Accelerating Life Cycle Testing for Battery Cell Manufacturing

Gigafactories have helped boost Lithium-ion (Li-ion) battery cell production from a capacity of 4 to 10 GWh per year to 40 to 80 GWh per year. Bringing cost efficiency to the forefront, high-volume battery cell production has led to a 90% plunge in EV battery prices over the past decade.

Scaling up battery cell production requires high precision, automation, and quality checks at every step of a highly complex process across the gigafactory to produce the battery cell, the basic unit of battery modules that make up battery packs to power electric drivetrains.

With expectations for longer driving ranges, faster charging, and continued reduction in EV battery costs, determining the longevity of an EV battery has become more important than ever. Most EV batteries have 1500 to 2000 charge cycles, depending on the battery pack design and EV range. Detecting defects during the production process and checking quality during the pre-charging / formation and aging steps are critical to ensuring the batteries function and fulfill their longevity as designed. Figure 1 shows a simplified diagram of a typical Li-ion cell fabrication process.

Detecting Manufacturing Defects During Battery Cell Production

Battery cell performance can differ from its original design specifications due to electrochemical or mechanical defects introduced during the manufacturing process.

Humidity, trace particle contaminants, and other factors adversely affect the cell, leading to faster discharge and cell failure. For example, tiny mechanical imperfections in a cell’s structure can generate significant deformities with each charge-discharge cell cycle, leading to shorter battery cell life.

Another example of defects entering the cell manufacturing process is during the tab-soldering step, where a mild tab burr can develop during tab welding. Tabs connect the anode and cathode layers to the cell’s external terminals. A burr can cause an internal short, leading to thermal runaway.

Easing “Bottlenecks” in the Gigafactory Workflow

In the high-volume battery gigafactory manufacturing environment, throughput is a vital barometer of productivity. Cell formation and aging however, are two time-consuming steps in the cell-finishing process. A published study on current and future Li-ion battery manufacturing showed that the cell formation and aging processes incur the highest cost, almost a third of the total manufacturing process cost (see Table 1).

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Table 1: Cost, throughput, and energy consumption of Li-ion battery manufacturing processes

Most of the total aging time comes from the process of determining whether the cell’s self-discharge behavior falls within acceptable limits. During cell aging, manufacturers must measure the cell’s self-discharge rate even when it is not connected to any device. The purpose is to sieve out errant cells that exhibit abnormal or excessive self-discharge, since such “bad” cells adversely affect the performance of modules and packs.

A cell can take days, weeks, or months to exhibit its self-discharge characteristics. However, in a time and cost-sensitive manufacturing environment, the traditional way of tracking self-discharge over a long duration is impractical.

Instead, some manufacturers now use a relatively new potentiostatic measurement method to directly measure the cell’s internal self-discharge current. This method typically takes hours or less compared with the traditional method of waiting for days or weeks to log the cell’s self-discharge performance, thereby saving time and precious floor space for holding the cells for this vital quality gate check. Reducing the time cells spend in the aging step also provides savings that flow directly to the gigafactory’s bottom line.

Future-Ready Cell Cyclers for Growing Capacity

As new technology and cell chemistries come together to create more powerful batteries that can charge faster, these cells must also undergo cell cycling tests. In these tests, the battery cells are repeatedly charged and discharged against different parameters to evaluate their performance and determine when the tested battery capacity fails to reach the specified capacity.

In a modern gigafactory line, it is impossible to perform cell cycling quality checks on every cell. Prolonged cycling also degrades the cell. Hence, manufacturers conduct this quality gate testing offline on a sampling basis, using a cell cycler to characterize the cell’s response over time through a series of charge / discharge cycles with capacity and efficiency.

Demand for electric vehicles with longer driving ranges and faster charging capabilities puts pressure on cell researchers and manufacturers, who have to cope with testing a greater variety of cells more quickly to meet time-to-market schedules. In the lab or on the production floor, this translates to the need to invest in cell cycling equipment with the flexibility to handle different cell types, and the capacity to source and sink larger currents. With growing cost of electricity, these cell cyclers will also need to be cost-effective to operate, and be easy enough for quick deployment in large numbers with production ramp-ups. Let’s look at a comparative case study involving traditional cell cyclers and a modern modular life cycle test chamber.

Improving Cell Cycling Efficiency with Modular Lifetime Test Chambers

There is much ongoing effort to develop innovative ways of boosting efficiencies, lowering operating costs, and further reducing the cost of the EV battery at the operational level for the battery gigafactory.  

Figure 2 shows a typical cell cycling test setup, with each chamber capable of holding 10 to 20 cells. Each chamber uses about 10 kW of power, not including the energy that the cycler needs for charging and discharging the cells.

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Figure 2. A traditional cell cycling test setup

In a new way of implementing quality gates for cell manufacturing, a modular cycle lifetime test chamber can help reduce floor space utilization and power consumption in the gigafactory. Figure 3 shows the novel modular quality gate setup from battery test solutions provider Keysight – created in partnership with Proventia. Each container-sized chamber holds up to 200 350A channels for prismatic cells and 100 10A channels for cylindrical cells, with safety and temperature control systems.

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Figure 3. A compact footprint modular lifetime test chamber for cell cycling. (Image credit: Proventia)

In the project by Keysight and Proventia, the collaborators found that for a traditional quality gate that would have required 18 traditional cyclers and their accompanying thermal chambers, the lifetime test chamber occupied only one-third of the floor space while providing equivalent channel capacity (see Figure 4).

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Figure 4. The novel modular lifetime test chamber (left) occupies 33% of the floor space that traditional chambers and cyclers would need to provide the same channel capacity

If higher testing capacity is needed, additional chamber blockscan be installed on-site more quickly to enable capacity expansion in tandem with manufacturing growth. Complete with safety contingency systems for Level 5 hazard control in the event of severe safety issues such as over temperature, system failure, or smoke, gases, and fire, this “super test chamber” provides a cost and time-saving alternative for EV cell manufacturers.

Operational Efficiency Saves Resources, Time, and Cost

Another cost-saving component of this modular lifetime test chamber as a cell cycling quality gate is that the cyclers use regenerative power. Energy from the discharging cells is recycled back to the AC power line. These regenerative power systems can recycle 75% or more of clean power. Besides lowering the electric bill, much less heat dissipation reduces stress on the system components, increasing equipment reliability. Less heat generation also allows for smaller system designs, further lowering floor space utilization. A comparative study of the modular lifetime cycling test chamber showed that it required only 50 kW of power versus 180 kW to run 18 traditional cell cyclers.

Besides lowering overhead costs with regenerative power, automating tests and leveraging test data are ways to develop better batteries faster. Most modern battery development and production facilities employ operations management tools that provide a 360-degree view of the battery test lab assets, software, test plans, results, and reports. Such data sets help battery developers gain insights into existing cell designs and how to improve new designs.

The Road to Profitability

As vehicle electrification gains momentum, automotive manufacturers and their battery supply chain will continue to look for ways to lower battery cost. Lower battery costs will help drive the switch from the commonplace internal combustion engine (ICE) to EVs for the mass market. Innovative ways of implementing cost-saving quality gates such as using compact, energy-efficient lifetime test chambers, along with regenerative power and software-centric operations, can boost efficiencies, lower operating costs, and help shorten the road to profitability while accelerating EV adoption.

Keysight Technologies