What happens when a lithium cell fails, and what to do about it

Treat the sections below as a sequence. With battery failure, getting the early decisions right makes the later ones much easier.

The failure begins as a short circuit

A lithium cell holds a great deal of energy separated by a thin polymer separator, and puncturing, crushing or manufacturing contamination can breach it. Current then flows internally through the breach, generating heat in a very small volume. Metallic lithium deposited by charging when cold or too fast can grow structures that pierce the separator over time, producing the same result with no external damage.

This is why a cell can fail weeks after the abuse that caused it.

Runaway is a feedback loop

Above a threshold temperature the cell materials begin decomposing exothermically, producing more heat than the cell can shed. That raises the temperature further, accelerating the reaction, which is why the process is called thermal runaway. Some cathode chemistries release oxygen as they decompose, which allows the reaction to sustain itself without air.

That is the reason smothering is ineffective and the reason iron phosphate chemistry is considered safer.

Cooling is the intervention, not smothering

Because the reaction supplies its own heat and in some chemistries its own oxygen, removing air does little. Large volumes of water are effective mainly as a heat sink, cooling adjacent cells enough to prevent propagation.

At the protocol level, fire services train specifically for this and treat it as a chemical fire with a long re-ignition risk. A pack that appears extinguished can re-ignite hours later as heat migrates to cells that had not yet reacted.

Warning signs worth acting on

Swelling is the clearest: gases produced by internal decomposition inflate the cell, lifting screens, separating cases and jamming trackpads. Unusual heat while idle, rapid unexplained capacity loss, or a device that will not hold charge are weaker but real indicators. A dropped device that subsequently behaves oddly around charging deserves suspicion even if it looks intact.

Any of these means stop charging it, stop carrying it, and arrange disposal.

Handling and disposal

Do not puncture, crush or attempt to prise out a swollen cell, and do not put it in general waste or in a domestic bin. Battery fires in waste collection vehicles and recycling facilities are a documented and growing problem worldwide caused by exactly this. Most countries have designated collection points for batteries and electronics, and many retailers accept them.

Store a suspect device somewhere non-combustible and away from anything flammable until it can be handed in.

Reducing the risk in the first place

Use certified chargers and cables, since inadequate regulation is a common precipitating factor. Avoid charging in bed, under pillows or on soft furnishings where heat cannot escape.

In practice, avoid extreme temperatures, and never charge a cell that has been cold-soaked until it has warmed. Buy replacement cells from reputable sources, because counterfeit cells with no protection circuitry are widely sold and indistinguishable by appearance.

The takeaway

Swelling means stop. The reaction feeds itself, so prevention is the only reliable control.

Once you know what it is trading away, the design stops looking arbitrary.