The significance of nail penetration testing
Nail Penetration Test is an internal short circuit testing method that tests the safety of lithium-ion batteries to withstand internal short circuits. Use steel nails to penetrate the battery, simulate an internal short circuit, and conduct a test to confirm if the battery is smoking, catching fire, or breaking. In addition, nail penetration testing is not only a test to confirm the safety of the battery, but also a test to understand the basic properties of the battery.
Under normal conditions, the positive and negative electrode plates of lithium-ion batteries are insulated by a polymer insulating film in the organic electrolyte – a separator. In this state, inserting the steel nail into the interior of the lithium-ion battery creates a short circuit between the positive and negative electrode plates, forcing an internal short circuit test. The characteristic of this testing method is that it can adjust the testing conditions such as the diameter, material, insertion depth, insertion position, and insertion speed of the steel nails inserted into the battery.
The danger of nail penetration
The need for nail penetration testing in lithium-ion batteries means that they are prone to short circuits inside, and if a short circuit occurs, they will be in a very dangerous state. Lithium ion batteries have the characteristics of high energy density, low internal resistance, and allowing for high current flow, making them highly hazardous. Generally speaking, internal short circuits in batteries during use are caused by the presence of conductive foreign objects during the manufacturing process, or by external impact or stress.
In reality, once a product is manufactured, it is difficult to take measures for internal short circuits in control systems, including battery charging and discharging circuits. When an internal short circuit occurs, a huge short-circuit current is generated inside the battery, which in turn generates Joule heat. This heat causes a reaction of flammable organic electrolytes, producing high-temperature gases, and increasing the likelihood of thermal runaway. When thermal runaway occurs, smoke and fire may occur, and in severe cases, it may rupture, endangering the personal safety of the user.
From the user’s perspective, ensuring the safety of the battery is crucial for the application of lithium-ion batteries. Needle testing involves inserting steel nails into the battery, which can easily create an internal short circuit between the positive and negative electrodes. However, when the nail is inserted into the battery to form a hole on the surface of the battery, this hole will release high-temperature gas, causing a change in the internal heat dissipation state of the battery, which may differ from the actual internal short circuit situation.
Blunt nail test
The mandatory internal short circuit test uses a spherical pin at the top, which is a testing method that can create an internal short circuit (small short circuit) without causing perforation on the battery surface, known as the Blunt Nail Test. This method creates a short circuit between the electrode materials (positive and negative electrode plates) of the battery through the pressure of nails, resulting in only a slight deformation of the tested battery. Compared to the usual nail penetration test, this method can create a state closer to the actual internal short circuit.
Preparation of test chamber
To conduct this test, manually inserting nails into lithium-ion batteries is definitely not feasible. Do not attempt it easily – nails may fly out, causing hand and body injuries. Therefore, it is necessary for us to use a needle testing machine. The DGBELL nail penetration testing chamber used here (with a maximum pressure of 20 k N and a stroke of 200 mm) has been made to be able to install steel nails of two different diameters（ φ 3 mm φ 5 mm, 100 mm)
Security measures
For safety reasons, the testing machine has an explosion-proof function to prevent battery fires and explosions from endangering the safety of testing personnel. Due to price considerations, the testing machine has options for hydraulic motors and computer servo motors. The hydraulic motors are easy to operate, while the servo motors can be set with more detailed testing conditions, resulting in higher accuracy. Equipped with a temperature and voltage acquisition system, the battery temperature data is collected through K-type thermocouples, plug and play, with a measurement range of 0-600 ℃. The voltage collection range is 0-50 V. The test box is equipped with fixtures that can fix batteries and steel needles of different sizes. The insertion speed of the nail is adjustable from 0.1 to 80 mm/s.
Related precautions
Here, we test three different types of stacked batteries (300 m Ah, 1000 m Ah, 2000 m Ah). The data to be measured is as follows:
- The variation of battery terminal voltage over time
- Battery surface temperature
- Changes in the appearance and shape of the battery after nail penetration
Before conducting the experiment, it is necessary to carefully prepare fire prevention and safety measures, such as preventing splashing, leaving enough maintenance space for the testing machine, and ensuring that the fire extinguishing system can operate normally.
Differences caused by nail penetration conditions
Based on several experimental results, including the above test results, the following facts can be inferred.
(1) At the puncture point, the terminal voltage drops significantly and gradually drops to 0 V. When the voltage drops to a certain extent, there will be significant fluctuations in the voltage curve.
(2) At the stage of significant voltage changes from the needle point to the terminal, the test results of different battery types and needle conditions also vary, but the terminal voltage will drop to 0 V after 10-20 seconds.
(3) When testing batteries of the same capacity, the thicker the nail, the greater the change in terminal voltage.
Measurement of battery surface temperature
After nail penetration, the surface temperature of the stacked battery can be obtained over time through a temperature acquisition system. Using φ The surface temperature change measured during the puncture test of a 2000 m Ah stacked battery with a 5 mm steel nail.
Temperature rise after nail penetration
During the puncture test of a 2000 m Ah stacked battery with a 5 mm steel nail, there was a situation of accompanying fire and temperature exceeding 200 ℃. In addition, in the testing of 1000 m Ah battery samples φ A fire broke out when a 3 mm steel nail was punctured. Using φ In the puncture test conducted with a 5 mm steel nail, not only did a fire occur, but the battery temperature also increased significantly. In addition, we tested multiple battery samples using steel nails with different diameters, and the trend of temperature rise over time is similar to the above figure. Moreover, the highest temperature measured for different battery cells can also be below 100 ℃.
Summary
This test, by short circuiting the positive and negative electrodes, instantly converts the stored electrical energy inside the battery into thermal energy, generating enough heat to cause the electrolyte to burn and explode. From an energy perspective, the heat generated by igniting gasoline is almost the same. What a dangerous test this is! I believe you already have some understanding, do not easily implement this test. However, as a safety assessment for designing and manufacturing batteries, nail penetration testing can be implemented without relying on a large amount of funding, and the measurement results can be reflected in the production. Finally, lithium-ion batteries are safe to use within the manufacturer’s specified rated range. Exceeding the rated range, such as repeated overcharging or discharging, can cause the battery to catch fire or be damaged.