Battery cell formation and testing follow cell assembly and are critical, helping ensure high-performance and cost-effective electric vehicle (EV) battery packs. Formation activates the materials, enabling the cell to act as a rechargeable battery.
The following article reviews:
- The battery cell formation process
- The requirements for automated production and testing equipment
- Battery formation and testing system architectures
- The importance of energy recycling.
The first step after battery cell assembly is ambient aging, giving the cell time to fully absorb the electrolyte before cycling for the first time during formation. Battery formation involves precisely charging and discharging the battery. The solid electrolyte interphase (SEI) on the anode and the cathode electrolyte interface (CEI) are formed during this formation process.
The SEI and CEI formation process is sensitive to several factors. For some types of lithium-ion (Li-ion) cells, formation can take several days and the cell’s performance can be significantly degraded if improperly formed. The process typically uses a 0.1C current rate, where “C” is the cell capacity — taking up to 20 hours for a single charge and discharge cycle.
After formation, high-temperature (HT) aging occurs at 30° to 50° C. During HT aging, chemical reactions can occur due to the Li-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 1C and a discharge rate of 0.5C for several cycles.
Battery formation and testing require voltage and current tolerances of better than ±0.02% in the specified temperature range. The testing or grading process is critical in fabricating Li-ion cells for EV battery packs and ensures the uniform performance needed to support reliable operation.
Figure 1. EV battery cell formation and testing are complex and precise processes that benefit from high levels of automation. (Image: Chroma ATE).
During testing, the cells are graded into three categories: A, B, and C. Only A-graded cells are suitable for EV battery packs. About 90% of the cells will be A-grade with a properly optimized cell assembly process. 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 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) 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 than high-capacity cells. Energy costs account for up to 30% of the production cost for large cells, making energy recycling critical to holding down the cost of manufacturing large cells. It’s 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 be fed back onto the ac grid.
Table 1. Formation and testing systems for large EV battery cells (right-hand column) require fewer channels but demand higher accuracy and energy recycling (Table: Analog Devices).
Summary
Efficient Li-ion cell formation and testing are essential for ensuring high-performance EV battery packs. Automated production lines are often used to support 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.
References
- 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
Filed Under: Batteries, Battery Power - EV Engineering, Charging, FAQs, Featured, Safety and Reliability