagation that can spread through a battery pack if the construction offers insufficient resistance to it. This is exactly the behaviour that IEC 62933-5-2 tests and assesses when certifying a stationary BESS: how the system behaves when one cell enters thermal runaway, and whether/how quickly that propagates to the rest of the pack.

The gas risk before any flame is visible

Before a cell in thermal runaway actually catches fire, flammable and partly toxic gas (electrolyte vapour, carbon monoxide, hydrogen, hydrocarbons) already escapes through the cell's vent. In an enclosed or poorly ventilated space this gas can accumulate into an explosive mixture even before there is visible smoke or flame — a risk separate from the eventual fire hazard, which makes ventilation of the installation space just as relevant as for the stationary lead-acid batteries in the guide on battery-room ventilation — albeit with a different gas composition and a different underlying standard.

Why an ordinary hand extinguisher is not enough here

CO₂ does not extinguish reliably: a burning lithium-ion cell produces its own oxygen during the reaction, independent of the surrounding air — the smothering principle a CO₂ extinguisher normally relies on (displacing ambient oxygen) therefore does not work reliably, and the fire can reignite after appearing to be out.
Powder extinguishers do not cool: a powder extinguisher interrupts a flame at the surface, but does not cool the cell itself — with an underlying thermal-runaway reaction in a battery pack, cooling is exactly what is needed to slow further cell propagation, not merely suppressing visible flames.
Large quantities of water for cooling are, in practice, the most common response of fire brigades to a BESS or EV battery fire: not to directly stop the chemical reaction, but to lower the temperature of neighbouring cells and thereby slow further propagation — often combined with prolonged cooling and monitoring, because a lithium-ion battery fire can reignite hours after appearing to have been extinguished.
Specialised extinguishing agents (based on encapsulating additives in water) have been developed specifically for lithium-ion applications and are cited by fire-safety organisations as more effective than standard agents, but they are not a substitute for limiting the risk through the installation itself.

Practical relevance

For siting and periodic assessment of a BESS (home battery or larger scale), relevant factors include sufficient distance to escape routes and combustible material, a layout that limits propagation to other parts of the pack or adjacent spaces (compartmentation), functioning ventilation against gas accumulation, and — just as important — awareness among users and first responders that a BESS in thermal runaway is not a situation for a small hand extinguisher: in an actual thermal-runaway event, evacuating and calling the fire brigade is the first step, not attempting to extinguish it yourself.

Common mistakes

Treating a lithium-ion BESS fire as an ordinary electrical fire with a CO₂ or powder extinguisher, assuming this is sufficient — the self-sustaining oxygen production during thermal runaway makes this unreliable.
Not providing sufficient ventilation for the installation space, risking gas accumulation before there is visible smoke or flame.
Not accounting for cell propagation in the physical siting of a BESS (too close to combustible material, no compartmentation between multiple systems) — IEC 62933-5-2 certification of the system itself does not replace careful installation-level siting.
Assuming an apparently extinguished BESS fire is definitely over — reignition hours later is a well-known phenomenon in lithium-ion thermal-runaway incidents, which makes prolonged monitoring after an incident relevant.