Lithium-ion battery manufacturing process, pole piece production is completed, the positive and negative pole piece and the diaphragm winding or laminated together, the separator will be positive and negative pole pieces isolated in the battery charge and discharge process, the diaphragm isolation Electrode conduction between positive and negative plates, allowing lithium ions to pass through.
After winding or laminating, the bare cell needs to be hot-pressed to shape the tabs and the diaphragm so that they are in closer contact with each other to reduce the resistance of the lithium-ion transmission, and at the same time of hot pressing, the bare cell is often insulated Withstand voltage test, mainly to determine whether there is foreign particles inside the cell caused by short circuit.
Insulation withstand voltage test commonly used safety device, the test, the instrument to the battery plus a voltage, the voltage for a specified period of time, and then detect the amount of leakage current is maintained within the specified range, determine the positive and negative batteries have No short circuit. In general, the applied voltage shown in Figure 1:
Figure 1 insulation withstand voltage test diagram
① Within a certain period of time t1, the voltage applied to the cell increases from 0 to U.
② voltage U to maintain a period of time to t2.
③ test is completed, cut off the test voltage, and stray capacitance batteries discharge.
In the test, positive and negative pole pieces close to each other, only 15-30μm, the bare cell core will form a certain capacitance (stray capacitance), due to the existence of capacitance, the test voltage must start from 'zero', the slow rise, In order to avoid the charging current is too large, the greater the capacity required for the ramp-up time t1, the voltage can be increased at a lower charge current is too large, it will certainly lead to misjudgment of the tester, the test results Incorrect. Once the stray capacitance of the tested cell is full, only the actual leakage current will remain. Since DC withstand voltage test will charge the tested cell, after the test, it is necessary to discharge the tested cell .
The diaphragm has a certain dielectric strength, when the load voltage is too high, it can certainly penetrate the diaphragm, the formation of leakage current. Therefore, the first cell insulation test voltage is lower than the breakdown voltage shown in Figure 2, when the positive and negative Between the absence of foreign body, the leakage current under the test voltage is less than the specified value, to determine the cell qualified.And if there is a certain size between the positive and negative foreign body, the diaphragm is squeezed, the gap between the positive and negative decreases, Breakdown voltage between positive and negative will drop, if the same voltage is also loaded, the leakage current may exceed the set alarm value by setting the test voltage and other parameters, you can statistical analysis to determine the size of foreign body batteries, and then according to Actual production status and quality requirements, you can set the test parameters, to develop quality standards.
Figure 2 foreign body size and withstand voltage test example (the value of the assumed value)
In the test, the main parameters include the voltage ramp-up time t1, voltage hold time t2, the load voltage U, and alarm leakage current.As mentioned earlier, t1 and U and the stray capacitance of the batteries, the larger the required capacitance The longer the ramp-up time t1 is, the lower the load voltage U is, and U is also related to the dielectric strength of the separator itself. Generally, the thinner the diaphragm, the lower the dielectric strength is and the lower the test voltage page U. The setting of the alarm current Need to consider the capacitor charging current, the greater the charging current, the alarm current should be set larger, otherwise it will lead to miscarriage of justice, qualified products will be judged as unqualified, affecting the passing rate of the product.If the diaphragm moisture content is also easy to form a larger Leakage current. If there is foreign matter inside the test cell, resulting in an internal short circuit, the diaphragm is breakdown, the specific situation shown in Figure 3.
Figure 3 cell withstand voltage test internal foreign body short circuit
Therefore, the bare cell insulation withstand voltage test is an important step in the process of product testing, you can detect unqualified products, improve the safety factor of the final battery products.Practical testing need to consider many factors such as parameter setting, criteria and so on.
