Completing Li-Ion Cell Self-Discharge Testing in Just Two Hours

Keysight Success Stories: Li-Ion self-discharge

Completing Li-Ion Cell Self-Discharge Testing in Just Two Hours

Propelled by the growing demand for electric vehicles (EVs), the worldwide market for lithium-ion (Li-Ion) batteries is projected to pass $40 billion by 2024. The main applications will be EVs, hybrid electric vehicles (HEVs), and a variety of batterybased energy storage systems (ESSs).

During normal usage, a cell loses stored energy in a process called self-discharge. Losing charge over time can occur even when a cell is not in use. Causes range from chemically unstable electrodes, impurities in the electrolyte, or simple aging of the battery. Warm temperatures will cause cells to self-discharge at a faster rate.

In manufacturing, individual batches of cells may exhibit different self-discharge behavior within the same production process. Consequently, battery makers choose to thoroughly test their cells and batteries as a way to assess product quality, reliability, and performance.

This situation faced one of the world’s leading original equipment manufacturers (OEMs) of Li-Ion cells. Its traditional approach to self-discharge testing was taking up to two weeks, and this was making it difficult to keep up with increasing order growth, product mix, and production volume.

The Challenge: Testing Faster to Meet Tighter Deadlines

Cells that exhibit high levels of self-discharge are more likely to fail. These cells must be identified and the underlying cause determined (e.g., faulty material, process or test). Traditional test methods relied on measuring the gradual decline in open-circuit voltage (OCV), typically measured passively over a period of several weeks (Figure 1).

A typical 100 kWh EV battery pack might have 8,000 standard “18650” cells (18 mm x 65 mm). For this OEM, order growth was ramping quickly and there was tremendous pressure to perform thorough testing in less time to meet increasingly tight delivery deadlines. As a leader in the industry, its approach to self-discharge testing was taking just two weeks—but a game-changing decrease in test time would provide a significant competitive advantage.

The OEM in this study has a global manufacturing footprint, with facilities located in some of today’s fastest-growing markets for EV, HEV, and ESS. Not only did it require a quick test solution, but the answer needed to meet global deployment requirements:

• Rapidly deployable
• Test plans are easy to replicate across different batches and different locations
• Have a compact form factor that is easy to integrate into the available space