How insulation testers reduce the risk of battery insulation fires

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Continued growth in energy demand (energy demand) and performance improvements have led to the rapid maturation of lithium-ion batteries in large energy-intensive applications such as electric vehicles (EVs) and various renewable energy storage systems (ESSs). However, with the rapid growth in battery usage, energy density, and high-speed charging and discharging has come an increase in fire and explosion incidents.

1. Why is it difficult to analyze the cause of most lithium-ion battery fires?

Commonly reported are errors in the electrical control system (electrical control system errors) or lithium metal deposits (lithium metal deposition) build up over time and grow into lithium dendrites (lithium dendrite), leading to short circuits inside the battery (short-circuits). While these are certainly possible factors, most accidents caused by rapid series chemical combustion are difficult to determine the specific factors.

2. What are the usual root causes of lithium-ion battery fires?

The majority of fire accidents in lithium-ion battery products (including electric vehicles) occur during charging. The main reason for this is that the negative material (graphite or hybrid silicon) continues to expand after repeated charging cycles, shortening the distance between the positive and negative charges. Due to the presence of electrical burrs or metal particles, this results in the effective distance between these electrics becoming shorter than originally designed, thus increasing the risk of internal short circuits.

3. Why is it so difficult to detect defects, such as partial piercing of the diaphragm by electrical burrs or metal particles, during the general production process?

A common problem in cell production inspection is the low insulation test voltage (<350V) for dry cells (jelly rolls) and the inability to detect electrical flashover caused by electrical burrs or metal particles during the insulation test. From the fire prevention point of view, the insulation test of lithium-ion battery cells should check the distance between the electricity rather than measuring the insulation resistance.

From the data of air breakdown voltage and distance, even if the gap distance is as short as a few um, the breakdown voltage is higher than 350V, so the insulation voltage test should be conducted at a voltage higher than 350V.

4. How should the test voltage be set to detect poor withstand voltages?

According to Chroma's recent test results on the withstand voltage (WV) of commercial diaphragms, the WV distribution of normal products is about 800V - 1200V; short-circuited products can be easily detected with <250V.

For abnormal products with diaphragm breakdown, the WV distribution is 400V - 600V, which seems to be consistent with the relationship between air breakdown voltage and gap. The WV distribution of products with high risk of partial breakdown is about 450V to 700V, which is between normal products and abnormal products with diaphragm breakdown. It can be seen that the usual test voltage setting of ≤250V can only recognize the short-circuit abnormal products, but cannot detect the diaphragm breakdown abnormal products and partially broken high-risk products.

5. Chroma ATE solution 11210 battery cell insulation tester

In order to improve the insulation quality test of Li-ion batteries, Chroma has specially designed a built-in +Flash test function, which can be set to test whether the electrical distance is sufficient by using a short-term high voltage first and then applying a higher voltage in the second stage.

In addition, there is a long-time low voltage test to detect abnormal leakage current that may be caused by other fine particles or defects in the diaphragm. The +Flash test function allows both test conditions to be met throughout the test, enabling a fast, complete solution for inspecting the insulation quality of lithium-ion batteries and reducing the risk of lithium-ion battery fires.