Battery cell formation and testing follow cell assembly and are critical processes to ensure high-performance and cost-effective electric vehicle (EV) battery packs. Formation activates the materials enabling the cell to act as a rechargeable battery. This FAQ reviews the cell formation process and the requirements for automated production and testing equipment, looks at battery formation, and testing system architectures including the importance of energy recycling.
The first step after cell assembly is ambient aging which gives the cell time to fully absorb the electrolyte prior to cycling for the first time during formation. Battery formation involves precisely charging and discharging the battery. During the formation process, the solid electrolyte interphase (SEI) on the anode and the cathode electrolyte interface (CEI) are formed. The formation of the SEI and CEI is sensitive to numerous factors. If they are not properly formed, cell performance can be significantly degraded. Formation can take several days for some types of Li-ion cells. It often uses a 0.1 C current rate, where C is the cell capacity, taking up to 20 hours for a single charge and discharge cycle.
High temperature (HT) aging at 30 to 50 °C takes place after formation. During HT aging, chemical reactions can occur due to lithium-ion concentration balancing in the electrodes, final SEI stabilization reactions, and final electrode wetting processes. Final testing and sorting can take several additional hours using a charge rate of 1 C and a discharge rate of 0.5 C for several cycles.
Battery formation and testing requires voltage and current tolerances of better than ±0.02% in the specified temperature range. The testing or grading process is a critical step in the fabrication of Li-ion cells for EV battery packs and ensures the uniform performance needed to support reliable operation. During testing, the cells are graded into three categories, A, B, and C. Only A-graded cells are suitable for use in EV battery packs. With a properly optimized cell assembly process, about 90% of the cells will be A-grade. B-grade cells can be used in battery packs for less demanding applications like battery energy storage systems while C-grade cells are only suitable for single-cell portable applications. The combination of high complexity, high precision, and high production throughput requires the use of automated battery cell formation and testing equipment (Figure 1).
Formation and testing systems topologies
Formation and testing systems are designed using different topologies optimized for specific types of cells. For small cells (under 5 Ah) that are used in portable electronics, manufacturing efficiency, and high-volume production are more important than cost. High-capacity cells used in EV battery packs demand higher levels of performance consistency.
The number of instrumentation channels for cell monitoring and testing is much higher when producing small- and medium-sized cells compared to high-capacity cells. Energy costs account for up to 30% of the production cost for large cells making energy recycling critical to hold down the cost of manufacturing large cells, and it is not used when producing small or medium-sized cells (Table 1). Depending on the system architecture, the recycled energy can be used within the system to support battery charging, or it can be fed back onto the AC grid.
Efficient Li-ion cell formation and testing are key activities to ensure high-performance EV battery packs. Automated production lines are often used that can support the high production volumes and provide the precision processing needed. In addition, EV battery cell formation benefits from energy recycling since energy can account for up to 30% of the manufacturing costs.
A Complete, Automated Solution For Battery Formation, Chroma ATE
Battery formation, Infineon
Power Efficient Battery Formation, Analog Devices
Ways of battery formation and difference comparison, Tycorun Energy
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