Venting - Battery Design

Gases can build up in a cell over time or suddenly when the cell fails. At some point, and depending on the cell design, the gas pressure will cause the safety valve in the cell to release or the cell case to fail.

We tend to use the description “Soft Venting” for a gas release from a cell that is not alight, “Hard Venting” is when a cell is in thermal runaway and the gases are combusting. These terms can be misinterpreted and hence it is best to fully describe the event.

A cell venting doesn’t necessarily mean it will go into thermal runaway, but a thermal runaway will be predated by the cell venting.

Gas Composition

The gases being vented will depend of a number of factors:

chemistry of the cell
trigger mechanism for gas evolution
age of the cell
state of charge of the cell
temperature of the cell

Typically the vented material will be composed of:

electrolyte
carbon dioxide
carbon monoxide
hydrogen
various VOC’s
hydrogen fluoride

This shows:

Increasing SOC => increasing fraction of hydrogen and carbon-monoxide
Increasing SoC => decreasing fraction of the inert carbon-dioxide decreases
- as CO2 decreases the overall hazard increases as there is less inert gas
Hydrocarbons are relatively constant versus SoC:
- 10-15% for NCA and LFP
- 20-25% for LCO

This table only shows the Volume Fraction and not the total amount of gas.

The gas composition, surrounding gas composition, laminar flame speed, lower flammability limit and maximum overpressure all need to be evaluated to understand the likelihood and severity of combustion.

The combustion metrics that were evaluated show that NCA and LCO vented gases produce higher flame speeds and maximum overpressures relative to LFP vent gases. LFP cells also have a higher LFL, which likely reduces the probability of a flammable ignition.
Austin R. Baird, Erik J. Archibald, Kevin C. Marr, Ofodike A. Ezekoye, Explosion Hazards from Lithium-Ion Battery Vent Gas, SAND2019-6428J

Cell Vent Design

The cell vent needs to remain intact at normal cell operational pressures and operate with a safety margin at a pressure that is lower than that which will cause an uncontrolled structural failure.

Electrical Creepage

Battery packs have a unique micro-environment because lithium-ion cells contain liquid electrolytes made of highly volatile organic solvents and conductive salts (like lithium hexafluorophosphate, LiPF6).

If a cell malfunctions and undergoes “soft venting,” it releases these vaporized solvents into the sealed battery enclosure.
When this vapor condenses on cooler insulating plastics (like cell holders, busbar mounts, or BMS boards), it leaves a highly conductive, moisture-absorbing salt residue.
This instantly changes the local environment from a clean state (Pollution Degree 2) to a highly contaminated state (Pollution Degree 3 or 4). If the battery pack was designed using low-CTI materials (like standard FR4), the existing creepage distances will be insufficient to halt the current. The conductive liquid film will initiate surface leakage and dry-band arcing, rapidly carbonizing the plastic. This forms a massive high-voltage short circuit that can melt components, ignite adjacent cells, and trigger a systemic thermal runaway.

Gas Volume

The volume of gas released is typically 1 to 2 litres per Ah of electrical capacity. This is just a rough estimate.

Sturk et al [1] measured the release of gas during thermal runaway in an inert gas chamber and got a significant difference in the volume of gas released for NMC/NCO compared to LFP.

NMC/LMO = 780 litres/kg
LFP = 42 litres/kg

Rupture Discs

In a pack design the vent gases will need to be released in a controlled manner.

OsecoElfab DualGard breather and vent disc

Pressure Equalization vs. Pressure Relief

Pressure equalization, commonly referred to as “breathing,” aims to maintain pressure balance within a battery enclosure due to environmental or elevation changes. Think of a plastic gas can in the garage in the summer months vs winter (it can be expanded or contracted in shape). However, pressure equalization alone does not mitigate overpressure risks. Rupture discs provide active pressure relief by opening at predetermined thresholds, exhausting excess pressure, and preventing catastrophic explosions.

Soft and Hard Venting

Thermal Runaway

If you heat a battery cell to somewhere above 130°C then exothermic chemical reactions inside the cell will increase the temperature and further reactions will take place. The result is an uncontrolled runaway and increase in temperature. The cell should vent in a controlled manner with fire and molten material. In severe cases the cell may explode. The energy released from one cell failing is likely to heat neighbouring cells that again could be triggered into thermal propagation.

Vent Clearance

The minimum physical gap that must be left between adjacent cells so that a venting cell’s escaping gas and pressure aren’t blocked by its neighbour or by a the wall of an enclosure.