Battery outgassing - causes, warning signs and safe handling

Battery venting – known as ‘venting’ in technical jargon – refers to the controlled or uncontrolled release of gases from a lithium-ion cell. Modern 18650, 20700 or 21700 cells have a predetermined breaking point on the positive terminal, through which excess pressure is released. This battery venting occurs when the internal pressure rises too high due to overheating, deep discharge or an internal short circuit.

The escaping gases are a mixture of electrolyte vapours, hydrogen, carbon monoxide and other organic compounds – flammable and harmful to health. Venting must be distinguished from thermal runaway: in a pure venting incident, only gases escape; the cell remains intact enough to prevent more serious consequences. In the case of thermal runaway, the cell continues to heat up until flames or a flash fire occur. Degassing is therefore the final protective mechanism before a cell fails completely.

The most common triggers are found in everyday life – and almost all of them can be avoided.

Mechanical damage

A dropped battery, a dented casing or damaged heat-shrink tubing can cause internal short circuits. Loose metal objects (coins, keys) in a trouser pocket are also a classic cause of spontaneous venting.

Overloading and incorrect settings

Anyone operating in the sub-ohm range below 0.15 ohms will quickly draw currents exceeding 25 A. If this value exceeds the cell’s CDR (Continuous Discharge Rate), the cell will overheat. Power outputs exceeding 80 W per individual cell also cause stress in unbalanced dual setups.

Incorrect charging

Non-certified chargers, faulty USB cables or charging below 0 °C damage the cell chemistry. Particularly risky: charging 18650s in a mod that does not have a balanced dual-charging circuit.

Ageing

Batteries lose significant capacity after 300–500 cycles. A cell that drops below 3.6 V within a few hours of charging should be recycled.

Most incidents can be prevented by consistent battery safety measures. Pay attention to the following points:

• Transport only in hard-shell cases: Silicone sleeves or plastic boxes prevent contact with metal. Never carry them loose in a bag or rucksack.
• Check the heat-shrink tubing: As soon as the wrap shows cracks, cuts or discolouration, it must be replaced. A bare negative terminal edge is a direct short-circuit risk.
• Respect the CDR: Read the data sheets from reputable manufacturers (Molicel, Samsung, Sony/Murata, LG). Specifications such as ‘40 A’ on no-name cells are often exaggerated.
• Observe temperature range: Charge between 10 and 30 °C, store at around 40% charge. Never leave in the car in summer heat.
• Pair cells: In dual mods, only use cells of the same type, age and charge level (difference < 0.05 V).
• Use original chargers: An external charger with individual cell monitoring (e.g. with a balancer and cut-off function) is safer than USB charging in the mod.

If you notice warning signs – unusual heat, hissing, a sweet smell, a swollen cell – take the battery outside immediately, place it on a non-flammable surface (concrete, sand) and keep your distance. Water is of limited use in lithium fires; a metal fire extinguisher (Class D) or plenty of sand are more suitable.

Can a battery simply start to off-gas?

Spontaneous outgassing without external influence is rare but possible – usually due to manufacturing defects or severely aged cells. In over 90% of cases, incidents are caused by mechanical damage, incorrect use or unsuitable chargers.

Are built-in batteries in pod systems safer?

Built-in batteries usually have protective electronics with overcurrent, overheating and short-circuit protection. The risk is lower, but not zero. Never leave these devices to charge overnight on hard, flammable surfaces without supervision.

What should you do if a battery is already emitting gas?

Ventilate the room immediately, remove people and pets, and take the cell outside. Do not touch it with bare hands – the cell can reach temperatures of over 150 °C. Do not inhale any fumes.

How do I dispose of a suspicious battery?

In Switzerland, any battery retailer will take back damaged cells (INOBAT system). Before doing so, place the cell in a fireproof container filled with sand or vermiculite; never put it in the household waste.

Battery gassing is not a fault that suddenly appears out of nowhere – it is almost always the result of several minor oversights. If you transport cells carefully, check the wrap regularly and use only certified charging equipment, you’ll reduce the risk to a minimum. You’ll find suitable charging accessories and protective cases in our range, as well as fresh branded cells in the battery mod accessories section. Take your time to browse and choose the components that suit your setup and usage profile.