If the case of your bike, scooter or power tool battery has deformed, cracked, or looks swollen, that is an immediate danger signal. Swelling means gas is being released inside the lithium-ion or lithium-polymer cells, and it comes directly before thermal runaway and fire.
Here is why a lithium battery starts to gas, why it must be neither recharged nor kept in your home, and how to react to protect your home and the people in it.
Why does a lithium battery swell?
Swelling comes from chemical degradation of the electrolyte, which produces flammable gases inside a cell that is supposed to be sealed.
1. Cylindrical cells and soft Li-Po cells are not the same
E-bikes mostly use cylindrical cells in a rigid metal can, in 18650 or 21700 format. That metal body does not deform: under critical pressure a safety vent (CID/PRV) opens and releases the gas.
Some compact batteries and scooters, however, use soft lithium-polymer cells (pouch cells), wrapped in nothing more than an aluminised film. Those are the ones that can inflate like a balloon and split the plastic or aluminium case around them.
The NORDIX experience
We come across these soft-cell assemblies on certain scooters, notably from E-Twow. Swelling remains rare there: only two cases since our workshop opened.
One engineering rule then applies without exception: even if a single cell has swollen, the entire battery is scrapped and sent for recycling. Unlike standard cylindrical cells, a replacement pouch cell is almost never available — the formats are rare, and the remaining cells have never degraded identically. The pack is written off as a whole.
2. Excessive ageing and deep discharge
Leaving a battery fully discharged for a long period — a whole winter, for example — dissolves the copper current collectors inside the cells. On recharging, parasitic reactions start, with heavy gas release and a risk of short circuit.
3. Overheating and unsuitable chargers
Cheap or faulty chargers are one of the main causes of overcharge and boiling electrolyte.
The NORDIX experience
These chargers often have no serious output voltage regulation. We recently bench-tested a whole batch of third-party chargers for 13S batteries (36 V). Off load they correctly delivered 54.6 V, the expected 4.2 V per cell.
But under load these units kept pushing maximum current at a voltage above 56 V. That overcharge takes the cells past their permissible voltage, boils the electrolyte and leads to fire. The entire batch was rejected and returned to the supplier.
4. Mechanical shock
A heavy fall or an accident can deform cells with nothing visible from outside. If the internal structure and the separator between anode and cathode are damaged, a micro short circuit sets in. That area heats locally and gasses continuously, until the envelope swells.
Why a swollen battery must never be ignored
A swollen battery holds flammable gas and unstable lithium under pressure.
- Thermal runaway. At the slightest internal short circuit, the cell reaches 600–800 °C in an instant and ignites its neighbours.
- Household extinguishers are useless. A lithium-ion fire responds to neither powder nor CO₂: burning lithium does not need oxygen from the air to keep going.
The NORDIX experience
Tests in our former workshop left no room for doubt: simply to slow down — not even stop — the combustion of a single soft cell, it has to be fully immersed in a mass of water 40 to 60 times its own. No household extinguisher has any effect on a lithium fire.
The 4 steps to take immediately
If you find the battery deformed, cracked or swollen:
Unplug the charger immediately and do not try to switch the vehicle on.
Do not press on the case and never pierce the envelope to "let the gas out".
The NORDIX experience
Piercing the envelope makes a short circuit between the internal layers unavoidable: a single 15 Ah soft cell fills a 40 m² room entirely with dense, toxic smoke in 20 to 30 seconds.
Isolate the battery. Carefully take it outside, or place it on a non-combustible surface — concrete, tile — away from living areas, curtains and vehicles.
Do not put it in household waste. Damaged lithium thrown into a bin will start a fire in the collection truck or at the sorting centre.
Can a swollen battery be repaired?
A swollen cell cannot be "deflated" and cannot be repaired. Its chemical structure is irreversibly destroyed.
The only possible restoration is a full reconditioning, and only if the original case has not suffered from the deformation.
The NORDIX experience
Our workshop applies strict fire safety rules: all batteries are stored in a dedicated fire-rated room, and damaged packs are isolated from one another.
As soon as dangerous degradation is found — or merely suspected — the battery is immediately removed from the work area and handed to an approved recycler. We never try to "save" a swollen cell.
Swollen or deformed battery? Have it made safe by a partner workshop
Do not gamble with your home or your health. If your battery shows signs of swelling or serious overheating, describe the situation.
We will point you to the nearest partner in Bourgogne-Franche-Comté, working to the protocols for handling damaged lithium. NORDIX engineers handle safe dismantling, send the hazardous chemistry for recycling, and can propose a full reconditioning with new certified cells.