What causes e-bike battery fires

Battery cells fail for a small number of reasons. A dropped or crushed battery carries an internal short that can sit dormant for weeks. A non-original charger pushes current that the battery was never built to accept. Every battery has a management system (the electronics that regulate charging and keep cells balanced), and a mismatched charger can overwhelm it. Cheap replacement batteries and aftermarket conversion kits often carry no such system at all, which is why London Fire Brigade, FDNY and Toronto Fire Services highlight uncertified and second-hand purchases as the common thread.

Age and heat compound all of it: a cell several hundred cycles old tolerates far less than a new one. Charging is one of the points at which these failures surface. A battery spends six to eight hours plugged in, and for most riders it's left unattended, usually indoors. That's a window in which a fault becomes a fire.

Why a lithium-ion fire behaves differently

The mechanism is thermal runaway: a failing cell heats, vents flammable gas, and drives its neighbours into the same reaction. Because the cell releases its own oxidiser, the fire sustains itself without drawing oxygen from the room. Water cools it but does not smother it, and a standard extinguisher does neither well.

"When these batteries go into thermal runaway, they are almost impossible to put out."

Marla Friebe, Deputy Fire Chief of Community Risk Reduction, Toronto Fire Services

Firefighters contain the spread by cooling the fire, waiting for the battery to burn itself out.

But before any of that happens, the battery cell off-gasses: it vents hydrogen and carbon monoxide. This gap between venting and flame is short, and it is the only chance to intervene. While a smoke detector waits for the fire, a gas sensor catches the off-gassing.

Where the risk sits in a building

Riders detach the battery and carry it to wherever charging is possible, which is rarely where it is safe. In an apartment building, that means wherever a socket is accessible, from the balcony to the broom closet. In an office, it means on the desk, by the cupboard, in a shared kitchen or wherever an extension lead can run through.

The theft risk compounds it. A battery is the most valuable removable part of an e-bike, so riders bring it indoors rather than leave it unattended. Wherever the battery ends up, it is inside the building with everyone else.

"Firefighters are currently attending an e-bike or e-scooter fire every other day, on average. These fires can be explosive and have devastating consequences."

Spencer Sutcliff, Deputy Commissioner and Operational Director for Prevention, Protection and Policy, London Fire Brigade

E-bike numbers are set to double this decade, from more than 36.5 million worldwide to roughly 77.3 million by 2030. With most being urban commuting bikes, a building is no longer managing one person's battery but dozens, whether they charge overnight at home or from 9 to 5 at the office.

Who is responsible

The answer depends on the type of building, and both versions land in the same place.

In a workplace or a commercial building, the operator controls the premises and carries the duty of care. If a rider charges on a socket you provided, in a space you manage, the liability sits with the building rather than the rider.

In residential buildings the picture is messier. Someone charging inside their own home is largely their own business, but common areas, storage rooms and shared corridors belong to whoever owns or manages the building. Blanket bans are common and rarely work, because a resident who cannot charge in the common areas charges in the hallway or the apartment instead.

Insurers tend to move first, because they don't need permission to change a policy. Where a policy attaches conditions to how and where batteries charge, a safety question becomes a financial one. Hotel Okura Amsterdam, a luxury hotel, reached that point when guests began bringing batteries into the building and charging them under their chairs during dinner in the hotel's Michelin-starred restaurants. The hotel's insurer issued an ultimatum: batteries out of the building, or cover was at risk. Rather than ban e-bikes, management installed two charging lockers on the exterior premises, one for guests and another for staff, next to their bike parking.

What reduces the risk

Five measures do most of the work, and they run in sequence: sense the failure, cut the power, clear the gas, contain what is left, and tell somebody.

Sensing has to happen at the gas stage rather than the smoke stage, which means detectors for hydrogen, CO2 and CO alongside temperature. Power has to cut automatically at threshold, per compartment, because a control that waits for a person is not a control at night. Extraction has to remove the gas that would otherwise ignite. Containment has to be built into the compartment itself, with passive propagation resistance so that one cell failure cannot reach the battery beside it. And someone has to be told, by alarm on site and by alert off it.

Containment also does the heaviest lifting, because a battery does not have to be charging to fail. FDNY found that 59 percent of New York's lithium-ion battery fires in 2023 started when the battery was not charging, which makes secure storage as much of the answer as controlled charging.

Not every site needs all five measures. A locker that sits outside the building can rely more heavily on containment, because a failure has nowhere to spread and nobody to reach. A locker inside an occupied building needs the full sequence, since ventilation, distance and open air can no longer be assumed. The trade-offs between indoor and outdoor charging depend on the site.

This is the difference between a battery charging locker built as infrastructure and a cupboard with sockets in it. Both hold batteries. Only one assumes a battery will fail. PowerShelter lockers are tested under thermal runaway conditions to VDMA 24994 methodology, informed by the UL 1487 and UL 4900 safety frameworks. The full specification sits on our e-bike battery charging safety page.

Hotel Okura Amsterdam, ROC Hilversum, Thales and a national heritage institution in Amsterdam all reached the same conclusion: give riders somewhere better to charge than the building itself.

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