In general production testing, dry cells for lithium-ion batteries are typically tested with a singular focus on withstand voltage or insulation. Applying a sufficiently high test voltage and detecting electrical flashovers during the test can help to identify defective cells that pose a risk of future internal short circuits (due to partial punctures or perforations in the separator). However, manufacturers are often reluctant to raise the test voltage as a means to reduce internal short-circuit risks. This is because stacking and winding processes frequently cause the dielectric strength of semi-finished dry cell products to drop significantly compared to that of the original intact separator, it becomes difficult to implement, and the root causes is also hard to identity. In fact, the dry cell structure of a lithium-ion battery is essentially a plastic film capacitor. By measuring its capacitance (C), one can manage related manufacturing processes and material quality by understanding how C relates to material properties and physical structure.

From equation (1), we can identify several parameters related to capacitance (C):
①.    Separator: Incoming material (thickness, porosity), overheating during processing
②.    Winding/Stacking: Wrinkles, looseness, electrode alignment
③.    Fixture pressure

For example, when a lot exhibits abnormally low capacitance values, the possible causes and their impact on battery quality can be summarized as shown in Table 1.

▲【Table 1】Possible Causes and Quality Impacts of Abnormally Low Capacitance

As an example, consider the commonly overlooked factor of  "heating" and its impact on the insulation withstand performance of dry battery cells. During the winding and stacking processes, heating is often applied to bond and shape the electrodes and separator. Additionally, heating is used during the drying process. However, few studies address how such heating affects the withstand voltage of the separator. Figure 1 shows a real-world example of changes in a dry cell before and after heating/drying. Capacitance C dropped by about 20%, and the withstand voltage unexpectedly decreased by approximately 150V, reaching the air breakdown voltage of 350V.

▲【Figure 1】Possible Causes and Quality Impacts of Abnormally Low Capacitance

The cause in this case is believed to be the enlargement of pores within the separator due to heating (thus reducing capacitance). When multiple pore diameters exceed the separator thickness, the effective dielectric strength approaches that of air, resulting in a loss of the separator's original dielectric capabilities. Without relevant analysis, the common response may be to reduce the test voltage, unknowingly weakening the ability to detect high-risk defects like partial punctures or perforation, and thereby reducing battery safety. By managing the distribution of capacitance values, it becomes possible to monitor process and material consistency, detect anomalies during design changes or process modifications, and even identify early signs of aging in winding or stacking equipment. This approach helps mitigate risks and product losses during the early stages of cell manufacturing.

The Chroma 11210 Battery Cell Insulation Tester, combined with the A112101 PD/Flashover Analyzer, now features capacitance measurement functionality integrated directly into the testing process. This addition requires no extra test stations or test time, providing users with a more convenient tool for managing battery cell winding and stacking quality—ultimately enhancing battery cell safety and reliability.

For Chroma 11210's detailed product information, please visit Chroma's official website linked below.