​​Charging Ahead: Battery Cell Quality Testing in EV Production​

Validation engineers use various tests to verify aspects of battery cell quality and performance. Each test has different objectives, advantages, and disadvantages. 

Visual Inspection

Human visual inspection is a simple test to check for physical defects or abnormalities, although it’s prone to subjectivity and human error. An inspector may check electrode coating, weld integrity, and the cell structure.

As automation becomes more prevalent in battery cell manufacturing, robotic systems with cameras and computer vision algorithms can perform visual inspections with speed, consistency, and precision. Automated visual inspection is a valuable tool for streamlining quality control processes, enhancing inspection capabilities, and boosting manufacturing efficiency.

DCIR Impedance Tests

Direct current internal resistance (DCIR) tests measure resistance to direct current flow. The battery is injected with a relatively high intensity current pulse to measure the ohmic response of the cell. Pulses are applied when the battery is at different states of charge to map the response across the cell range, both charging and discharging. This process provides insights into the internal condition of the cell and its functionality.

While DCIR doesn’t provide the most comprehensive analysis, it’s a quick and simple approach to get a baseline measurement of internal resistance. Alternating current internal resistance (ACIR) and electrochemical impedance spectroscopy (EIS) tests provide more information about battery impedance, but they are more time-consuming and require specialized equipment. 

ACIR Impedance Tests

Unlike DCIR, which measures resistance to direct current, ACIR evaluates the impedance of the battery under alternating current flows. A sinusoidal current is applied to the cell, and the voltage reaction is measured at specific frequencies (typically 1 kHz). 

By analyzing the voltage response, engineers can characterize the AC dynamic of the cell and detect critical defects. The results reveal information about various electrochemical processes, aging effects, and internal resistance variations across different frequencies.

EIS Impedance Tests

EIS tests are the most advanced type of impedance test. Instead of applying the sinusoid at one frequency and amplitude like an ACIR test, the stimuli are applied at many frequencies, typically from 0.1 Hz to 10 kHz or more. 

Defects that would otherwise go undetected by ACIR and DCIR tests are captured with EIS because these tests evaluate performance across a larger spectrum and wider frequency range. This method can help identify issues like premature aging. These insights improve the lab’s diagnostic capabilities and provide deeper insights into battery behavior. 

OCV Tests

Open circuit voltage (OCV) tests measure the cell voltage when no load is applied to it and no current is flowing outside the battery. This simple, non-intrusive test indicates whether a cell was correctly activated and characterizes how the cell degrades over time. However, this test provides limited insights into the cell’s internal condition. A cell with an OCV that is too low or too high indicates a cell quality concern.

OCV tests are often used to estimate the leakage current of a cell in the formation, pack assembly, and throughout the battery's lifecycle, rating its performance and checking its health. For example, OCV tests are used to sort and match cells during pack assembly.

Leakage Tests  

Leakage tests are helpful for the early detection of leaks, cell damage, or deterioration. These tests are performed in numerous ways to assess the quality of battery cells and packs. Most commonly, periodic OCV tests are performed to detect leakage when the cell is left to age in a climatically controlled environment. 

The industry is researching advanced methods. For example, a voltage can be imposed on a cell, and an additional current is supplied to stabilize that voltage. The additional current required to stabilize the voltage is an estimate of the internal leakage current. Unfortunately, this technique is time-consuming and does not produce repeatable results.  

Another method under investigation is similar to an EIS test. The cell is scanned with different current profiles at various states of charge and the response is measured. This approach is more promising, providing more consistent results, but still under development.

Most people associate quality with safety and performance, which are undoubtedly vital, but battery cell quality is also critical for scaling up production and responding to customer demands.

McKinsey estimates that global battery producers only have about 10 percent of the capacity required to meet 2030 targets. The availability and production of better batteries are critical for helping the automotive industry stay on course and meet its projected demands. 

By performing the right tests as early as possible in production processes, issues are detected before investing costly raw materials into a potentially faulty product. However, due to high volumes, long testing times, and the footprint of the production line, battery cell testing is adding complexity to traditional production challenges.

An integrated test solution comprised of hardware, software, and automation allows you to analyze trends, identify crucial steps in the process for testing, streamline workflows, and tackle bottlenecks. An investment in modernizing test will boost production throughput with less rework while reducing scrap and raw material costs. 

Keeping up with today’s market demands is challenging when you are also trying to grow and innovate. Battery technology is changing faster than industry players can dissect test data and establish standards. 

Many of the industry’s current test procedures evolved from legacy technology, but emerging battery technology demands we change the way we test. As technology takes strides forward, it’s critical to continuously evaluate your test strategy and systems.

Hyper-automation, artificial intelligence, and machine learning are integral components of a long-term test strategy that require action today. Incorporating these technologies will improve accuracy, optimize efficiency, and produce quantifiable business results for battery providers who invest.

Fragmented systems often lead to inconsistent data storage practices and manual analysis. Over time, this limits your organization’s ability to efficiently perform root cause analysis, solve problems, and innovate. 

As the ability to connect and collect vast amounts of data has increased, advanced analytics has worked to solve fragmentation issues. Advanced analytics is an approach to data analysis leveraging statistical, machine learning, and operational research techniques to discover patterns, gain insights, and predict outcomes from large data sets. 

By tracking volumes of data across production lines, from the cell level to the pack level with advanced analytics, you can connect and analyze test results to make data-driven decisions, prevent defects, and improve battery performance. A robust battery analytics solution streamlines collection, aggregation, and analysis for identifying urgent red flags as well as long-term trends. 

The tiniest variables and environmental factors can impact battery performance; tight controls are essential to reduce noise in test results and improve defect detection. The consistency of your testing hardware, software, and processes helps with the comparability and reproducibility of results across different batches, products, or suppliers.

Additionally, standardization is critical to meeting various national and international regulatory requirements and audits. 

Batteries are complicated, the market is competitive, and the technology is evolving quickly. An up-to-date battery cell quality test strategy is critical to delivering higher-performance batteries faster with automation, optimized workflows, and powerful data analytics.

Companies that modernize their testing approaches can improve battery cell quality and turn test into a strategic advantage that sets them apart in this highly competitive and rapidly evolving market.

NI’s extensive Guide to Testing Battery Cell Quality walks you through the basics of battery production, compares testing methods, and discusses advanced testing solutions. This white paper equips test engineers and plant managers with foundational knowledge and emerging trends to inform a future-proof battery cell quality test strategy. 

• Watch the webinar on determining battery cell quality
• Read the Guide to Testing Battery Cell Quality
• Read more content from the H2 2023 NI Automotive Journal