Battery Fire Propagation Explained: Why One Failing Cell Rarely Stays Isolated

Propagation is the real failure mode

In lithium-ion battery incidents, the initial failure is rarely the most dangerous part.

The real risk is propagation; the process by which a single failing cell triggers failure in neighboring cells, escalating into a system-level event. Propagation occurs only after a cell enters thermal runaway, which marks the beginning of internal failure and heat generation.

This escalation mechanism is a key reason why EV fires behave differently than gasoline fires, particularly in terms of duration, re-ignition risk, and suppression demands.

Understanding propagation explains why battery fires:

• escalate so rapidly

• overwhelm suppression efforts

• are difficult to contain once they begin

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What is battery fire propagation?

Propagation occurs when heat, gas, or flame from one failing cell causes adjacent cells to reach their own failure thresholds.

In battery systems, cells are:

• densely packed

• thermally coupled

• electrically interconnected

This makes them especially vulnerable to cascading failure.

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How propagation begins

Heat transfer between cells drives propagation in dense battery systems.

1. Heat transfer between cells

When a cell enters thermal runaway, its temperature rises rapidly.

That heat transfers to adjacent cells through:

• direct physical contact

• module structures

• enclosure materials

If neighboring cells reach critical temperature, they also enter thermal runaway, even if they were initially undamaged.

2. OFF-GAS ignition accelerates spread

As a failing cell releases OFF-GAS, flammable gases can ignite.

This ignition:

• dramatically increases local temperature

• exposes multiple cells simultaneously

• accelerates heat transfer across the pack

This is why propagation often accelerates suddenly after initial venting.

👉 For context, see Battery OFF-GAS Explained.

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Why propagation is difficult to stop once it starts

Propagation is challenging because it is self-reinforcing:

• Heat causes failure

• Failure produces more heat and gas

• Gas ignition increases temperature further

• Adjacent cells reach failure threshold faster

By the time visible flames appear, multiple cells may already be compromised internally.

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Delay is not the same as prevention

Many battery safety strategies focus on delaying propagation.

Delay can be valuable — it buys time for:

• evacuation

• emergency response

• system shutdown

However, delay alone does not prevent propagation if:

• heat continues to accumulate

• OFF-GAS is not neutralized

• internal reactions remain active

True prevention requires intervention before adjacent cells reach critical conditions.

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Why propagation drives long-duration battery fires

Propagation explains why battery fires can last hours:

• Cells fail sequentially, not simultaneously

• Each new failure releases additional heat and gas

• Suppression must continuously manage new ignition sources

This behavior is fundamentally different from fuel-limited fires and is a key reason why battery incidents are so resource-intensive.

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Where suppression strategies struggle

Conventional suppression strategies face challenges during propagation because:

• Cooling must reach internal cell structures

• Suppressing visible flames does not stop internal heat transfer

• Re-ignition can occur as new cells fail

This is why battery fire safety requires early-stage intervention, not just reactive suppression.

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The critical takeaway

Propagation is what turns a single battery failure into a system-level event.

Any effective battery fire safety strategy must account for:

• thermal runaway initiation

• OFF-GAS generation

• and propagation between cells

Ignoring propagation means planning for containment too late in the event timeline.