Cell Vent - Battery Design

The cell vent design is a critical aspect of the overall design. Get this right and when things go wrong the cell will vent in a controlled manner. The cell vent is designed to operate at a particular pressure:

above the normal operating pressure
below the maximum cell strength limit

As the cell ages the internal pressure increases due to electrolyte decomposition to high vapour pressure gasses like CO₂, CO, CH₄, H₂, C₂H₄ etc. if you designed the cell with too low vent pressure the cell would vent way too early in its normal operating life. Additionally increased temperatures cause the internal pressure to rise so it needs a margin of safety to account for all these possible scenarios.

The cell vent needs to consider a number of aspects:

The design of the vent is mainly considered from two points: size and opening pressure, excluding information such as material, CID, etc. The area requirement can meet the exhaust rate requirements of cell failure, and the logic of the opening pressure is that it needs to be greater than the internal pressure of the cell EOL, but less than the strength requirements of the cell cover and can, of course, it also includes the strength of the packaging position, and this parameter takes into account the pressure tolerance of the vent. This can not only prevent the vent from being opened before the cell EOL, but also play a safety role.

Cylindrical Cell Sidewall Rupture

Cylindrical cell sidewall rupture is a significant issue in most module designs as it nearly always results in thermal propagation to neighbouring cells.

At present, the vent pressure value of most prismatic LFP cells is 0.6-0.7Mpa, while the NCM cell reaches 0.9Mpa, and the vent pressure of cylindrical cells is greater than this.
There is a certain empirical formula relationship between the cross-sectional size of the vent and the capacity of the cell.
In the early stage of design, it is necessary to consider the size, material selection, thickness, pressure value, opening position, patent status, and connection and fixing method of the top cover of the vent from a chemical and mechanical perspective. Some simulation and experimental design work is necessary.
Based on the use of the vent, some opening pressure tests, cyclic breathing tests, functional tests under different environmental conditions, etc. are usually performed. These will be required to be completed by the supplier or designer in the vent DVP. In addition, since the cell cycle is a long process, the creep of the vent material also needs to be considered.
The vent pressure is determined for extreme situations such as exposure to fire, internal short circuit, nail penetration. It is enough that this pressure is actually lower than the splitting of the case or the release of crimping. A cid pressure is sufficient to ensure safety under normal usage conditions.
Cell venting depends also on the space into which it is venting. This space and flow of gas impacts the back pressure and hence will influence the rate at which the gas is expelled.
The venting relationship around potted battery or using TIM as it directly impact the venting path inside the battery!