Lithium battery fires: Understanding thermal runaway and how to protect yourself

A lithium battery catches fire when one of its cells enters a thermal runaway state: it heats up, its chemical composition breaks down, releasing even more heat, and the reaction sustains itself before spreading to neighboring cells. This scenario is still rare, but it is violent, and most cases have an identifiable cause: a low-quality battery, an unsuitable charger, a shock, water damage, or improper charging practices. Understanding the mechanism allows us to know where to intervene to prevent it from ever happening.

Thermal runaway, explained simply

A lithium-ion cell contains two electrodes separated by a very thin membrane, immersed in a flammable electrolyte. As long as the cell remains within its voltage and temperature limits, everything is fine. However, if it exceeds a certain temperature threshold, internal reactions are triggered, which in turn generate heat. The temperature rises, which accelerates the reactions, and so on: this is thermal runaway.

The cell then releases flammable and toxic gases, can rupture violently, and reaches extremely high temperatures. In an electric scooter battery, the cells are packed tightly together: the heat from a single cell experiencing thermal runaway can trigger its neighbors. This propagation is what transforms a defect in one cell into a fire that engulfs the entire battery.

A lithium battery fire is difficult to extinguish: the reaction doesn't require oxygen from the air to continue within the cells, and a battery that appears to be extinguished can reignite long after.

What triggers a runaway condition

Protect yourself from the moment of purchase

A significant portion of the risk occurs even before the first charge:

• Branded cells, with publicly available specifications and a known origin. Recovered cells or those of questionable origin are the primary cause of defects.
• A reliable BMS (Battery Management System) that monitors each cell group, the current, and the temperature, and cuts off power in case of overvoltage. Our guide on the BMS and balancing details these protections.
• Careful assembly: Insulation between cells and towards the casing, appropriately sized connections, vibration dampening, as described in our guide on battery pack assembly.
• A battery sized for the required power: An original battery pushed by a more powerful controller overheats with each use.

All of our electric scooter batteries are assembled in our workshop in France, using only branded cells, and are equipped with a BMS (Battery Management System) that manages charging, discharging, balancing, and thermal protection. For machines exposed to water and mud, we use sealed enclosures, as with our Performance battery for Talaria, which features a waterproof enclosure with a pressure valve.

Protect yourself during charging

Charging poses significant risks: the battery is under stress, and a latent defect may become apparent. The INRS (National Institute for Occupational Safety and Health) recommends, among other things, never charging a damaged battery, using the original charger or a compatible charger, respecting charging temperatures, avoiding covering the battery, and disconnecting it at the end of the charging cycle. The DGCCRF (Directorate General for Competition Policy, Consumer Affairs and Fraud Prevention) has also published "golden rules" that include not charging in a location that blocks an emergency exit. Specifically:

• Charge the device on a non-flammable surface, away from textiles, wood, and cardboard
• Never in a hallway, stairwell, or in front of an emergency exit
• In a room equipped with a smoke detector, and while staying within close proximity
• The battery is cooled down after a ride, and it never gets below 0°C.

The details are in our guide on how to charge your electric scooter battery: best practices.

Protect yourself every day

• After a fall or impact to the underside of the deck, have the battery inspected before recharging it.
• After submersion or heavy rain, do not charge the device until you have verified that there is no water inside the battery pack.
• Do not ride with a battery that frequently cuts out or overheats.
• Store the scooter away from exits and flammable materials.

The warning signs and the appropriate course of action are detailed in our guide on swollen, overheating, or odor-emitting batteries.

If a fire breaks out:

Safety is paramount. Do not attempt to move a battery that is smoking or on fire. Evacuate everyone, close the door to the room to contain the smoke, and call the fire department (18, or 112 from a mobile phone), specifying that it is a lithium battery. In a professional setting, the INRS (National Institute for Occupational Safety and Health) recommends completely covering a small battery with sand or immersing it in a large volume of water. If this cannot be done safely, isolate the battery from any combustible materials and allow it to burn. For a multi-kilowatt-hour scooter battery in a home environment, this is not a task for the average person.