Meaning
Measured force applied to a group of rectangular battery cells held together in a module. This prismatic cell stack pressure is necessary to maintain the structural integrity and the electrochemical performance of the cells. It prevents the internal layers of the cell from separating as they expand and contract during use.
The boundary of this measurement is the contact area between the cells and the end plates of the module. Manufacturers specify an ideal pressure range that must be maintained throughout the life of the battery. It is a key parameter in the assembly of large format energy storage systems.
Interface Resistance
Keeping the cells under the correct pressure ensures that the active materials stay in contact with the current collectors. When prismatic cell stack pressure is too low, the internal resistance of the cells increases and leads to higher heat generation. This heat further accelerates the degradation of the chemistry and reduces the capacity.
If the pressure is too high, it can damage the separator and cause an internal short circuit. The pressure also affects the flow of the electrolyte within the cell. A consistent pressure across the entire surface of the cell is required for uniform current distribution.
This uniformity prevents the formation of hot spots that can lead to early failure of the battery.
Compression Fixture
The mechanical hardware used to apply the force must be robust and stable over time. This prismatic cell stack pressure is typically generated by metal bands, bolts or spring systems. The fixture must be able to handle the additional force created by the cells as they swell during charging.
This is known as the dynamic pressure, and it can be much higher than the initial assembly pressure. The choice of materials for the end plates is critical for preventing warping. If the plates bend, the pressure will be uneven and will cause the cells to age at different rates.
The assembly process includes a step to verify the pressure using load cells or torque wrenches.
Cycle Retention
Maintaining the proper compression is one of the best ways to extend the life of a prismatic cell. This prismatic cell stack pressure helps to prevent the delamination of the electrodes over many thousands of cycles. It ensures that the battery can meet the long term performance requirements for electric vehicles and grid storage.
The data sheets for the cells provide the recommended pressure for both the beginning of life and the end of life. These figures are used by the mechanical engineers to design the module enclosure. The final assembly provides a stable and high performance environment for the battery cells.
