Battery testing becomes key factor in electric vehicle industry development

Electric vehicles (EVs) will account for 13% of global auto sales in 2022 and are expected to account for 30% of global auto sales by 2030. The industry needs to continue to increase research and development efforts, launch more affordable models, and promote the transformation of the fuel vehicle market into the electric vehicle market. In the process of market transformation, there is a huge demand for power batteries with stronger performance and lower prices . Currently, power batteries are still the main factor leading to the high price of electric vehicles.

According to McKinsey & Company, the battery cell market is expected to grow at an annual rate of more than 20% and will reach US$410 billion by 2030. From 2020 to 2030, the market size will increase 10 times.

When designing a power battery, rigorous testing must be conducted to understand the battery's performance.

performance goals

Promoting the advancement of power batteries is the key to improving vehicle economy, social acceptance, and environmental sustainability. The U.S. Advanced Battery Coalition (USABC) has released guidance specifying performance and cost targets for next-generation electric vehicle batteries . As the automotive industry explores new battery chemistries and cell technologies, three main goals need to be achieved to increase EV adoption:

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• Reduce the cost of power batteries to no more than $100 per kilowatt-hour - with the ultimate goal of $75.

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• Increase electric vehicle range to 300 miles.

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• Reduce charging time to less than 15 minutes.

Only through rigorous testing can we ensure that each manufactured cell can meet mandatory performance standards. There is a lot of testing required during design, validation, and production, but this article will highlight a few key tests that help you understand battery quality.

Open circuit voltage (OCV) measurement

Batteries store energy , creating a voltage potential between the positive and negative terminals. We use this energy in circuits . When the battery is not connected to any circuit, this potential is called the open circuit voltage  (OCV). This value directly reflects the battery's state of charge, which is a measure of how much energy the battery contains.

The OCV of a battery changes during charging and discharging. Monitor the status of the battery during charging and discharging to ensure that the battery is neither overcharged nor discharged. Batteries must be charged and discharged multiple times during the manufacturing process, and OCV monitoring is part of the validation process and final application. A battery pack consisting of a large number of cells contains a management chip that tracks the OCV of the cells and modules to report their status.

When the battery is disconnected from the load, there is still a small amount of current flowing internally. This is called self-discharge current. The OCV of a battery cell is relatively constant, but can vary slightly by tens to hundreds of microvolts over a period of weeks. For poor quality batteries, this number will be higher. OCV measurements can detect battery self-discharge and identify defective cells.

OCV is a relatively simple measurement. As shown in Figure 1, connect a digital multimeter  (DMM) directly to the battery and measure the DC voltage .

Figure 1. Performing an open circuit voltage measurement on a cell requires connecting a multimeter to the positive and negative terminals of the battery.

For applications that only require OCV confirmation, any 6.5-digit DMM will do the job. For applications such as self-discharge testing, where small changes in voltage need to be detected, a higher accuracy (7 and a half digits) DMM will be required.

For example, for a good-quality battery core, the charge loss caused by self-discharge within four weeks is usually small, generally in the range of a few millivolts to tens of millivolts. However, with failed or defective cells, this loss can amount to hundreds of millivolts. A few microvolts can be lost every day. Performing OCV measurements on 3.7 V cells, a typical 6.5-digit DMM has an error of 142 µV over 1 year of calibration. However, the 7.5-bit DMM has an error of 63.8 µV under the same conditions.

Internal resistance and load resistance

We can simply think of a battery as a cup filled with energy. When we need energy, connect the circuit and let the energy flow out. However, this analogy does not take into account the internal resistance of the battery . A battery is more appropriately compared to a water bottle. When a bottle filled with water is turned upside down, the water does not

Let it flow freely because the spout or neck of the bottle will block the flow of water. Similarly, batteries have internal resistance that blocks the flow of energy due to factors such as age, material quality, and structural defects. This internal resistance contains not only a resistive component but also a capacitive component, so it is difficult to measure.

Like OCV, internal resistance reflects the quality of the battery and how its performance changes over its lifetime. Batteries with higher internal resistance are less efficient and more likely to fail. Excessively high internal resistance will also generate excessive heat during battery operation. If the battery thermally loses control, it will pose a huge safety hazard. Measuring internal resistance before use can help identify cells that may be at risk of failure. For lithium-ion batteries , the internal resistance of a good-quality cell can reach 100 mΩ, while a poor-quality or failed cell may reach several hundred milliohms. There are several methods of characterizing internal resistance that are used to evaluate different aspects of performance.

Electrochemical Impedance Spectroscopy (EIS)

The first technique is electrochemical impedance spectroscopy (EIS). In this test method, an AC signal (usually a few hundred milliamps, but in some cases several amps) is applied to the battery over a broad frequency spectrum (0.5 Hz to over 100 kHz) and the battery's response is measured. This test can take anywhere from a few minutes to a few hours (the lower the frequency, the longer the test), but can yield a full picture of the impedance behavior within the battery.

AC internal resistance (ACIR)

The most common method is called AC Internal Resistance (ACIR). Since this is an AC technology, it does characterize internal resistance. ACIR is a subset of the EIS process that measures at a single frequency (usually 1 kHz). This test characterizes small signal performance, provides a perfect indication of battery quality, and is faster than the full EIS process. The short occupancy time makes it a popular testing method in production, and every battery must pass this test.

DC internal resistance (DCIR)

The final method is direct current internal resistance (DCIR), also known as pulse characterization. In this method, only the resistive component is measured because we assume that the battery is represented by an ideal open circuit voltage and series resistance, as shown in Figure 2.

Figure 2. The DCIR battery model includes an ideal voltage source and an internal resistor .

DC current is applied to the battery for a certain period of time. Measure the change in battery voltage to calculate resistance. The chart in Figure 3 demonstrates this.

Figure 3 DCIR measurement method measures the resistance component of a battery cell.

The current used in the DCIR method (several amps) is usually much larger than the ACIR method (100 mA) and is closer to the actual application scenario, since the battery is often subjected to sudden high currents. The internal resistance of the battery is the biggest limiting factor in the battery's ability to output high current. Therefore, it is important to identify batteries that cannot operate at high currents.

Test early and often

The validation process requires the implementation of a large number of tests, starting from chemical and material manufacturing (measuring isolation after electrolyte filling, cell charge and discharge, short circuit conditions, etc.) until the cells are ready to be packaged and shipped to the end application, such as grid energy storage, consumer electronics and electric vehicles. The goal of verification and validation testing is always to identify faulty cells before they proceed to the next process, avoiding wastage of material and time. Testing at every step is essential because there are many factors that may affect battery performance.

On the surface, open-circuit voltage testing and internal resistance testing have the same goal: to identify cells that fail to meet performance expectations. However, one cannot exist without the other as they each test different indicators. Open circuit voltage testing focuses on capacitance and self-discharge, properties that can arise from defects such as impurities in the separator or faulty formation during manufacturing. High impedance can be caused by other factors. The cause of the high impedance can only be found through direct impedance measurement. Such as a bad solder from electrode to tag.

For power batteries, the goals set by the United States Advanced Battery Consortium (USABC) for optimizing battery safety, performance and reliability should serve as a good guide for current compound batteries as well as emerging technologies beyond lithium-ion. The key to effective performance characterization is to start with

Effectiveness measurement begins. With the proper equipment and testing, manufacturers, researchers, designers and even end users can gain a complete understanding of the future performance of their batteries.